linux-kernel - Re: [PATCH 2/3] sched, timer: Use atomics for thread_group

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Date:	Wed, 15 Apr 2015 14:15:33 -0700
From:	Jason Low <jason.low2@...com>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mel Gorman <mgorman@...e.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	hideaki.kimura@...com, Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>, jason.low2@...com
Subject: Re: [PATCH 2/3] sched, timer: Use atomics for thread_group_cputimer
 to improve scalability

On Wed, 2015-04-15 at 07:23 -0700, Davidlohr Bueso wrote:
> 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?

Yes, this also helps increase throughput since the scalability issue
causes a performance degradation in the database workload. I currently
don't have the exact  numbers though.

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