Re: [RFC PATCH] sched, timer: Use atomics for thread_group_cputimer stats

Peter Zijlstra
2015-01-23T08:57:14Z

On Thu, Jan 22, 2015 at 07:31:53PM -0800, Jason Low wrote:
> When 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 the 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%.

I'll have to look; I worry about consistency between the values. But why
would any self respecting piece of software use this crap stuff? Its a
guaranteed scalability fail.