5.3.1 couchbase#8
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jimwwalker merged 5 commits intoJul 6, 2026
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Make the file autoconf-generated. In the follow-up change, the representation of the mallocx flags will be made configurable at compile-time. (cherry picked from commit 82caac4)
In the Couchbase Server Data Service (KV Engine), we use one tcache per arena, per thread (multipies). We have one arena per bucket, and we currently support 30 buckets, but might eventually bump this up to 100 buckets per instance. However, because we allocate a tcache per thread as well, the number of tcache which we use ends up being num-buckets x num-threads. Tcaches are allocated when a thread decides to use a bucket's arena, allocating tcaches lazily, so not all threads will need a tcache, but most will. jemalloc can automatically allocate and make use of a tcache, but we've found that can result in incorrect accounting of memory stats per-arena, which is a deal-breaker for us, because we rely on some of these stats to calculate per-arena memory fragmentation. The old limit on the number of tcaches was largely dependant on the representation of the tcache ID in the 32-bit flags through which it was specified to je_mallocx. --- Flag bits: a: arena t: tcache .: configurable, dependent on --with-lg-tcache-limit: The higher this value, the more of the configurable bits will be teated as tcache bits (the rest are arena bits). 0: unused z: zero n: alignment Old representation: aaaaaaaa aaaatttt tttttttt 0znnnnnn New representation: aaaaaaa. ........ ....tttt 0znnnnnn Before this change, we had 12 bits for the arena index, 12 bits for the tcache ID, giving us 4094 (tcache 0 and 1 reserved) tcaches and 4096 arenas. By using more bits from the arena index representation, we can increase the representation of the tcache ID to be wider/narrower, allowing up to 2^17 valid tcache IDs to be specified (but 0 and 1 are reserved by jemalloc). In this example, however, the number of areans falls to 128. (cherry picked from commit 8903343)
As seen in our tests when we override the global new allocator and perform allocations from the context we're cleaning up thread local data we may end up calling je_mallocx and explicitly specify arena 0 which cause jemalloc to fail an assert (before it explicitly change the allocation to use arena 0). This asssert is currently blocking our upgrade of folly, and the effect of allowing arena 0 to be specified is harmless in this context (as jemalloc would change it to 0 on the next statement) See jemalloc#2791 See https://jira.issues.couchbase.com/browse/MB-65040 (cherry picked from commit 13f495b)
On Windows, we have seen that the tls callback is called before C++ thread_local destructors run. This means that malloc_tsd_dalloc is called before potential further alloc/free is done, leaving us with a dangling tsd_t returned by tsd_wrapper_get(). In order to workaround this, first we need to reset the tls storage to NULL. It can now get re-created if needed. Also, provide a new exported symbol je_tls_cleanup() which allows users which alloc/free at thread_local destruction to manually cleanup a potential re-created tsd_t. The contract for the je_thread_cleanup() method is that it can be called at any point of the thread lifetime, and it will clear the thread tsd_t. The tsd_t will be re-created automatically if required again. Calling je_thread_cleanup() at the end of the thread lifetime ensures that there is not a memory leak in threads which allocate during thread exit. (cherry picked from commit fc0eda6)
Otherwise, the symbol name is not correct on macOS. Before: nm -gU lib/libjemalloc.a | grep je_th 0000000000006c74 T _je_je_thread_cleanup 0000000000000818 D _je_thp_mode_names After: nm -gU lib/libjemalloc.a | grep je_th 0000000000006c74 T _je_thread_cleanup 0000000000000818 D _je_thp_mode_names (cherry picked from commit 59aa12e)
jimwwalker
marked this pull request as ready for review
July 6, 2026 12:28
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Create a 5.3.1 couchbase branch copying the customisation from 5.3.0-couchbase