runtime: use a smaller arena size on Wasm

On Wasm, some programs have very small heap. Currently,
we use 4 MB arena size (like all other 32-bit platforms).
For a very small program, it needs to allocate one heap
arena, 4 MB size at a 4 MB aligned address. So we'll
need 8 MB of linear memory, whereas only a smaller
portion is actually used by the program. On Wasm, samll
programs are not uncommon (e.g. WASI plugins), and
users are concerned about the memory usage.

This CL switches to a smaller arena size, as well as a
smaller page allocator chunk size (both are now 512 KB).
So the heap will be grown in 512 KB granularity. For a
helloworld program, it now uses less than 3 MB of
linear memory, instead of 8 MB.

Change-Id: Ibd66c1fa6e794a12c00906cbacc8f2e410f196c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/683296
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

0 files changed, 0 insertions, 0 deletions