Currently that just means max_alloc, but we're about to add more. While we're
touching these lines anyways, tweak things to be more in line with testing.
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 saves us a cache miss when lookup up the arena bin offset in a remote
arena during tcache flush. All arenas share the base offset, and so we don't
need to look it up repeatedly for each arena. Secondarily, it shaves 288 bytes
off the arena on, e.g., x86-64.
The items we pick to flush matter a lot, but the order in which they get flushed
doesn't; just use forward scans. This simplifies the accessing code, both in
terms of the C and the generated assembly (i.e. this speeds up the flush
pathways).
By carefully force-inlining the division constants and the operation sum count,
we can eliminate redundant operations in the arena-level dalloc function. Do
so.
This frontloads more of the miss latency. It also moves it to a pathway where
we have not yet acquired any locks, so that it should (hopefully) reduce hold
times.
In practice, many rtree_leaf_elm accesses are cache misses. By restructuring,
we can make it more likely that these misses occur without blocking us from
starting later lookups, taking more of those misses in parallel.
