netdev - Re: [PATCH v5 00/45] CPU hotplug: stop

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Date:	Fri, 08 Feb 2013 23:39:54 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	linux-doc@...r.kernel.org, peterz@...radead.org,
	fweisbec@...il.com, linux-kernel@...r.kernel.org,
	walken@...gle.com, mingo@...nel.org, linux-arch@...r.kernel.org,
	xiaoguangrong@...ux.vnet.ibm.com, wangyun@...ux.vnet.ibm.com,
	paulmck@...ux.vnet.ibm.com, nikunj@...ux.vnet.ibm.com,
	linux-pm@...r.kernel.org, Rusty Russell <rusty@...tcorp.com.au>,
	rostedt@...dmis.org, rjw@...k.pl, namhyung@...nel.org,
	tglx@...utronix.de, linux-arm-kernel@...ts.infradead.org,
	netdev@...r.kernel.org, oleg@...hat.com, sbw@....edu,
	tj@...nel.org, akpm@...ux-foundation.org,
	linuxppc-dev@...ts.ozlabs.org, vincent.guittot@...aro.org
Subject: Re: [PATCH v5 00/45] CPU hotplug: stop_machine()-free CPU hotplug

On 02/08/2013 10:14 PM, Srivatsa S. Bhat wrote:
> On 02/08/2013 09:11 PM, Russell King - ARM Linux wrote:
>> On Thu, Feb 07, 2013 at 11:41:34AM +0530, Srivatsa S. Bhat wrote:
>>> On 02/07/2013 09:44 AM, Rusty Russell wrote:
>>>> "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com> writes:
>>>>> On 01/22/2013 01:03 PM, Srivatsa S. Bhat wrote:
>>>>>                  Avg. latency of 1 CPU offline (ms) [stop-cpu/stop-m/c latency]
>>>>>
>>>>> # online CPUs    Mainline (with stop-m/c)       This patchset (no stop-m/c)
>>>>>
>>>>>       8                 17.04                          7.73
>>>>>
>>>>>      16                 18.05                          6.44
>>>>>
>>>>>      32                 17.31                          7.39
>>>>>
>>>>>      64                 32.40                          9.28
>>>>>
>>>>>     128                 98.23                          7.35
>>>>
>>>> Nice!
>>>
>>> Thank you :-)
>>>
>>>>  I wonder how the ARM guys feel with their quad-cpu systems...
>>>>
>>>
>>> That would be definitely interesting to know :-)
>>
>> That depends what exactly you'd like tested (and how) and whether you'd
>> like it to be a test-chip based quad core, or an OMAP dual-core SoC.
>>
> 
> The effect of stop_machine() doesn't really depend on the CPU architecture
> used underneath or the platform. It depends only on the _number_ of
> _logical_ CPUs used.
> 
> And stop_machine() has 2 noticeable drawbacks:
> 1. It makes the hotplug operation itself slow
> 2. and it causes disruptions to the workloads running on the other
> CPUs by hijacking the entire machine for significant amounts of time.
> 
> In my experiments (mentioned above), I tried to measure how my patchset
> improves (reduces) the duration of hotplug (CPU offline) itself. Which is
> also slightly indicative of the impact it has on the rest of the system.
> 
> But what would be nice to test, is a setup where the workloads running on
> the rest of the system are latency-sensitive, and measure the impact of
> CPU offline on them, with this patchset applied. That would tell us how
> far is this useful in making CPU hotplug less disruptive on the system.
> 
> Of course, it would be nice to also see whether we observe any reduction
> in hotplug duration itself (point 1 above) on ARM platforms with lot
> of CPUs. [This could potentially speed up suspend/resume, which is used
> rather heavily on ARM platforms].
> 
> The benefits from this patchset over mainline (both in terms of points
> 1 and 2 above) is expected to increase, with increasing number of CPUs in
> the system.
> 

Adding Vincent to CC, who had previously evaluated the performance and
latency implications of CPU hotplug on ARM platforms, IIRC.

Regards,
Srivatsa S. Bhat

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