linux-kernel - Re: [RFC PATCH] sched, timer: Use atomics for thread_group

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Date:	Fri, 23 Jan 2015 09:57:05 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Low <jason.low2@...com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Scott J Norton <scott.norton@...com>,
	Chegu Vinod <chegu_vinod@...com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] sched, timer: Use atomics for thread_group_cputimer
 stats

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.
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