On Wed, 2015-04-29 at 10:38 -0400, Rik van Riel wrote: > On 04/28/2015 04:00 PM, Jason Low wrote: > > While running a database workload, we found a scalability issue with > > itimers. > > > > Much of the problem was caused by the thread_group_cputimer spinlock. > > Each time we account for group system/user time, we need to obtain a > > thread_group_cputimer's spinlock to update the timers. On larger systems > > (such as a 16 socket machine), this caused more than 30% of total time > > spent trying to obtain this kernel lock to update these group timer stats. > > > > This patch converts the timers to 64 bit atomic variables and use > > atomic add to update them without a lock. With this patch, the percent > > of total time spent updating thread group cputimer timers was reduced > > from 30% down to less than 1%. > > > > Note: On 32 bit systems using the generic 64 bit atomics, this causes > > sample_group_cputimer() to take locks 3 times instead of just 1 time. > > However, we tested this patch on a 32 bit system ARM system using the > > generic atomics and did not find the overhead to be much of an issue. > > An explanation for why this isn't an issue is that 32 bit systems usually > > have small numbers of CPUs, and cacheline contention from extra spinlocks > > called periodically is not really apparent on smaller systems. > > I don't see 32 bit systems ever getting so many CPUs > that this becomes an issue :)
Yeah, the generic 64 bit atomics are meant to be used on systems with (<=4 or so) CPUs. > > Signed-off-by: Jason Low <jason.l...@hp.com> > > Acked-by: Rik van Riel <r...@redhat.com> Thanks! -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
