runtime: break out new minimum heap size into a goexperiment
The new minimum heap of 512 KiB has been the cause of some build
slowdown (~1%) and microbenchmark slowdown (usually ~0%, up to ~50%)
because of two reasons:
1. Applications with lots of small short-lived processes execute many
more GC cycles.
2. Applications with heaps <4 MiB GC up to 8x more often.
In many ways these consequences are inevitable given how GOGC works,
however we need to investigate more as to whether the apparent slowdowns
are indeed unavoidable or if the GC has issues scaling down, which it's
too late for for this release.
Given that this release is already huge, it's OK to push this back.
We'll take a closer look at it next cycle, so place block it behind a
new goexperiment to allow users and ourselves to easily experiment with
it.
Fixes #49744.
Updates #44167.
Change-Id: Ibad51f7873de7517490c89802f3c593834e77ff0
Reviewed-on: https://go-review.googlesource.com/c/go/+/368137
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Austin Clements <austin@google.com> Reviewed-by: David Chase <drchase@google.com>