When we attempt to allocate an N page span (either for a large
allocation or when an mcentral runs dry), we first try to sweep spans
to release N pages. Currently, this can be extremely expensive:
sweeping a span to emptiness is the hardest thing to ask for and the
sweeper generally doesn't know where to even look for potentially
fruitful results. Since this is on the critical path of many
allocations, this is unfortunate.

This CL changes how we reclaim empty spans. Instead of trying lots of
spans and hoping for the best, it uses the newly introduced span marks
to efficiently find empty spans. The span marks (and in-use bits) are
in a dense bitmap, so these spans can be found with an efficient
sequential memory scan. This approach can scan for unmarked spans at
about 300 GB/ms and can free unmarked spans at about 32 MB/ms. We
could probably significantly improve the rate at which is can free
unmarked spans, but that's a separate issue.

Like the current reclaimer, this is still linear in the number of
spans that are swept, but the constant factor is now so vanishingly
small that it doesn't matter.

The benchmark in #18155 demonstrates both significant page reclaiming
delays, and object reclaiming delays. With "-retain-count=20000000
-preallocate=true -loop-count=3", the benchmark demonstrates several
page reclaiming delays on the order of 40ms. After this change, the
page reclaims are insignificant. The longest sweeps are still ~150ms,
but are object reclaiming delays. We'll address those in the next
several CLs.

Updates #18155.

Fixes #21378 by completely replacing the logic that had that bug.

Change-Id: Iad80eec11d7fc262d02c8f0761ac6998425c4064
Reviewed-on: https://go-review.googlesource.com/c/138959
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>