Adding guarded extents, which are regular extents surrounded by guard pages
(mprotected). To reduce syscalls, small guarded extents are cached as a
separate eset in ecache, and decay through the dirty / muzzy / retained pipeline
as usual.
This mallctl accepts an arena_config_t structure which
can be used to customize the behavior of the arena.
Right now it contains extent_hooks and a new option,
metadata_use_hooks, which controls whether the extent
hooks are also used for metadata allocation.
The medata_use_hooks option has two main use cases:
1. In heterogeneous memory systems, to avoid metadata
being placed on potentially slower memory.
2. Avoiding virtual memory from being leaked as a result
of metadata allocation failure originating in an extent hook.
Existing backtrace implementations skip native stack frames from runtimes like
Python. The hook allows to augment the backtraces to attribute allocations to
native functions in heap profiles.
This change allows every allocator conforming to PAI communicate that it
deferred some work for the future. Without it if a background thread goes into
indefinite sleep, there is no way to notify it about upcoming deferred work.
Previously the calculation of sleep time between wakeups was implemented within
background_thread. This resulted in some parts of decay and hpa specific
logic mixing with background thread implementation. In this change, background
thread delegates this calculation to arena and it, in turn, delegates it to PAI.
The next step is to implement the actual calculation of time until deferred work
in HPA.
Retained pages are those which haven't been touched and are unbacked from OS
perspective. For a pageslab their number should equal "total pages in slab"
minus "touched pages".
By force-inlining everything that would otherwise be a macro, we get the same
effect (it's not clear in the first place that this is actually a good idea, but
it avoids making any changes to the existing performance profile).
This makes the code more maintainable (in anticipation of subsequent changes),
as well as making performance profiles and debug info more readable (we get
"real" line numbers, instead of making everything point to the macro definition
of all associated functions).
The edata_cache_small had a fill/flush heuristic. In retrospect, this was a
premature optimization; more testing indicates that an unbounded cache is
effectively fine here, and moreover we spend a nontrivial amount of time doing
unnecessary filling/flushing.
As the HPA takes on a larger and larger fraction of all allocations, any
theoretical differences in allocation patterns should shrink. The HPA is more
efficient with its metadata in general, so it still comes out ahead on metadata
usage anyways.
We wait a while after deciding a huge extent should get hugified to see if it
gets purged before long. This avoids hugifying extents that might shortly get
dehugified for purging.
Rename and use the hpa_dehugification_threshold option support code for this,
since it's now ignored.
This is a simple multi-producer, single-consumer queue. The intended use case
is in the HPA, as we begin supporting hpdatas that move between hpa_shards. We
take just a single CAS as the cost to send a message (or a batch of messages) in
the low-contention case, and lock-freedom lets us avoid some lock-ordering
issues.
Now that all merging go through try_acquire_edata_neighbor, the mergeablility
checks (including head state checking) are done before reaching the merge hook.
In other words, merge hook will never be called if the head state doesn't agree.
Instead of passing down the new_addr, pass down the active edata which allows us
to always use a neighbor-acquiring semantic. In other words, this tells us both
the original edata and neighbor address. With this change, only neighbors of a
"known" edata can be acquired, i.e. acquiring an edata based on an arbitrary
address isn't possible anymore.
This avoids the addr-based mutexes (i.e. the mutex_pool), and instead relies on
the metadata tracked in rtree leaf: the head state and extent_state. Before
trying to access the neighbor edata (e.g. for coalescing), the states will be
verified first -- only neighbor edatas from the same arena and with the same
state will be accessed.
When retain is on, when extent_grow_retained failed (e.g. due to split hook
failures), we'll try extent_alloc_wrapper as the last resort. Set the is_head
bit in that case to be consistent. The allocated extent in that case will be
retained properly, but not merged with other extents.
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 change pulls the SEC options into a struct, which simplifies their handling
across various modules (e.g. PA needs to forward on SEC options from the
malloc_conf string, but it doesn't really need to know their names). While
we're here, make some of the fixed constants configurable, and unify naming from
the configuration options to the internals.
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.
