Messages in this thread | | | Date | Thu, 1 Sep 2016 11:49:06 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 1/3] sched/cputime: Improve scalability of times()/clock_gettime() on 32 bit cpus |
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On Thu, Sep 01, 2016 at 11:27:42AM +0200, Stanislaw Gruszka wrote: > My previous commit: > > a1eb1411b4e4 ("sched/cputime: Improve scalability by not accounting thread group tasks pending runtime") > > helped to achieve good performance of SYS_times() and > SYS_clock_gettimes(CLOCK_PROCESS_CPUTIME_ID) on 64 bit architectures. > However taking task_rq_lock() when reading t->se.sum_exec_runtime on > 32 bit architectures still make those syscalls slow. > > The reason why we take the lock is to make 64bit sum_exec_runtime > variable consistent. While a inconsistency scenario is very very unlike, > I assume it still may happen at least on some 32 bit architectures. > > To protect the variable I introduced new seqcount lock. Performance > improvements on machine with 32 cores (32-bit cpus) measured by > benchmarks described in commit:
No,.. running 32bit kernels on a machine with 32 cores is insane, full stop.
You're now making rather hot paths slower to benefit a rather slow path, that too is backwards.
[ also, seqcount is not a lock ].
Really, people should not expect process wide numbers to be fast.
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