linux-kernel - Re: [PATCH 0/3] timer: Improve itimers scalability

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Date:	Wed, 26 Aug 2015 09:33:24 -0700
From:	Jason Low <jason.low2@...com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Steven Rostedt <rostedt@...dmis.org>,
	Terry Rudd <terry.rudd@...com>, Rik van Riel <riel@...hat.com>,
	Scott J Norton <scott.norton@...com>, jason.low2@...com
Subject: Re: [PATCH 0/3] timer: Improve itimers scalability

Hi Andrew,

On Tue, 2015-08-25 at 20:27 -0700, Andrew Morton wrote:
> On Tue, 25 Aug 2015 20:17:45 -0700 Jason Low <jason.low2@...com> wrote:
> 
> > When running a database workload on a 16 socket machine, there were
> > scalability issues related to itimers.
> > 
> > Commit 1018016c706f addressed the issue with the thread_group_cputimer
> > spinlock taking up a significant portion of total run time.
> > 
> > This patch series address the other issue where a lot of time is spent
> > trying to acquire the sighand lock. It was found in some cases that
> > 200+ threads were simultaneously contending for the same sighand lock,
> > reducing throughput by more than 30%.
> 
> Does this imply that the patchset increased the throughput of this
> workload by 30%?
> 
> And is this test case realistic?  If not, what are the benefits on a
> real-world workload?

Yes, the test case with the database workload is realistic. We did write
a simple micro-benchmark that just generates the contention in this code
path to quickly test experimental patches, since the database takes
longer to set up and run. However, the performance issues and numbers
mentioned here are for the database workload.

These patches should also be beneficial for other multi-threaded
applications which uses process-wide timers particularly on systems with
a lot of cores.

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