Commit Graph

20 Commits

Author SHA1 Message Date
David Goldblatt
73ca4b8ef8 HPA: Use dirtiest-first purging.
This seems to be practically beneficial, despite some pathological corner cases.
2021-02-19 15:10:54 -08:00
David Goldblatt
0f6c420f83 HPA: Make purging/hugifying more principled.
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.
2021-02-19 15:10:54 -08:00
David Goldblatt
56e85c0e47 HPA: Use a whole-shard purging heuristic.
Previously, we used only hpdata-local information to decide whether to purge.
2021-02-04 20:58:31 -08:00
David Goldblatt
dc886e5608 hpdata: Return the number of pages to be purged.
We'll use this in the next commit.
2021-02-04 20:58:31 -08:00
David Goldblatt
da63f23e68 HPA: Track pending purges/hugifies in the psset.
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).
2021-02-04 20:58:31 -08:00
David Goldblatt
bf64557ed6 Move empty slab tracking to the psset.
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.
2021-02-04 20:58:31 -08:00
David Goldblatt
99fc0717e6 psset: Reconceptualize insertion/removal.
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.
2021-02-04 20:58:31 -08:00
David Goldblatt
68a1666e91 hpdata: Rename "dirty" to "touched".
This matches the usage in the rest of the codebase.
2021-02-04 20:58:31 -08:00
David Goldblatt
94cd9444c5 HPA: Some minor reformattings. 2021-02-04 20:58:31 -08:00
David Goldblatt
30b9e8162b HPA: Generalize purging.
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.
2021-02-04 20:58:31 -08:00
David Goldblatt
70692cfb13 hpdata: Add state changing helpers.
We're about to allow hugepage subextent purging; get as much of our metadata
handling ready as possible.
2021-02-04 20:58:31 -08:00
David Goldblatt
2ae966222f hpdata: track per-page dirty state. 2021-02-04 20:58:31 -08:00
David Goldblatt
ff4086aa6b hpdata: count active pages instead of free ones.
This will be more consistent with later naming choices.
2021-02-04 20:58:31 -08:00
David Goldblatt
3624dd42ff hpdata: Add a comment for hpdata_consistent. 2021-02-04 20:58:31 -08:00
David Goldblatt
a559caf74a hpdata: Strengthen assertions.
Now that we have flat bitmap bit counting functions, we can easily assert that
nfree is always correct.  While we're tightening up this code, enforce
consistency on API boundaries as well.
2020-12-07 06:21:08 -08:00
David Goldblatt
d9f7e6c668 hpdata: Add a test.
We're about to make the functionality here more complicated; testing hpdata
directly (rather than relying on user's tests) will make debugging easier.
2020-12-07 06:21:08 -08:00
David Goldblatt
0971e1e4e3 hpdata: Use addr/size instead of begin/npages.
This is easier for the users of the hpdata.
2020-12-07 06:21:08 -08:00
David Goldblatt
5228d869ee psset: Use fit/insert/remove as basis functions.
All other functionality can be implemented in terms of these; doing so (while
retaining the same API) will be convenient for subsequent refactors.
2020-12-07 06:21:08 -08:00
David Goldblatt
089f8fa442 Move hpdata bitmap logic out of the psset. 2020-12-07 06:21:08 -08:00
David Goldblatt
ca30b5db2b Introduce hpdata_t.
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.
2020-12-07 06:21:08 -08:00