runtime: make the scavenger and allocator respect the memory limit

This change does everything necessary to make the memory allocator and
the scavenger respect the memory limit. In particular, it:

- Adds a second goal for the background scavenge that's based on the
  memory limit, setting a target 5% below the limit to make sure it's
  working hard when the application is close to it.
- Makes span allocation assist the scavenger if the next allocation is
  about to put total memory use above the memory limit.
- Measures any scavenge assist time and adds it to GC assist time for
  the sake of GC CPU limiting, to avoid a death spiral as a result of
  scavenging too much.

All of these changes have a relatively small impact, but each is
intimately related and thus benefit from being done together.

For #48409.

Change-Id: I35517a752f74dd12a151dd620f102c77e095d3e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/397017
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

0 files changed, 0 insertions, 0 deletions