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

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Date:	Thu, 15 Oct 2015 10:47:02 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Jason Low <jason.low2@...com>
Cc:	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


* 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?

Thanks,

	Ingo
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