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7aa77c8e2a
The noinline accessor puts an opaque call on the malloc/free fastpath.
test/stress/microbench (malloc(1)/free pairs, pinned core, clang):
unpatched accessor this commit
malloc, no LTO 7.0 ns/op 9.8 (+40%) 7.0 (+-0%)
free, no LTO 6.5 ns/op 9.2 (+40%) 6.5 (+-0%)
malloc, ThinLTO 7.0 ns/op 9.0 (+28%) 7.6 (~+8%)
free, ThinLTO 6.5 ns/op 8.4 (+28%) 7.0 (~+8%)
Under a static TLS model the tsd address is `thread_pointer + offset` with a
thread-independent offset. Capture the offset once (a noinline helper, lazy and
sentinel-initialized) and re-read the thread pointer per call with a `volatile`
asm the optimizer may not hoist or CSE (per-arch reads from mimalloc's
mi_prim_tls_slot). Both inputs are hoist-proof: the tp read is volatile, the
offset is thread-independent.
The offset global is read and written with relaxed atomics: threads racing
their first allocation init it concurrently with the same value, so a plain
access would be a benign-but-UB data race (TSan-reported). Relaxed adds no
fence -- it compiles to the same load/store on the fast path.
The offset must NOT be computed inline as `&tsd_tls - __builtin_thread_pointer()`:
the terms hoist independently, and clang ThinLTO keeps the stale `&tsd_tls` in a
callee-saved register while re-reading the thread pointer, so `fresh_tp +
(stale_addr - fresh_tp)` cancels back to the stale address. Capturing behind a
call boundary evaluates both terms at one point on one thread.
Gated on JEMALLOC_TLS_MODEL_INITIAL_EXEC (a new configure define, set whenever
jemalloc applies its default initial-exec model), GNU asm, a known arch, and
!_WIN32 (MinGW's thread pointer lives in the TEB, not fs/gs:0); everything else
keeps the noinline accessor. Verified with the fiber-migration reproducer under
clang ThinLTO.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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| TUNING.md | ||
jemalloc is a general purpose malloc(3) implementation that emphasizes fragmentation avoidance and scalable concurrency support. jemalloc first came into use as the FreeBSD libc allocator in 2005, and since then it has found its way into numerous applications that rely on its predictable behavior. In 2010 jemalloc development efforts broadened to include developer support features such as heap profiling and extensive monitoring/tuning hooks. Modern jemalloc releases continue to be integrated back into FreeBSD, and therefore versatility remains critical. Ongoing development efforts trend toward making jemalloc among the best allocators for a broad range of demanding applications, and eliminating/mitigating weaknesses that have practical repercussions for real world applications. The COPYING file contains copyright and licensing information. The INSTALL file contains information on how to configure, build, and install jemalloc. The ChangeLog file contains a brief summary of changes for each release. URL: https://jemalloc.net/