1. `thread_tcache_ncached_max_read_sizeclass` allows users to get the
ncached_max of the bin with the input sizeclass, passed in through
oldp (will be upper casted if not an exact bin size is given).
2. `thread_tcache_ncached_max_write` takes in a char array
representing the settings for bins in the tcache.
When using metadata_thp, allocate tcache bin stacks from base0, which means they
will be placed on huge pages along with other metadata, instead of mixed with
other regular allocations.
In order to do so, modified the base allocator to support limited reuse: freed
tcached stacks (from thread termination) will be returned to base0 and made
available for reuse, but no merging will be attempted since they were bump
allocated out of base blocks. These reused base extents are managed using
separately allocated base edata_t -- they are cached in base->edata_avail when
the extent is all allocated.
One tricky part is, stats updating must be skipped for such reused extents
(since they were accounted for already, and there is no purging for base). This
requires tracking the "if is reused" state explicitly and bypass the stats
updates when allocating from them.
1. add tcache_max and nhbins into tcache_t so that they are per-tcache,
with one auto tcache per thread, it's also per-thread;
2. add mallctl for each thread to set its own tcache_max (of its auto tcache);
3. store the maximum number of items in each bin instead of using a global storage;
4. add tests for the modifications above.
5. Rename `nhbins` and `tcache_maxclass` to `global_do_not_change_nhbins` and `global_do_not_change_tcache_maxclass`.
Following from PR #2481, we replace all integer-to-pointer casts [which
hide pointer provenance information (and thus inhibit
optimizations)](https://clang.llvm.org/extra/clang-tidy/checks/performance/no-int-to-ptr.html)
with equivalent operations that preserve this information. I have
enabled the corresponding clang-tidy check in our static analysis CI so
that we do not get bitten by this again in the future.
On deallocation, sampled pointers (specially aligned) get junked and stashed
into tcache (to prevent immediate reuse). The expected behavior is to have
read-after-free corrupted and stopped by the junk-filling, while
write-after-free is checked when flushing the stashed pointers.
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.
This fixes an incorrect debug-mode assert:
- T1 starts an arena stats update and reads stack_head from another thread's
cache bin, when that cache bin has 1 item in it.
- T2 allocates from that cache bin. The cache_bin's stack_head now points to a
NULL pointer, since the cache bin is empty.
- T1 Re-reads the cache_bin's stack_head to perform an assertion check (since it
previously saw that the bin was empty, whatever stack_head points to should be
non-NULL).
Previously all the small size classes were cached. However this has downsides
-- particularly when page size is greater than 4K (e.g. iOS), which will result
in much higher SMALL_MAXCLASS.
This change allows tcache_max to be set to lower values, to better control
resources taken by tcache.
For locality reasons, tcache bins are integrated in TSD. Allowing all size
classes to be cached has little benefit, but takes up much thread local storage.
In addition, it complicates the layout which we try hard to optimize.
Without a lock held continuously between checking tcaches_past and incrementing
it, it's possible for two threads to go down manual creation path
simultaneously. If the number of tcaches is one less than the maximum, it's
possible for both to create a tcache and increment tcaches_past, with the second
thread returning a value larger than TCACHES_MAX.
In making these settings configurable, 634afc4124
unintentially changed a tuning parameter (reducing the "goal" max by a factor of
4). This commit undoes that change.
This lets us put more allocations on an "almost as fast" path after a flush.
This results in around a 4% reduction in malloc cycles in prod workloads
(corresponding to about a 0.1% reduction in overall cycles).
