Re: [PATCH v2 3/5] sched, timer: Use atomics in thread_group_cputimer to improve scalability

[Hillf Danton]

> 
> 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.
> 
FYI, another cache line problem encountered by Mel,
a368ab67aa
mm: move zone lock to a different cache line than order-0 free page lists

> 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.
> 

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