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futexes: Increase hash table size for better performance
Currently, the futex global hash table suffers from its fixed, smallish (for today's standards) size of 256 entries, as well as its lack of NUMA awareness. Large systems, using many futexes, can be prone to high amounts of collisions; where these futexes hash to the same bucket and lead to extra contention on the same hb->lock. Furthermore, cacheline bouncing is a reality when we have multiple hb->locks residing on the same cacheline and different futexes hash to adjacent buckets. This patch keeps the current static size of 16 entries for small systems, or otherwise, 256 * ncpus (or larger as we need to round the number to a power of 2). Note that this number of CPUs accounts for all CPUs that can ever be available in the system, taking into consideration things like hotpluging. While we do impose extra overhead at bootup by making the hash table larger, this is a one time thing, and does not shadow the benefits of this patch. Furthermore, as suggested by tglx, by cache aligning the hash buckets we can avoid access across cacheline boundaries and also avoid massive cache line bouncing if multiple cpus are hammering away at different hash buckets which happen to reside in the same cache line. Also, similar to other core kernel components (pid, dcache, tcp), by using alloc_large_system_hash() we benefit from its NUMA awareness and thus the table is distributed among the nodes instead of in a single one. For a custom microbenchmark that pounds on the uaddr hashing -- making the wait path fail at futex_wait_setup() returning -EWOULDBLOCK for large amounts of futexes, we can see the following benefits on a 80-core, 8-socket 1Tb server: +---------+--------------------+------------------------+-----------------------+-------------------------------+ | threads | baseline (ops/sec) | aligned-only (ops/sec) | large table (ops/sec) | large table+aligned (ops/sec) | +---------+--------------------+------------------------+-----------------------+-------------------------------+ | 512 | 32426 | 50531 (+55.8%) | 255274 (+687.2%) | 292553 (+802.2%) | | 256 | 65360 | 99588 (+52.3%) | 443563 (+578.6%) | 508088 (+677.3%) | | 128 | 125635 | 200075 (+59.2%) | 742613 (+491.1%) | 835452 (+564.9%) | | 80 | 193559 | 323425 (+67.1%) | 1028147 (+431.1%) | 1130304 (+483.9%) | | 64 | 247667 | 443740 (+79.1%) | 997300 (+302.6%) | 1145494 (+362.5%) | | 32 | 628412 | 721401 (+14.7%) | 965996 (+53.7%) | 1122115 (+78.5%) | +---------+--------------------+------------------------+-----------------------+-------------------------------+ Reviewed-by:Darren Hart <dvhart@linux.intel.com> Reviewed-by: