linux-kernel - Re: [PATCH v2 0/4] timer: Improve itimers scalability

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Date:	Thu, 15 Oct 2015 12:01:16 -0700
From:	Jason Low <jason.low@....com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Jason Low <jason.low2@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	George Spelvin <linux@...izon.com>, hideaki.kimura@....com,
	terry.rudd@....com, scott.norton@....com
Subject: Re: [PATCH v2 0/4] timer: Improve itimers scalability

On Thu, 2015-10-15 at 10:47 +0200, Ingo Molnar wrote:
> * Jason Low <jason.low2@...com> wrote:
> 
> > While running a database workload on a 16 socket machine, there were
> > scalability issues related to itimers. The following link contains a
> > more detailed summary of the issues at the application level.
> > 
> > https://lkml.org/lkml/2015/8/26/737
> > 
> > Commit 1018016c706f addressed the issue with the thread_group_cputimer
> > spinlock taking up a significant portion of total run time.
> > This patch series addresses the secondary 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%.
> > 
> > With this patch set (along with commit 1018016c706f mentioned above),
> > the performance hit of itimers almost completely goes away on the
> > 16 socket system.
> > 
> > Jason Low (4):
> >   timer: Optimize fastpath_timer_check()
> >   timer: Check thread timers only when there are active thread timers
> >   timer: Convert cputimer->running to bool
> >   timer: Reduce unnecessary sighand lock contention
> > 
> >  include/linux/init_task.h      |    3 +-
> >  include/linux/sched.h          |    9 ++++--
> >  kernel/fork.c                  |    2 +-
> >  kernel/time/posix-cpu-timers.c |   63 ++++++++++++++++++++++++++++-----------
> >  4 files changed, 54 insertions(+), 23 deletions(-)
> 
> Is there some itimers benchmark that can be used to measure the effects of these 
> changes?

Yes, we also wrote a micro benchmark which generates cache misses and
measures the average cost of each cache miss (with itimers enabled). We
used this while writing and testing patches, since it takes a bit longer
to set up and run the database.

Jason

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