Before this change, purge/hugify decisions had several sharp edges that could
lead to pathological behavior if tuning parameters weren't carefully chosen.
It's the first of a series; this introduces basic "make every hugepage with
dirty pages purgeable" functionality, and the next commit expands that
functionality to have a smarter policy for picking hugepages to purge.
Previously, the dehugify logic would *never* dehugify a hugepage unless it was
dirtier than the dehugification threshold. This can lead to situations in which
these pages (which themselves could never be purged) would push us above the
maximum allowed dirty pages in the shard. This forces immediate purging of any
pages deallocated in non-hugified hugepages, which in turn places nonobvious
practical limitations on the relationships between various config settings.
Instead, we make our preference not to dehugify to purge a soft one rather than
a hard one. We'll avoid purging them, but only so long as we can do so by
purging non-hugified pages. If we need to purge them to satisfy our dirty page
limits, or to hugify other, more worthy candidates, we'll still do so.
It tracks pageslabs. Soon, we'll have another bitmap (to track dirty pages)
that we want to disambiguate.
While we're here, fix an out-of-date comment.
This finishes the refactoring of the HPA/psset interactions the past few commits
have been building towards.
Rather than the HPA removing and then reinserting hpdatas, it simply begins
updates and ends them. These updates can set flags on the hpdata that prevent
it from being returned for certain types of requests. For example, it can call
hpdata_alloc_allowed_set(hpdata, false) during an update, at which point the
given hpdata will no longer be returned for psset_pick_alloc requests.
This has various of benefits:
- It maintains stats correctness during purges and hugifies.
- It allows simpler and more explicit concurrency control for the various
special cases (e.g. allocations are disallowed during purge, but not during
hugify).
- It lets allocations and deallocations avoid disturbing the purging and
hugification orderings. If an hpdata "loses its place" in one of the queues
just do to an alloc / dalloc, it can result in pathological edge cases where
very hot, very full hugepages never get hugified (and cold extents on the
same hugepage as hot ones never get purged).
The key benefit though is that tracking hpdatas to be purged / hugified in a
principled way will let us do delayed purging and hugification. Eventually this
will let us move these operations to background threads, but in the short term
the benefit is that it will let us have global purging policies (e.g. purge when
the entire arena has too many dirty pages, rather than any particular hugepage).
We're moving towards a world in which purging decisions are less rigidly
enforced at a single-hugepage level. In that world, it makes sense to keep
around some hpdatas which are not completely purged, in which case we'll need to
track them.
Really, this isn't a functional change, just a naming change. We start thinking
of pageslabs as being always in the psset. What we used to think of as removal
is now thought of as being in the psset, but in the process of being updated
(and therefore, unavalable for serving new allocations).
This is in preparation of subsequent changes to support deferred purging;
allocations will still be in the psset for the purposes of choosing when to
purge, but not for purposes of allocation/deallocation.
This is really only useful for human consumption. Correspondingly, emit it only
in the human-readable stats, and let everybody else compute from the hugepage
size and nactive.
Previously, we would purge a hugepage only when it's completely empty. With
this change, we can purge even when only partially empty. Although the
heuristic here is still fairly primitive, this infrastructure can scale to
become more advanced.
This is no longer part of the "core" functionality; we only need the stub
implementations as an end-to-end test of hpdata + psset interactions when
metadata is being modified. Treat them accordingly.
Using an edata_t both for hugepages and the allocations within those hugepages
was convenient at first, but has outlived its usefulness. Representing
hugepages explicitly, with their own data structure, will make future
development easier.
This functions more like the serial number strategy of the ecache and
hpa_central_t. Longer-lived slabs are more likely to continue to live for
longer in the future.
This introduces a new sort of edata_t; a pageslab, and a set to manage them.
This is part of a series of a commits to implement a hugepage allocator; the
pageset will be per-arena, and track small page allocations requests within a
larger extent allocated from a centralized hugepage allocator.
