We eagerly coalesce large buffers when deallocating, however the previous logic
around this introduced extra lock overhead -- when coalescing we always lock the
neighbors even if they are active, while for active extents nothing can be done.
This commit checks if the neighbor extents are potentially active before
locking, and avoids locking if possible. This speeds up large_dalloc by ~20%.
It also fixes some undesired behavior: we could stop coalescing because a small
buffer was merged, while a large neighbor was ignored on the other side.
When retain is enabled, the default dalloc hook does nothing (since we avoid
munmap). But the overhead preparing the call is high, specifically the extent
de-register and re-register involve locking and extent / rtree modifications.
Bypass the call with retain in this diff.
The global data is mostly only used at initialization, or for easy access to
values we could compute statically. Instead of consuming that space (and
risking TLB misses), we can just pass around a pointer to stack data during
bootstrapping.
The largest small class, smallest large class, and largest large class may all
be needed down fast paths; to avoid the risk of touching another cache line, we
can make them available as constants.
This class removes almost all the dependencies on size_classes.h, accessing the
data there only via the new module sc.h, which does not depend on any
configuration options.
In a subsequent commit, we'll remove the configure-time size class computations,
doing them at boot time, instead.
Before this commit jemalloc produced many warnings when compiled with -Wextra
with both Clang and GCC. This commit fixes the issues raised by these warnings
or suppresses them if they were spurious at least for the Clang and GCC
versions covered by CI.
This commit:
* adds `JEMALLOC_DIAGNOSTIC` macros: `JEMALLOC_DIAGNOSTIC_{PUSH,POP}` are
used to modify the stack of enabled diagnostics. The
`JEMALLOC_DIAGNOSTIC_IGNORE_...` macros are used to ignore a concrete
diagnostic.
* adds `JEMALLOC_FALLTHROUGH` macro to explicitly state that falling
through `case` labels in a `switch` statement is intended
* Removes all UNUSED annotations on function parameters. The warning
-Wunused-parameter is now disabled globally in
`jemalloc_internal_macros.h` for all translation units that include
that header. It is never re-enabled since that header cannot be
included by users.
* locally suppresses some -Wextra diagnostics:
* `-Wmissing-field-initializer` is buggy in older Clang and GCC versions,
where it does not understanding that, in C, `= {0}` is a common C idiom
to initialize a struct to zero
* `-Wtype-bounds` is suppressed in a particular situation where a generic
macro, used in multiple different places, compares an unsigned integer for
smaller than zero, which is always true.
* `-Walloc-larger-than-size=` diagnostics warn when an allocation function is
called with a size that is too large (out-of-range). These are suppressed in
the parts of the tests where `jemalloc` explicitly does this to test that the
allocation functions fail properly.
* adds a new CI build bot that runs the log unit test on CI.
Closes#1196 .
Previously, we would leak the extent and memory associated with a salvageable
portion of an extent that we were trying to split in three, in the case where
the first split attempt succeeded and the second failed.
preserve_lru feature adds lots of complication, for little value.
Removing it means merged extents are re-added to the lru list, and may
take longer to madvise away than they otherwise would.
Canaries after removal seem flat for several services (no change).
"always" marks all user mappings as MADV_HUGEPAGE; while "never" marks all
mappings as MADV_NOHUGEPAGE. The default setting "default" does not change any
settings. Note that all the madvise calls are part of the default extent hooks
by design, so that customized extent hooks have complete control over the
mappings including hugepage settings.
We compute the max size required to satisfy an alignment. However this can be
quite pessimistic, especially with frequent reuse (and combined with state-based
fragmentation). This commit adds one more fit step specific to aligned
allocations, searching in all potential fit size classes.
When purging, large allocations are usually the ones that cross the npages_limit
threshold, simply because they are "large". This means we often leave the large
extent around for a while, which has the downsides of: 1) high RSS and 2) more
chance of them getting fragmented. Given that they are not likely to be reused
very soon (LRU), let's over purge by 1 extent (which is often large and not
reused frequently).
Coalescing is a small price to pay for large allocations since they happen less
frequently. This reduces fragmentation while also potentially improving
locality.
When allocating from dirty extents (which we always prefer if available), large
active extents can get split even if the new allocation is much smaller, in
which case the introduced fragmentation causes high long term damage. This new
option controls the threshold to reuse and split an existing active extent. We
avoid using a large extent for much smaller sizes, in order to reduce
fragmentation. In some workload, adding the threshold improves virtual memory
usage by >10x.
Added an upper bound on how many pages we can decay during the current run.
Without this, decay could have unbounded increase in stashed, since other
threads could add new pages into the extents.
This option controls the max size when grow_retained. This is useful when we
have customized extent hooks reserving physical memory (e.g. 1G huge pages).
Without this feature, the default increasing sequence could result in fragmented
and wasted physical memory.
Before this commit, extent_recycle_split intermingles the splitting of an extent
and the return of parts of that extent to a given extents_t. After it, that
logic is separated. This will enable splitting extents that don't live in any
extents_t (as the grow retained region soon will).
There does not seem to be any overlap between usage of
extent_avail and extent_heap, so we can use the same hook.
The only remaining usage of rb trees is in the profiling code,
which has some 'interesting' iteration constraints.
Fixes#888
If we guarantee no malloc activity in extent hooks, it's possible to make
customized hooks working on arena 0. Remove the non-a0 assertion to enable such
use cases.
When retain is enabled, we should not attempt mmap for in-place expansion
(large_ralloc_no_move), because it's virtually impossible to succeed, and causes
unnecessary syscalls (which can cause lock contention under load).
This issue caused the default extent alloc function to be incorrectly
used even when arena.<i>.extent_hooks is set. This bug was introduced
by 411697adcd (Use exponential series to
size extents.), which was first released in 5.0.0.
Fix management of extent_grow_next to serialize operations that may grow
retained memory. This assures that the sizes of the newly allocated
extents correspond to the size classes in the intended growth sequence.
Fix management of extent_grow_next to skip size classes if a request is
too large to be satisfied by the next size in the growth sequence. This
avoids the potential for an arbitrary number of requests to bypass
triggering extent_grow_next increases.
This resolves#858.
Instead of embedding a lock bit in rtree leaf elements, we associate extents
with a small set of mutexes. This gets us two things:
- We can use the system mutexes. This (hypothetically) protects us from
priority inversion, and lets us stop doing a backoff/sleep loop, instead
opting for precise wakeups from the mutex.
- Cuts down on the number of mutex acquisitions we have to do (from 4 in the
worst case to two).
We end up simplifying most of the rtree code (which no longer has to deal with
locking or concurrency at all), at the cost of additional complexity in the
extent code: since the mutex protecting the rtree leaf elements is determined by
reading the extent out of those elements, the initial read is racy, so that we
may acquire an out of date mutex. We re-check the extent in the leaf after
acquiring the mutex to protect us from this race.
This lets us specify whether and how mutexes of the same rank are allowed to be
acquired. Currently, we only allow two polices (only a single mutex at a given
rank at a time, and mutexes acquired in ascending order), but we can plausibly
allow more (e.g. the "release uncontended mutexes before blocking").
Add the extent_destroy_t extent destruction hook to extent_hooks_t, and
use it during arena destruction. This hook explicitly communicates to
the callee that the extent must be destroyed or tracked for later reuse,
lest it be permanently leaked. Prior to this change, retained extents
could unintentionally be leaked if extent retention was enabled.
This resolves#560.
Control use of munmap(2) via a run-time option rather than a
compile-time option (with the same per platform default). The old
behavior of --disable-munmap can be achieved with
--with-malloc-conf=munmap:false.
This partially resolves#580.
This avoids creating clean committed pages as a side effect of aligned
allocation. For configurations that decommit memory, purged pages are
decommitted, and decommitted extents cannot be coalesced with committed
extents. Unless the clean committed pages happen to be selected during
allocation, they cause unnecessary permanent extent fragmentation.
This resolves#766.
Rather than using a LIFO queue to track available extent_t structures,
use a red-black tree, and always choose the oldest/lowest available
during reuse.
This is a biggy. jemalloc_internal.h has been doing multiple jobs for a while
now:
- The source of system-wide definitions.
- The catch-all include file.
- The module header file for jemalloc.c
This commit splits up this functionality. The system-wide definitions
responsibility has moved to jemalloc_preamble.h. The catch-all include file is
now jemalloc_internal_includes.h. The module headers for jemalloc.c are now in
jemalloc_internal_[externs|inlines|types].h, just as they are for the other
modules.
Compact extent_t to 128 bytes on 64-bit systems by moving
arena_slab_data_t's nfree into extent_t's e_bits.
Cacheline-align extent_t structures so that they always cross the
minimum number of cacheline boundaries.
Re-order extent_t fields such that all fields except the slab bitmap
(and overlaid heap profiling context pointer) are in the first
cacheline.
This resolves#461.
For extents which do not delay coalescing, use first fit layout policy
rather than first-best fit layout policy. This packs extents toward
older virtual memory mappings, but at the cost of higher search overhead
in the common case.
This resolves#711.
Expand and restructure the rtree API such that all common operations can
be achieved with minimal work, regardless of whether the rtree leaf
fields are independent versus packed into a single atomic pointer.
This allows leaf elements to differ in size from internal node elements.
In principle it would be more correct to use a different type for each
level of the tree, but due to implementation details related to atomic
operations, we use casts anyway, thus counteracting the value of
additional type correctness. Furthermore, such a scheme would require
function code generation (via cpp macros), as well as either unwieldy
type names for leaves or type aliases, e.g.
typedef struct rtree_elm_d2_s rtree_leaf_elm_t;
This alternate strategy would be more correct, and with less code
duplication, but probably not worth the complexity.
Rather than storing usize only for large (and prof-promoted)
allocations, store the size class index for allocations that reside
within the extent, such that the size class index is valid for all
extents that contain extant allocations, and invalid otherwise (mainly
to make debugging simpler).
Split decay-based purging into two phases, the first of which uses lazy
purging to convert dirty pages to "muzzy", and the second of which uses
forced purging, decommit, or unmapping to convert pages to clean or
destroy them altogether. Not all operating systems support lazy
purging, yet the application may provide extent hooks that implement
lazy purging, so care must be taken to dynamically omit the first phase
when necessary.
The mallctl interfaces change as follows:
- opt.decay_time --> opt.{dirty,muzzy}_decay_time
- arena.<i>.decay_time --> arena.<i>.{dirty,muzzy}_decay_time
- arenas.decay_time --> arenas.{dirty,muzzy}_decay_time
- stats.arenas.<i>.pdirty --> stats.arenas.<i>.p{dirty,muzzy}
- stats.arenas.<i>.{npurge,nmadvise,purged} -->
stats.arenas.<i>.{dirty,muzzy}_{npurge,nmadvise,purged}
This resolves#521.
This has the dual advantages of allowing for sparsely used large
allocations, and relying on the kernel to supply zeroed pages, which
tends to be very fast on modern systems.
In the process, we can do some strength reduction, changing the fetch-adds and
fetch-subs to be simple loads followed by stores, since the modifications all
occur while holding the mutex.
This simplifies what would be pairing heap operations to the equivalent
of LIFO queue operations. This is a complementary optimization in the
context of delayed coalescing for cached extents.
Rather than purging uncoalesced extents, perform just enough incremental
coalescing to purge only fully coalesced extents. In the absence of
cached extent reuse, the immediate versus delayed incremental purging
algorithms result in the same purge order.
This resolves#655.
Remove obsolete unit test scaffolding for extent quantization. Remove
redundant assertions. Add an assertion to
extents_first_best_fit_locked() that should help prevent aligned
allocation regressions.
Extent splitting and coalescing is a major component of large allocation
overhead, and disabling coalescing of cached extents provides a simple
and effective hysteresis mechanism. Once two-phase purging is
implemented, it will probably make sense to leave coalescing disabled
for the first phase, but coalesce during the second phase.
Rather than dynamically building a table to aid per level computations,
define a constant table at compile time. Omit both high and low
insignificant bits. Use one to three tree levels, depending on the
number of significant bits.
Read adjacent rtree elements while holding element locks, since the
extents mutex only protects against relevant like-state extent mutation.
Fix management of the 'coalesced' loop state variable to merge
forward/backward results, rather than overwriting the result of forward
coalescing if attempting to coalesce backward. In practice this caused
no correctness issues, but could cause extra iterations in rare cases.
These regressions were introduced by
d27f29b468 (Disentangle arena and extent
locking.).
Set extent as active prior to registration so that other threads can't
modify it in the absence of locking.
This regression was introduced by
d27f29b468 (Disentangle arena and extent
locking.), via non-obvious means. Removal of extents_mtx protection
during extent_grow_retained() execution opened up the race, but in the
presence of that locking, the code was safe.
This resolves#599.
Refactor arena and extent locking protocols such that arena and
extent locks are never held when calling into the extent_*_wrapper()
API. This requires extra care during purging since the arena lock no
longer protects the inner purging logic. It also requires extra care to
protect extents from being merged with adjacent extents.
Convert extent_t's 'active' flag to an enumerated 'state', so that
retained extents are explicitly marked as such, rather than depending on
ring linkage state.
Refactor the extent collections (and their synchronization) for cached
and retained extents into extents_t. Incorporate LRU functionality to
support purging. Incorporate page count accounting, which replaces
arena->ndirty and arena->stats.retained.
Assert that no core locks are held when entering any internal
[de]allocation functions. This is in addition to existing assertions
that no locks are held when entering external [de]allocation functions.
Audit and document synchronization protocols for all arena_t fields.
This fixes a potential deadlock due to recursive allocation during
gdump, in a similar fashion to b49c649bc1
(Fix lock order reversal during gdump.), but with a necessarily much
broader code impact.
Add/rename related mallctls:
- Add stats.arenas.<i>.base .
- Rename stats.arenas.<i>.metadata to stats.arenas.<i>.internal .
- Add stats.arenas.<i>.resident .
Modify the arenas.extend mallctl to take an optional (extent_hooks_t *)
argument so that it is possible for all base allocations to be serviced
by the specified extent hooks.
This resolves#463.
Split purging into lazy and forced variants. Use the forced variant for
zeroing dss.
Add support for NULL function pointers as an opt-out mechanism for the
dalloc, commit, decommit, purge_lazy, purge_forced, split, and merge
fields of extent_hooks_t.
Add short-circuiting checks in large_ralloc_no_move_{shrink,expand}() so
that no attempt is made if splitting/merging is not supported.
This resolves#268.
If virtual memory is retained, allocate extents such that their sizes
form an exponentially growing series. This limits the number of
disjoint virtual memory ranges so that extent merging can be effective
even if multiple arenas' extent allocation requests are highly
interleaved.
This resolves#462.
Add extent serial numbers and use them where appropriate as a sort key
that is higher priority than address, so that the allocation policy
prefers older extents.
This resolves#147.
Do not call s2u() during alloc_size computation, since any necessary
ceiling increase is taken care of later by extent_first_best_fit() -->
extent_size_quantize_ceil(), and the s2u() call may erroneously cause a
higher quantization result.
Remove an overly strict overflow check that was added in
4a7852137d (Fix extent_recycle()'s
cache-oblivious padding support.).
Add padding *after* computing the size class, so that the optimal size
class isn't skipped during search for a usable extent. This regression
was caused by b46261d58b (Implement
cache-oblivious support for huge size classes.).
Add an "over-size" extent heap in which to store extents which exceed
the maximum size class (plus cache-oblivious padding, if enabled).
Remove psz2ind_clamp() and use psz2ind() instead so that trying to
allocate the maximum size class can in principle succeed. In practice,
this allows assertions to hold so that OOM errors can be successfully
generated.
Fix extent_alloc_cache[_locked]() to support decommitted allocation, and
use this ability in arena_stash_dirty(), so that decommitted extents are
not needlessly committed during purging. In practice this does not
happen on any currently supported systems, because both extent merging
and decommit must be implemented; all supported systems implement one
xor the other.
Rather than protecting dss operations with a mutex, use atomic
operations. This has negligible impact on synchronization overhead
during typical dss allocation, but is a substantial improvement for
extent_in_dss() and the newly added extent_dss_mergeable(), which can be
called multiple times during extent deallocations.
This change also has the advantage of avoiding tsd in deallocation paths
associated with purging, which resolves potential deadlocks during
thread exit due to attempted tsd resurrection.
This resolves#425.
Avoid calling s2u() on raw extent sizes in extent_recycle().
Clamp psz2ind() (implemented as psz2ind_clamp()) when inserting/removing
into/from size-segregated extent heaps.
Fix a fundamental extent_split_wrapper() bug in an error path.
Fix extent_recycle() to deregister unsplittable extents before leaking
them.
Relax xallocx() test assertions so that unsplittable extents don't cause
test failures.
rtree-based extent lookups remain more expensive than chunk-based run
lookups, but with this optimization the fast path slowdown is ~3 CPU
cycles per metadata lookup (on Intel Core i7-4980HQ), versus ~11 cycles
prior. The path caching speedup tends to degrade gracefully unless
allocated memory is spread far apart (as is the case when using a
mixture of sbrk() and mmap()).
Extract szad size quantization into {extent,run}_quantize(), and .
quantize szad run sizes to the union of valid small region run sizes and
large run sizes.
Refactor iteration in arena_run_first_fit() to use
run_quantize{,_first,_next(), and add support for padded large runs.
For large allocations that have no specified alignment constraints,
compute a pseudo-random offset from the beginning of the first backing
page that is a multiple of the cache line size. Under typical
configurations with 4-KiB pages and 64-byte cache lines this results in
a uniform distribution among 64 page boundary offsets.
Add the --disable-cache-oblivious option, primarily intended for
performance testing.
This resolves#13.
Treat sizes that round down to the same size class as size-equivalent
in trees that are used to search for first best fit, so that there are
only as many "firsts" as there are size classes. This comes closer to
the ideal of first fit.
Extend per arena unused dirty page purging to manage unused dirty chunks
in aaddtion to unused dirty runs. Rather than immediately unmapping
deallocated chunks (or purging them in the --disable-munmap case), store
them in a separate set of trees, chunks_[sz]ad_dirty. Preferrentially
allocate dirty chunks. When excessive unused dirty pages accumulate,
purge runs and chunks in ingegrated LRU order (and unmap chunks in the
--enable-munmap case).
Refactor extent_node_t to provide accessor functions.
Convert configuration-related cpp conditional logic to use static
constant variables, e.g.:
#ifdef JEMALLOC_DEBUG
[...]
#endif
becomes:
if (config_debug) {
[...]
}
The advantage is clearer, more concise code. The main disadvantage is
that data structures no longer have conditionally defined fields, so
they pay the cost of all fields regardless of whether they are used. In
practice, this is only a minor concern; config_stats will go away in an
upcoming change, and config_prof is the only other major feature that
depends on more than a few special-purpose fields.
