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>