** Description changed: [Impact] * Jammy has a malloc() performance degradation if CPU affinity masks are used (not default). * The maximum number of arenas for malloc() is calculated based on the number of processors. However, glibc 2.34 changed that to be based on sched_getaffinity(), which is the number of processors available _to the process_ (i.e., based on CPU affinity masks). [0] Previously, glibc 2.33 instead used the of processors available _in the system_ (i.e., based on sysfs and procfs files). * This is not an issue by default, as without CPU affinity masks, the returned number of processors is the same as sysfs and procfs. But it _is_ an issue if CPU affinity masks are set, as it can increase lock contention (less arenas), and thus degrade performance. * CPU affinity can be set at the process-level (e.g., taskset, numactl, sched_setaffinity()) or at the system-level (kernel boot options). The latter is common in hypervisor and/or DPDK deployments, where CPU partitioning is applied with isolcpus, cpusets, systemd's CPUAffinity. [Test Plan] * The upstream bug report [1] has a reproducer, used in comment #5 to reproduce the problem, and in comment #6 to validate the fix patch. It is copied/attached to this bug as backup (test-glibc-malloc.c). The expected behavior is that these 2 steps (measuring the average time taken by 50.000 malloc+free calls, with one thread per CPU) take similar amounts of time with & without CPU affinity masks (parameter 2: true/false), in a system with a great number of CPUs. $ ./test-glibc-malloc $(nproc) false false $ ./test-glibc-malloc $(nproc) true false * glibc has a build-time test suite. * glibc has autopkgtests (rebuild, ie, above) and triggers autopkgtests in a great number of reverse test dependencies. [Regression Potential] * Theoretically, any fallout should be contained in malloc() and be related only to performance, not to functional errors. * This happens because this malloc() patch [2] changes only which method to get the number of processors. * The method it changes to is what has been already used by previous versions of glibc (up to 2.33), which has been adopted back (2.39) and backported to all glibc releases after that version (2.34-2.38), which includes the version in Jammy (2.35 [3]). * The method it changes to is also exercised in other code paths (not just malloc()), thus it is already used and tested in Jammy -- it is not something new. [Other Info] * For details and analysis of (no) required dependencies, see comments #1, #2, and #3. * Upstream bug report [1] - * Build-testing in PPA with supported archs - and only -security (ppa:mfo/lp2089789) [4] + * Build-tested in PPA with supported archs + and only -security (ppa:mfo/lp2089789) [4], + with successful build & test-suite results. [0] glibc 2.33: $ git log --oneline origin/release/2.33/master -- sysdeps/unix/sysv/linux/getsysstats.c | grep 'misc: Add __get_nprocs_sched' $ glibc 2.34: $ git log --oneline origin/release/2.34/master -- sysdeps/unix/sysv/linux/getsysstats.c | grep 'misc: Add __get_nprocs_sched' e870aac8974c misc: Add __get_nprocs_sched glibc 2.35: $ git log --oneline origin/release/2.35/master -- sysdeps/unix/sysv/linux/getsysstats.c | grep 'misc: Add __get_nprocs_sched' 11a02b035b46 misc: Add __get_nprocs_sched [1] https://sourceware.org/bugzilla/show_bug.cgi?id=30945 [2] https://sourceware.org/git/?p=glibc.git;a=commit;h=472894d2cfee5751b44c0aaa71ed87df81c8e62e [3] https://sourceware.org/git/?p=glibc.git;a=commit;h=d47c5e4db7924bb10efe14b787c4bd868b604e48 [4] https://launchpad.net/~mfo/+archive/ubuntu/lp2089789
-- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/2089789 Title: malloc performance degradation with CPU affinity masks To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/2089789/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
