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This is conceptually simpler, as the sweeper doesn't have to worry about
clearing them separately. It also doesn't have a use for them.
This will also be useful to avoiding unnecessary zeroing in
initInlineMarkBits at allocation time. Currently, because it's used in
both span allocation and at sweep time, we cannot blindly trust
needzero.
This change also renames mergeInlineMarkBits to moveInlineMarkBits to
make this change in semantics clearer from the name.
For #73581.
Change-Id: Ib154738a945633b7ff5b2ae27235baa310400139
Reviewed-on: https://go-review.googlesource.com/c/go/+/687936
Auto-Submit: Michael Knyszek <mknyszek@google.com>
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Add build tag gated Valgrind annotations to the runtime which let it
understand how the runtime manages memory. This allows for Go binaries
to be run under Valgrind without emitting spurious errors.
Instead of adding the Valgrind headers to the tree, and using cgo to
call the various Valgrind client request macros, we just add an assembly
function which emits the necessary instructions to trigger client
requests.
In particular we add instrumentation of the memory allocator, using a
two-level mempool structure (as described in the Valgrind manual [0]).
We also add annotations which allow Valgrind to track which memory we
use for stacks, which seems necessary to let it properly function.
We describe the memory model to Valgrind as follows: we treat heap
arenas as a "pool" created with VALGRIND_CREATE_MEMPOOL_EXT (so that we
can use VALGRIND_MEMPOOL_METAPOOL and VALGRIND_MEMPOOL_AUTO_FREE).
Within the pool we treat spans as "superblocks", annotated with
VALGRIND_MEMPOOL_ALLOC. We then allocate individual objects within spans
with VALGRIND_MALLOCLIKE_BLOCK.
It should be noted that running binaries under Valgrind can be _quite
slow_, and certain operations, such as running the GC, can be _very
slow_. It is recommended to run programs with GOGC=off. Additionally,
async preemption should be turned off, since it'll cause strange
behavior (GODEBUG=asyncpreemptoff=1).
Running Valgrind with --leak-check=yes will result in some errors
resulting from some things not being marked fully free'd. These likely
need more annotations to rectify, but for now it is recommended to run
with --leak-check=off.
Updates #73602
[0] https://valgrind.org/docs/manual/mc-manual.html#mc-manual.mempools
Change-Id: I71b26c47d7084de71ef1e03947ef6b1cc6d38301
Reviewed-on: https://go-review.googlesource.com/c/go/+/674077
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gcMarkTermination() ensures that all caches are flushed before continuing the GC cycle, thus preempting all goroutines.
However, inlining calls to lock() in bgsweep makes it non-preemptible for most of the time, leading to livelock.
This change adds explicit preemption to avoid this.
Fixes #73499.
Change-Id: I4abf0d658f3d7a03ad588469cd013a0639de0c8a
Reviewed-on: https://go-review.googlesource.com/c/go/+/668795
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Reviewed-by: Michael Pratt <mpratt@google.com>
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This change splits the finalizer and cleanup queues and implements a new
lock-free blocking queue for cleanups. The basic design is as follows:
The cleanup queue is organized in fixed-sized blocks. Individual cleanup
functions are queued, but only whole blocks are dequeued.
Enqueuing cleanups places them in P-local cleanup blocks. These are
flushed to the full list as they get full. Cleanups can only be enqueued
by an active sweeper.
Dequeuing cleanups always dequeues entire blocks from the full list.
Cleanup blocks can be dequeued and executed at any time.
The very last active sweeper in the sweep phase is responsible for
flushing all local cleanup blocks to the full list. It can do this
without any synchronization because the next GC can't start yet, so we
can be very certain that nobody else will be accessing the local blocks.
Cleanup blocks are stored off-heap because the need to be allocated by
the sweeper, which is called from heap allocation paths. As a result,
the GC treats cleanup blocks as roots, just like finalizer blocks.
Flushes to the full list signal to the scheduler that cleanup goroutines
should be awoken. Every time the scheduler goes to wake up a cleanup
goroutine and there were more signals than goroutines to wake, it then
forwards this signal to runtime.AddCleanup, so that it creates another
goroutine the next time it is called, up to gomaxprocs goroutines.
The signals here are a little convoluted, but exist because the sweeper
and the scheduler cannot safely create new goroutines.
For #71772.
For #71825.
Change-Id: Ie839fde2b67e1b79ac1426be0ea29a8d923a62cc
Reviewed-on: https://go-review.googlesource.com/c/go/+/650697
Reviewed-by: Michael Pratt <mpratt@google.com>
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It's the job of the last sweeper to emit the GC pacer trace. The last
sweeper can identify themselves by reducing the count of sweepers, and
also seeing that there's no more sweep work.
Currently this identification is broken, however, because the last
sweeper doesn't check the state they just transitioned sweeping into,
but rather the state they transitioned from (one sweeper, no sweep work
left). By design, it's impossible to transition *out* of this state,
except for another GC to start, but that doesn't take this codepath.
This means lines like
pacer: sweep done at heap size ...
were missing from the gcpacertrace output for a long time.
This change fixes this problem by having the last sweeper check the
state they just transitioned sweeping to, instead of the state they
transitioned from.
Change-Id: I44bcd32fe2c8ae6ac6c21ba6feb2e7b9e17f60cc
Reviewed-on: https://go-review.googlesource.com/c/go/+/670735
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Our current parallel mark algorithm suffers from frequent stalls on
memory since its access pattern is essentially random. Small objects
are the worst offenders, since each one forces pulling in at least one
full cache line to access even when the amount to be scanned is far
smaller than that. Each object also requires an independent access to
per-object metadata.
The purpose of this change is to improve garbage collector performance
by scanning small objects in batches to obtain better cache locality
than our current approach. The core idea behind this change is to defer
marking and scanning small objects, and then scan them in batches
localized to a span.
This change adds scanned bits to each small object (<=512 bytes) span in
addition to mark bits. The scanned bits indicate that the object has
been scanned. (One way to think of them is "grey" bits and "black" bits
in the tri-color mark-sweep abstraction.) Each of these spans is always
8 KiB and if they contain pointers, the pointer/scalar data is already
packed together at the end of the span, allowing us to further optimize
the mark algorithm for this specific case.
When the GC encounters a pointer, it first checks if it points into a
small object span. If so, it is first marked in the mark bits, and then
the object is queued on a work-stealing P-local queue. This object
represents the whole span, and we ensure that a span can only appear at
most once in any queue by maintaining an atomic ownership bit for each
span. Later, when the pointer is dequeued, we scan every object with a
set mark that doesn't have a corresponding scanned bit. If it turns out
that was the only object in the mark bits since the last time we scanned
the span, we scan just that object directly, essentially falling back to
the existing algorithm. noscan objects have no scan work, so they are
never queued.
Each span's mark and scanned bits are co-located together at the end of
the span. Since the span is always 8 KiB in size, it can be found with
simple pointer arithmetic. Next to the marks and scans we also store the
size class, eliminating the need to access the span's mspan altogether.
The work-stealing P-local queue is a new source of GC work. If this
queue gets full, half of it is dumped to a global linked list of spans
to scan. The regular scan queues are always prioritized over this queue
to allow time for darts to accumulate. Stealing work from other Ps is a
last resort.
This change also adds a new debug mode under GODEBUG=gctrace=2 that
dumps whole-span scanning statistics by size class on every GC cycle.
A future extension to this CL is to use SIMD-accelerated scanning
kernels for scanning spans with high mark bit density.
For #19112. (Deadlock averted in GOEXPERIMENT.)
For #73581.
Change-Id: I4bbb4e36f376950a53e61aaaae157ce842c341bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/658036
Auto-Submit: Michael Knyszek <mknyszek@google.com>
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We will want to reference these definitions from new generator programs,
and this is a good opportunity to cleanup all these old C-style names.
Change-Id: Ifb06f0afc381e2697e7877f038eca786610c96de
Reviewed-on: https://go-review.googlesource.com/c/go/+/655275
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The updates are:
- API documentation changes.
- Removal of the old package documentation discouraging linkname.
- Addition of new package documentation with some advice.
- Renaming of weak.Pointer.Strong -> weak.Pointer.Value.
Fixes #67552.
Change-Id: Ifad7e629b6d339dacaf2ca37b459d7f903e31bf8
Reviewed-on: https://go-review.googlesource.com/c/go/+/628455
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Instead, have the runtime build the gc bitmaps on demand
at runtime.
Change-Id: If7a245bc62e4bce3ce80972410b0ed307d921abe
Reviewed-on: https://go-review.googlesource.com/c/go/+/616255
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We can find a few issues finally turned out to be a race condition,
such as #47513. I believe such a tip can eliminate the need for developers
to file this kind of issue in the first place.
Change-Id: I1597fa09fde641882e8e87453470941747705272
GitHub-Last-Rev: 9f136f5b3bee78f90f434dcea1cabf397c6c05f2
GitHub-Pull-Request: golang/go#70331
Reviewed-on: https://go-review.googlesource.com/c/go/+/627816
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Currently freeSpan is called before large object stats are updated when
sweeping large objects. This means heapStats.inHeap might get subtracted
before the large object is added to the largeFree field. The end result
is that the /memory/classes/heap/unused:bytes metric, which subtracts
live objects (alloc-free) from inHeap may overflow.
Fix this by always updating the large object stats before calling
freeSpan.
Fixes #67019.
Change-Id: Ib02bd8dcd1cf8cd1bc0110b6141e74f678c10445
Reviewed-on: https://go-review.googlesource.com/c/go/+/583380
Auto-Submit: Michael Knyszek <mknyszek@google.com>
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This change adds expensive alloc/free events to traces, guarded by a
GODEBUG that can be set at run time by mutating the GODEBUG environment
variable. This supersedes the alloc/free trace deleted in a previous CL.
There are two parts to this CL.
The first part is adding a mechanism for exposing experimental events
through the tracer and trace parser. This boils down to a new
ExperimentalEvent event type in the parser API which simply reveals the
raw event data for the event. Each experimental event can also be
associated with "experimental data" which is associated with a
particular generation. This experimental data is just exposed as a bag
of bytes that supplements the experimental events.
In the runtime, this CL organizes experimental events by experiment.
An experiment is defined by a set of experimental events and a single
special batch type. Batches of this special type are exposed through the
parser's API as the aforementioned "experimental data".
The second part of this CL is defining the AllocFree experiment, which
defines 9 new experimental events covering heap object alloc/frees, span
alloc/frees, and goroutine stack alloc/frees. It also generates special
batches that contain a type table: a mapping of IDs to type information.
Change-Id: I965c00e3dcfdf5570f365ff89d0f70d8aeca219c
Reviewed-on: https://go-review.googlesource.com/c/go/+/583377
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
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allocfreetrace prints all allocations and frees to stderr. It's not
terribly useful because it has a really huge overhead, making it not
feasible to use except for the most trivial programs. A follow-up CL
will replace it with something that is both more thorough and also lower
overhead.
Change-Id: I1d668fee8b6aaef5251a5aea3054ec2444d75eb6
Reviewed-on: https://go-review.googlesource.com/c/go/+/583376
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This change adds the internal/weak package, which exposes GC-supported
weak pointers to the standard library. This is for the upcoming weak
package, but may be useful for other future constructs.
For #62483.
Change-Id: I4aa8fa9400110ad5ea022a43c094051699ccab9d
Reviewed-on: https://go-review.googlesource.com/c/go/+/576297
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This change removes the allocheaders, deleting all the old code and
merging mbitmap_allocheaders.go back into mbitmap.go.
This change also deletes the SetType benchmarks which were already
broken in the new GOEXPERIMENT (it's harder to set up than before). We
weren't really watching these benchmarks at all, and they don't provide
additional test coverage.
Change-Id: I135497201c3259087c5cd3722ed3fbe24791d25d
Reviewed-on: https://go-review.googlesource.com/c/go/+/567200
Reviewed-by: Keith Randall <khr@google.com>
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For #65355
Change-Id: I65dd090fb99de9b231af2112c5ccb0eb635db2be
Reviewed-on: https://go-review.googlesource.com/c/go/+/560155
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
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CL 555355 has a bug in it - the GC program flag was also used to decide
when to free the unrolled bitmap. After that CL, we just don't free any
unrolled bitmaps, leading to a memory leak.
Use a separate flag to track types that need to be freed when their
corresponding object is freed.
Change-Id: I841b65492561f5b5e1853875fbd8e8a872205a84
Reviewed-on: https://go-review.googlesource.com/c/go/+/555416
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The pointer stored in mspan.largeType is an invalid pointer when
the span is an arena. We need to make sure that pointer isn't seen
by the garbage collector, as it might barf on it. Make sure we
zero the pointer using a uintptr write so the old value isn't picked
up by the write barrier.
The mspan.largeType field itself is in a NotInHeap struct, so a heap
scan won't find it. The only way we find it is when writing it, or
when reading it and putting it in a GC-reachable location. I think we
might need to audit the runtime to make sure these pointers aren't
being passed in places where the GC might (non-conservatively) scan a
stack frame it lives in. (It might be ok, many such places are either
systemstack or nosplit.)
Change-Id: Ie059d054e0da4d48a4c4b3be88b8e1e46ffa7d10
Reviewed-on: https://go-review.googlesource.com/c/go/+/548535
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Currently the execution tracer synchronizes with itself using very
heavyweight operations. As a result, it's totally fine for most of the
tracer code to look like:
if traceEnabled() {
traceXXX(...)
}
However, if we want to make that synchronization more lightweight (as
issue #60773 proposes), then this is insufficient. In particular, we
need to make sure the tracer can't observe an inconsistency between g
atomicstatus and the event that would be emitted for a particular
g transition. This means making the g status change appear to happen
atomically with the corresponding trace event being written out from the
perspective of the tracer.
This requires a change in API to something more like a lock. While we're
here, we might as well make sure that trace events can *only* be emitted
while this lock is held. This change introduces such an API:
traceAcquire, which returns a value that can emit events, and
traceRelease, which requires the value that was returned by
traceAcquire. In practice, this won't be a real lock, it'll be more like
a seqlock.
For the current tracer, this API is completely overkill and the value
returned by traceAcquire basically just checks trace.enabled. But it's
necessary for the tracer described in #60773 and we can implement that
more cleanly if we do this refactoring now instead of later.
For #60773.
Change-Id: Ibb9ff5958376339fafc2b5180aef65cf2ba18646
Reviewed-on: https://go-review.googlesource.com/c/go/+/515635
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This change replaces the 1-bit-per-word heap bitmap for most size
classes with allocation headers for objects that contain pointers. The
header consists of a single pointer to a type. All allocations with
headers are treated as implicitly containing one or more instances of
the type in the header.
As the name implies, headers are usually stored as the first word of an
object. There are two additional exceptions to where headers are stored
and how they're used.
Objects smaller than 512 bytes do not have headers. Instead, a heap
bitmap is reserved at the end of spans for objects of this size. A full
word of overhead is too much for these small objects. The bitmap is of
the same format of the old bitmap, minus the noMorePtrs bits which are
unnecessary. All the objects <512 bytes have a bitmap less than a
pointer-word in size, and that was the granularity at which noMorePtrs
could stop scanning early anyway.
Objects that are larger than 32 KiB (which have their own span) have
their headers stored directly in the span, to allow power-of-two-sized
allocations to not spill over into an extra page.
The full implementation is behind GOEXPERIMENT=allocheaders.
The purpose of this change is performance. First and foremost, with
headers we no longer have to unroll pointer/scalar data at allocation
time for most size classes. Small size classes still need some
unrolling, but their bitmaps are small so we can optimize that case
fairly well. Larger objects effectively have their pointer/scalar data
unrolled on-demand from type data, which is much more compactly
represented and results in less TLB pressure. Furthermore, since the
headers are usually right next to the object and where we're about to
start scanning, we get an additional temporal locality benefit in the
data cache when looking up type metadata. The pointer/scalar data is
now effectively unrolled on-demand, but it's also simpler to unroll than
before; that unrolled data is never written anywhere, and for arrays we
get the benefit of retreading the same data per element, as opposed to
looking it up from scratch for each pointer-word of bitmap. Lastly,
because we no longer have a heap bitmap that spans the entire heap,
there's a flat 1.5% memory use reduction. This is balanced slightly by
some objects possibly being bumped up a size class, but most objects are
not tightly optimized to size class sizes so there's some memory to
spare, making the header basically free in those cases.
See the follow-up CL which turns on this experiment by default for
benchmark results. (CL 538217.)
Change-Id: I4c9034ee200650d06d8bdecd579d5f7c1bbf1fc5
Reviewed-on: https://go-review.googlesource.com/c/go/+/437955
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Change-Id: Ia238889a704812473b838b20efedfe9d24b1e26f
Reviewed-on: https://go-review.googlesource.com/c/go/+/534160
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mspan.freeindex and nelems can fit into uint16 for all possible
values. Use uint16 instead of uintptr.
Change-Id: Ifce20751e81d5022be1f6b5cbb5fbe4fd1728b1b
Reviewed-on: https://go-review.googlesource.com/c/go/+/451359
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This change replaces the statically sized pinnerBits with gcBits
based ones, that are copied in each GC cycle if they exist. The
pinnerBits now include a second bit per object, that indicates if a
pinner counter for multi-pins exists, in order to avoid unnecessary
specials iterations.
This is a follow-up to CL 367296.
Change-Id: I82e38cecd535e18c3b3ae54b5cc67d3aeeaafcfd
Reviewed-on: https://go-review.googlesource.com/c/go/+/493275
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This change adds traceBlockReason which leaks fewer implementation
details of the tracer to the runtime. Currently, gopark is called with
an explicit trace event, but this leaks details about trace internals
throughout the runtime.
This change will make it easier to change out the trace implementation.
Change-Id: Id633e1704d2c8838c6abd1214d9695537c4ac7db
Reviewed-on: https://go-review.googlesource.com/c/go/+/494185
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Also, clean up atomics on released-per-cycle while we're here.
For #57069.
Change-Id: I14026e8281f01dea1e8c8de6aa8944712b7b24d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/495916
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More tightening up of the tracer's interface.
Change-Id: I992141c7f30e5c2d5d77d1fcd6817d35bc6e5f6d
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[git-generate]
cd src/runtime
grep -l 'trace\.enabled' *.go | grep -v "trace.go" | xargs sed -i 's/trace\.enabled/traceEnabled()/g'
Change-Id: I14c7821c1134690b18c8abc0edd27abcdabcad72
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Auto-Submit: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
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This change makes it so that on Linux the Go runtime explicitly marks
page heap memory as either available to be backed by hugepages or not
using heuristics based on density.
The motivation behind this change is twofold:
1. In default Linux configurations, khugepaged can recoalesce hugepages
even after the scavenger breaks them up, resulting in significant
overheads for small heaps when their heaps shrink.
2. The Go runtime already has some heuristics about this, but those
heuristics appear to have bit-rotted and result in haphazard
hugepage management. Unlucky (but otherwise fairly dense) regions of
memory end up not backed by huge pages while sparse regions end up
accidentally marked MADV_HUGEPAGE and are not later broken up by the
scavenger, because it already got the memory it needed from more
dense sections (this is more likely to happen with small heaps that
go idle).
In this change, the runtime uses a new policy:
1. Mark all new memory MADV_HUGEPAGE.
2. Track whether each page chunk (4 MiB) became dense during the GC
cycle. Mark those MADV_HUGEPAGE, and hide them from the scavenger.
3. If a chunk is not dense for 1 full GC cycle, make it visible to the
scavenger.
4. The scavenger marks a chunk MADV_NOHUGEPAGE before it scavenges it.
This policy is intended to try and back memory that is a good candidate
for huge pages (high occupancy) with huge pages, and give memory that is
not (low occupancy) to the scavenger. Occupancy is defined not just by
occupancy at any instant of time, but also occupancy in the near future.
It's generally true that by the end of a GC cycle the heap gets quite
dense (from the perspective of the page allocator).
Because we want scavenging and huge page management to happen together
(the right time to MADV_NOHUGEPAGE is just before scavenging in order to
break up huge pages and keep them that way) and the cost of applying
MADV_HUGEPAGE and MADV_NOHUGEPAGE is somewhat high, the scavenger avoids
releasing memory in dense page chunks. All this together means the
scavenger will now more generally release memory on a ~1 GC cycle delay.
Notably this has implications for scavenging to maintain the memory
limit and the runtime/debug.FreeOSMemory API. This change makes it so
that in these cases all memory is visible to the scavenger regardless of
sparseness and delays the page allocator in re-marking this memory with
MADV_NOHUGEPAGE for around 1 GC cycle to mitigate churn.
The end result of this change should be little-to-no performance
difference for dense heaps (MADV_HUGEPAGE works a lot like the default
unmarked state) but should allow the scavenger to more effectively take
back fragments of huge pages. The main risk here is churn, because
MADV_HUGEPAGE usually forces the kernel to immediately back memory with
a huge page. That's the reason for the large amount of hysteresis (1
full GC cycle) and why the definition of high density is 96% occupancy.
Fixes #55328.
Change-Id: I8da7998f1a31b498a9cc9bc662c1ae1a6bf64630
Reviewed-on: https://go-review.googlesource.com/c/go/+/436395
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
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Change-Id: I18bb87bfdea8b6d7994091ced5134aa2549f221e
Reviewed-on: https://go-review.googlesource.com/c/go/+/472476
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
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The sweep assist computation is intentionally racy for performance,
since the specifics of sweep assist aren't super sensitive to error.
However, if overflow occurs when computing the live heap delta, we can
end up with a massive sweep target that causes the sweep assist to sweep
until sweep termination, causing severe latency issues. In fact, because
heapLive doesn't always increase monotonically then anything that
flushes mcaches will cause _all_ allocating goroutines to inevitably get
stuck in sweeping.
Consider the following scenario:
1. SetGCPercent is called, updating sweepHeapLiveBasis to heapLive.
2. Very shortly after, ReadMemStats is called, flushing mcaches and
decreasing heapLive below the value sweepHeapLiveBasis was set to.
3. Every allocating goroutine goes to refill its mcache, calls into
deductSweepCredit for sweep assist, and gets stuck sweeping until
the sweep phase ends.
Fix this by just checking for overflow in the delta live heap calculation
and if it would overflow, pick a small delta live heap. This probably
means that no sweeping will happen at all, but that's OK. This is a
transient state and the runtime will recover as soon as heapLive
increases again.
Note that deductSweepCredit doesn't check overflow on other operations
but that's OK: those operations are signed and extremely unlikely to
overflow. The subtraction targeted by this CL is only a problem because
it's unsigned. An alternative fix would be to make the operation signed,
but being explicit about the overflow situation seems worthwhile.
Fixes #57523.
Change-Id: Ib18f71f53468e913548aac6e5358830c72ef0215
Reviewed-on: https://go-review.googlesource.com/c/go/+/460376
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
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When the GC is scanning some memory (possibly conservatively),
finding a pointer, while concurrently another goroutine is
allocating an object at the same address as the found pointer, the
GC may see the pointer before the object and/or the heap bits are
initialized. This may cause the GC to see bad pointers and
possibly crash.
To prevent this, we make it that the scanner can only see the
object as allocated after the object and the heap bits are
initialized. Currently the allocator uses freeindex to find the
next available slot, and that code is coupled with updating the
free index to a new slot past it. The scanner also uses the
freeindex to determine if an object is allocated. This is somewhat
racy. This CL makes the scanner use a different field, which is
only updated after the object initialization (and a memory
barrier).
Fixes #54596.
Change-Id: I2a57a226369926e7192c253dd0d21d3faf22297c
Reviewed-on: https://go-review.googlesource.com/c/go/+/449017
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
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This bool doesn't seem to be used anymore. Remove it.
Change-Id: Ic73346a98513c392d89482c5e1d818a90d713516
Reviewed-on: https://go-review.googlesource.com/c/go/+/419654
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
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This change adds an API to the runtime for arenas. A later CL can
potentially export it as an experimental API, but for now, just the
runtime implementation will suffice.
The purpose of arenas is to improve efficiency, primarily by allowing
for an application to manually free memory, thereby delaying garbage
collection. It comes with other potential performance benefits, such as
better locality, a better allocation strategy, and better handling of
interior pointers by the GC.
This implementation is based on one by danscales@google.com with a few
significant differences:
* The implementation lives entirely in the runtime (all layers).
* Arena chunks are the minimum of 8 MiB or the heap arena size. This
choice is made because in practice 64 MiB appears to be way too large
of an area for most real-world use-cases.
* Arena chunks are not unmapped, instead they're placed on an evacuation
list and when there are no pointers left pointing into them, they're
allowed to be reused.
* Reusing partially-used arena chunks no longer tries to find one used
by the same P first; it just takes the first one available.
* In order to ensure worst-case fragmentation is never worse than 25%,
only types and slice backing stores whose sizes are 1/4th the size of
a chunk or less may be used. Previously larger sizes, up to the size
of the chunk, were allowed.
* ASAN, MSAN, and the race detector are fully supported.
* Sets arena chunks to fault that were deferred at the end of mark
termination (a non-public patch once did this; I don't see a reason
not to continue that).
For #51317.
Change-Id: I83b1693a17302554cb36b6daa4e9249a81b1644f
Reviewed-on: https://go-review.googlesource.com/c/go/+/423359
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
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Currently bgsweep attempts to be a low-priority background goroutine
that runs mainly when the application is mostly idle. To avoid
complicating the scheduler further, it achieves this with a simple
heuristic: call Gosched after each span swept. While this is somewhat
inefficient as there's scheduling overhead on each iteration, it's
mostly fine because it tends to just come out of idle time anyway. In a
busy system, the call to Gosched quickly puts bgsweep at the back of
scheduler queues.
However, what's problematic about this heuristic is the number of
tracing events it produces. Average span sweeping latencies have been
measured as low as 30 ns, so every 30 ns in the sweep phase, with
available idle time, there would be a few trace events emitted. This
could result in an overwhelming number, making traces much larger than
they need to be. It also pollutes other observability tools, like the
scheduling latencies runtime metric, because bgsweep stays runnable the
whole time.
This change fixes these problems with two modifications to the
heursitic:
1. Check if there are any idle Ps before yielding. If there are, don't
yield.
2. Sweep at least 10 spans before trying to yield.
(1) is doing most of the work here. This change assumes that the
presence of idle Ps means that there is available CPU time, so bgsweep
is already making use of idle time and there's no reason it should stop.
This will have the biggest impact on the aforementioned issues.
(2) is a mitigation for the case where GOMAXPROCS=1, because we won't
ever observe a zero idle P count. It does mean that bgsweep is a little
bit higher priority than before because it yields its time less often,
so it could interfere with goroutine scheduling latencies more. However,
by sweeping 10 spans before volunteering time, we directly reduce trace
event production by 90% in all cases. The impact on scheduling latencies
should be fairly minimal, as sweeping a span is already so fast, that
sweeping 10 is unlikely to make a dent in any meaningful end-to-end
latency. In fact, it may even improve application latencies overall by
freeing up spans and sweep work from goroutines allocating memory. It
may be worth considering pushing this number higher in the future.
Another reason to do (2) is to reduce contention on npidle, which will
be checked as part of (1), but this is a fairly minor concern. The main
reason is to capture the GOMAXPROCS=1 case.
Fixes #54767.
Change-Id: I4361400f17197b8ab84c01f56203f20575b29fc6
Reviewed-on: https://go-review.googlesource.com/c/go/+/429615
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
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Atomic operations are used even during STW for consistency.
For #53821.
Change-Id: Ibe7afe5cf893b1288ce24fc96b7691b1f81754ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/417775
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
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Change-Id: Iedf10558d9a1d3b80a151927b99660b688ed9ccb
Reviewed-on: https://go-review.googlesource.com/c/go/+/418585
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
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Currently the consistent total allocation stats are managed as uintptrs,
which means they can easily overflow on 32-bit systems. Fix this by
storing these stats as uint64s. This will cause some minor performance
degradation on 32-bit systems, but there really isn't a way around this,
and it affects the correctness of the metrics we export.
Fixes #52680.
Change-Id: I7e6ca44047d46b4bd91c6f87c2d29f730e0d6191
Reviewed-on: https://go-review.googlesource.com/c/go/+/403758
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Austin Clements <austin@google.com>
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Currently the runtime's scavenging algorithm involves running from the
top of the heap address space to the bottom (or as far as it gets) once
per GC cycle. Once it treads some ground, it doesn't tread it again
until the next GC cycle.
This works just fine for the background scavenger, for heap-growth
scavenging, and for debug.FreeOSMemory. However, it breaks down in the
face of a memory limit for small heaps in the tens of MiB. Basically,
because the scavenger never retreads old ground, it's completely
oblivious to new memory it could scavenge, and that it really *should*
in the face of a memory limit.
Also, every time some thread goes to scavenge in the runtime, it
reserves what could be a considerable amount of address space, hiding it
from other scavengers.
This change modifies and simplifies the implementation overall. It's
less code with complexities that are much better encapsulated. The
current implementation iterates optimistically over the address space
looking for memory to scavenge, keeping track of what it last saw. The
new implementation does the same, but instead of directly iterating over
pages, it iterates over chunks. It maintains an index of chunks (as a
bitmap over the address space) that indicate which chunks may contain
scavenge work. The page allocator populates this index, while scavengers
consume it and iterate over it optimistically.
This has a two key benefits:
1. Scavenging is much simpler: find a candidate chunk, and check it,
essentially just using the scavengeOne fast path. There's no need for
the complexity of iterating beyond one chunk, because the index is
lock-free and already maintains that information.
2. If pages are freed to the page allocator (always guaranteed to be
unscavenged), the page allocator immediately notifies all scavengers
of the new source of work, avoiding the hiding issues of the old
implementation.
One downside of the new implementation, however, is that it's
potentially more expensive to find pages to scavenge. In the past, if
a single page would become free high up in the address space, the
runtime's scavengers would ignore it. Now that scavengers won't, one or
more scavengers may need to iterate potentially across the whole heap to
find the next source of work. For the background scavenger, this just
means a potentially less reactive scavenger -- overall it should still
use the same amount of CPU. It means worse overheads for memory limit
scavenging, but that's not exactly something with a baseline yet.
In practice, this shouldn't be too bad, hopefully since the chunk index
is extremely compact. For a 48-bit address space, the index is only 8
MiB in size at worst, but even just one physical page in the index is
able to support up to 128 GiB heaps, provided they aren't terribly
sparse. On 32-bit platforms, the index is only 128 bytes in size.
For #48409.
Change-Id: I72b7e74365046b18c64a6417224c5d85511194fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/399474
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
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Fundamentally, all of these memstats exist to serve the runtime in
managing memory. For the sake of simpler testing, couple these stats
more tightly with the GC.
This CL was mostly done automatically. The fields had to be moved
manually, but the references to the fields were updated via
gofmt -w -r 'memstats.<field> -> gcController.<field>' *.go
For #48409.
Change-Id: Ic036e875c98138d9a11e1c35f8c61b784c376134
Reviewed-on: https://go-review.googlesource.com/c/go/+/397678
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
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This will be used by the memory limit computation to determine
overheads.
For #48409.
Change-Id: Iaa4e26e1e6e46f88d10ba8ebb6b001be876dc5cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/394220
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
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This change refactors the scavenger into a type whose methods represent
the actual function and scheduling of the scavenger. It also stubs out
access to global state in order to make it testable.
This change thus also adds a test for the scavenger. In writing this
test, I discovered the lack of a behavior I expected: if the
pageAlloc.scavenge returns < the bytes requested scavenged, that means
the heap is exhausted. This has been true this whole time, but was not
documented or explicitly relied upon. This change rectifies that. In
theory this means the scavenger could spin in run() indefinitely (as
happened in the test) if shouldStop never told it to stop. In practice,
shouldStop fires long before the heap is exhausted, but for future
changes it may be important. At the very least it's good to be
intentional about these things.
While we're here, I also moved the call to stopTimer out of wake and
into sleep. There's no reason to add more operations to a context that's
already precarious (running without a P on sysmon).
Change-Id: Ib31b86379fd9df84f25ae282734437afc540da5c
Reviewed-on: https://go-review.googlesource.com/c/go/+/384734
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
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It has been agreed that we should prefer the US spelling of words like
"canceling" over "cancelling"; for example, see https://go.dev/cl/14526.
Fix a few occurrences of the "canceling" inconsistency, as well as:
* signaling
* tunneling
* marshaling
Change-Id: I99f3ba0a700a9f0292bc6c1b110af31dd05f1ff0
Reviewed-on: https://go-review.googlesource.com/c/go/+/398734
Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
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A future change to gofmt will rewrite
// Doc comment.
//go:foo
to
// Doc comment.
//
//go:foo
Apply that change preemptively to all comments (not necessarily just doc comments).
For #51082.
Change-Id: Iffe0285418d1e79d34526af3520b415a12203ca9
Reviewed-on: https://go-review.googlesource.com/c/go/+/384260
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
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Change-Id: I213fa8aa9b9c2537a189677394ddd30c62312518
GitHub-Last-Rev: ccafdee9440b06232cdfca83099bf0aeff62a4c0
GitHub-Pull-Request: golang/go#50268
Reviewed-on: https://go-review.googlesource.com/c/go/+/373336
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Zhuo Meng <mzh@golangcn.org>
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Add explicit address sanitizer instrumentation to the runtime and
syscall packages. The compiler does not instrument the runtime
package. It does instrument the syscall package, but we need to add
a couple of cases that it can't see.
Refer to the implementation of the asan malloc runtime library,
this patch also allocates extra memory as the redzone, around the
returned memory region, and marks the redzone as unaddressable to
detect the overflows or underflows.
Updates #44853.
Change-Id: I2753d1cc1296935a66bf521e31ce91e35fcdf798
Reviewed-on: https://go-review.googlesource.com/c/go/+/298614
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: fannie zhang <Fannie.Zhang@arm.com>
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The sweeper's pacing state is global, so detangle it from the GC pacer's
state updates so that the GC pacer can be tested.
For #44167.
Change-Id: Ibcea989cd435b73c5891f777d9f95f9604e03bd1
Reviewed-on: https://go-review.googlesource.com/c/go/+/309273
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
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Currently, there is a chance that the sweep termination condition could
flap, causing e.g. runtime.GC to return before all sweep work has not
only been drained, but also completed. CL 307915 and CL 307916 attempted
to fix this problem, but it is still possible that mheap_.sweepDrained is
marked before any outstanding sweepers are accounted for in
mheap_.sweepers, leaving a window in which a thread could observe
isSweepDone as true before it actually was (and after some time it would
revert to false, then true again, depending on the number of outstanding
sweepers at that point).
This change fixes the sweep termination condition by merging
mheap_.sweepers and mheap_.sweepDrained into a single atomic value.
This value is updated such that a new potential sweeper will increment
the oustanding sweeper count iff there are still outstanding spans to be
swept without an outstanding sweeper to pick them up. This design
simplifies the sweep termination condition into a single atomic load and
comparison and ensures the condition never flaps.
Updates #46500.
Fixes #45315.
Change-Id: I6d69aff156b8d48428c4cc8cfdbf28be346dbf04
Reviewed-on: https://go-review.googlesource.com/c/go/+/333389
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
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[git-generate]
cd src/runtime
mv export_test.go export.go
GOROOT=$(dirname $(dirname $PWD)) rf '
add mheap.reclaimCredit \
// reclaimCredit is spare credit for extra pages swept. Since \
// the page reclaimer works in large chunks, it may reclaim \
// more than requested. Any spare pages released go to this \
// credit pool. \
reclaimCredit_ atomic.Uintptr
ex {
import "runtime/internal/atomic"
var t mheap
var v, w uintptr
var d uintptr
t.reclaimCredit -> t.reclaimCredit_.Load()
t.reclaimCredit = v -> t.reclaimCredit_.Store(v)
atomic.Loaduintptr(&t.reclaimCredit) -> t.reclaimCredit_.Load()
atomic.LoadAcquintptr(&t.reclaimCredit) -> t.reclaimCredit_.LoadAcquire()
atomic.Storeuintptr(&t.reclaimCredit, v) -> t.reclaimCredit_.Store(v)
atomic.StoreReluintptr(&t.reclaimCredit, v) -> t.reclaimCredit_.StoreRelease(v)
atomic.Casuintptr(&t.reclaimCredit, v, w) -> t.reclaimCredit_.CompareAndSwap(v, w)
atomic.Xchguintptr(&t.reclaimCredit, v) -> t.reclaimCredit_.Swap(v)
atomic.Xadduintptr(&t.reclaimCredit, d) -> t.reclaimCredit_.Add(d)
}
rm mheap.reclaimCredit
mv mheap.reclaimCredit_ mheap.reclaimCredit
'
mv export.go export_test.go
Change-Id: I2c567781a28f5d8c2275ff18f2cf605b82f22d09
Reviewed-on: https://go-review.googlesource.com/c/go/+/356712
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
|
|
[git-generate]
cd src/runtime
mv export_test.go export.go
GOROOT=$(dirname $(dirname $PWD)) rf '
add mheap.pagesSweptBasis pagesSweptBasis_ atomic.Uint64 // pagesSwept to use as the origin of the sweep ratio
ex {
import "runtime/internal/atomic"
var t mheap
var v, w uint64
var d int64
t.pagesSweptBasis -> t.pagesSweptBasis_.Load()
t.pagesSweptBasis = v -> t.pagesSweptBasis_.Store(v)
atomic.Load64(&t.pagesSweptBasis) -> t.pagesSweptBasis_.Load()
atomic.LoadAcq64(&t.pagesSweptBasis) -> t.pagesSweptBasis_.LoadAcquire()
atomic.Store64(&t.pagesSweptBasis, v) -> t.pagesSweptBasis_.Store(v)
atomic.StoreRel64(&t.pagesSweptBasis, v) -> t.pagesSweptBasis_.StoreRelease(v)
atomic.Cas64(&t.pagesSweptBasis, v, w) -> t.pagesSweptBasis_.CompareAndSwap(v, w)
atomic.Xchg64(&t.pagesSweptBasis, v) -> t.pagesSweptBasis_.Swap(v)
atomic.Xadd64(&t.pagesSweptBasis, d) -> t.pagesSweptBasis_.Add(d)
}
rm mheap.pagesSweptBasis
mv mheap.pagesSweptBasis_ mheap.pagesSweptBasis
'
mv export.go export_test.go
Change-Id: Id9438184b9bd06d96894c02376385bad45dee154
Reviewed-on: https://go-review.googlesource.com/c/go/+/356710
Reviewed-by: Austin Clements <austin@google.com>
Trust: Michael Knyszek <mknyszek@google.com>
|
|
[git-generate]
cd src/runtime
mv export_test.go export.go
GOROOT=$(dirname $(dirname $PWD)) rf '
add mheap.pagesSwept pagesSwept_ atomic.Uint64 // pages swept this cycle
ex {
import "runtime/internal/atomic"
var t mheap
var v, w uint64
var d int64
t.pagesSwept -> t.pagesSwept_.Load()
t.pagesSwept = v -> t.pagesSwept_.Store(v)
atomic.Load64(&t.pagesSwept) -> t.pagesSwept_.Load()
atomic.LoadAcq64(&t.pagesSwept) -> t.pagesSwept_.LoadAcquire()
atomic.Store64(&t.pagesSwept, v) -> t.pagesSwept_.Store(v)
atomic.StoreRel64(&t.pagesSwept, v) -> t.pagesSwept_.StoreRelease(v)
atomic.Cas64(&t.pagesSwept, v, w) -> t.pagesSwept_.CompareAndSwap(v, w)
atomic.Xchg64(&t.pagesSwept, v) -> t.pagesSwept_.Swap(v)
atomic.Xadd64(&t.pagesSwept, d) -> t.pagesSwept_.Add(d)
}
rm mheap.pagesSwept
mv mheap.pagesSwept_ mheap.pagesSwept
'
mv export.go export_test.go
Change-Id: Ife99893d90a339655f604bc3a64ee3decec645ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/356709
Trust: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Austin Clements <austin@google.com>
|
