Message-ID: <1429107823.6795.18.camel@stgolabs.net>
Subject: Re: [PATCH 2/3] sched, timer: Use atomics for thread_group_cputimer
 to improve scalability
From: Davidlohr Bueso <dave@stgolabs.net>
To: Jason Low <jason.low2@hp.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Mike Galbraith <umgwanakikbuti@gmail.com>,
        Frederic Weisbecker <fweisbec@gmail.com>, Mel Gorman <mgorman@suse.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Preeti U Murthy <preeti@linux.vnet.ibm.com>, hideaki.kimura@hp.com,
        Aswin Chandramouleeswaran <aswin@hp.com>,
        Scott J Norton <scott.norton@hp.com>
Date: Wed, 15 Apr 2015 07:23:43 -0700
In-Reply-To: <1429052986-9420-3-git-send-email-jason.low2@hp.com>
References: <1429052986-9420-1-git-send-email-jason.low2@hp.com>
	 <1429052986-9420-3-git-send-email-jason.low2@hp.com>
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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

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