Re: [PATCH 2/3] sched, timer: Use atomics for thread_group_cputimer to improve scalability

Davidlohr Bueso
2015-04-15T14:24:10Z

On Tue, 2015-04-14 at 16:09 -0700, 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%.

What does 30% less time spent dealing with the thread_group_cputimer's
spinlock buy us? iow, does this help DB benchmark throughput or such?

Thanks,
Davidlohr