linux-kernel - Re: [-mm] Add an owner to the mm

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Date:	Sat, 05 Apr 2008 23:18:25 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Paul Menage <menage@...gle.com>
CC:	Pavel Emelianov <xemul@...nvz.org>,
	Hugh Dickins <hugh@...itas.com>,
	Sudhir Kumar <skumar@...ux.vnet.ibm.com>,
	YAMAMOTO Takashi <yamamoto@...inux.co.jp>, lizf@...fujitsu.com,
	linux-kernel@...r.kernel.org, taka@...inux.co.jp,
	linux-mm@...ck.org, David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [-mm] Add an owner to the mm_struct (v8)

Paul Menage wrote:
> On Sat, Apr 5, 2008 at 7:47 AM, Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
>>  Repeating my question earlier
>>
>>  Can we delay setting task->cgroups = &init_css_set for the group_leader, until
>>  all threads have exited?
> 
> Potentially, yes. It also might make more sense to move the
> exit_cgroup() for all threads to a later point rather than special
> case delayed group leaders.
> 

Yes, that makes sense. I think that patch should be independent of this one
though? What do you think?

>> If the user is unable to remove a cgroup node, it will
>>  be due a valid reason, the group_leader is still around, since the threads are
>>  still around. The user in that case should wait for notify_on_release.
>>
>>  >
>>  > To me, it seems that setting up a *virtual address space* cgroup
>>  > hierarchy and then putting half your threads in one group and half in
>>  > the another is asking for trouble. We need to not break in that
>>  > situation, but I'm not sure it's a case to optimize for.
>>
>>  That could potentially happen, if the virtual address space cgroup and cpu
>>  control cgroup were bound together in the same hierarchy by the sysadmin.
> 
> Yes, I agree it could potentially happen. But it seems like a strange
> thing to do if you're planning to be not have the same groupings for
> cpu and va.
> 

It's easier to set it up that way. Usually the end user gets the same SLA for
memory, CPU and other resources, so it makes sense to bind the controllers together.

>>  I measured the overhead of removing the delay_group_leader optimization and
>>  found a 4% impact on throughput (with volanomark, that is one of the
>>  multi-threaded benchmarks I know of).
> 
> Interesting, I thought (although I've never actually looked at the
> code) that volanomark was more of a scheduling benchmark than a
> process start/exit benchmark. How frequently does it have processes
> (not threads) exiting?
> 

I could not find any other interesting benchmark for benchmarking fork/exits. I
know that volanomark is heavily threaded, so I used it. The threads quickly exit
after processing the messages, I thought that would be a good test to see the
overhead.

> How many runs was that over? Ingo's recently posted volanomark tests
> against -rc7 showed ~3% random variation between runs.

I ran the test four times. I took the average of runs, I did see some variation
between runs, I did not calculate the standard deviation.

-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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