linux-kernel - Re: [RFC] eventpoll: Move a kmem_cache_alloc and kmem_cache

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Date:	Thu, 19 Sep 2013 11:37:59 -0500
From:	Nathan Zimmer <nzimmer@....com>
To:	Jason Baron <jbaron@...mai.com>
CC:	<linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC] eventpoll: Move a kmem_cache_alloc and kmem_cache_free

On 09/18/2013 02:09 PM, Jason Baron wrote:
> On 09/13/2013 11:54 AM, Nathan Zimmer wrote:
>> We noticed some scaling issue in the SPECjbb benchmark.  Running perf
>> we found that the it was spending lots of time in SYS_epoll_ctl.
>> In particular it is holding the epmutex.
>> This patch helps by moving out the kmem_cache_alloc and kmem_cache_free out
>> from under the lock.  It improves throughput by around 15% on 16 sockets.
>>
>> While this patch should be fine as it is there are probably is more things
>> that can be done out side the lock, like wakeup_source_unregister, but I am
>> not familar with the area and I don't know of many tests.  I did find the
>> one posted by Jason Baron at https://lkml.org/lkml/2011/2/25/297.
>>
>> Any thoughts?
>>
> Hi,
>
> Intersting - I think its also possible to completely drop taking the
> 'epmutex' for EPOLL_CTL_DEL by using rcu, and restricting it on add
> to more 'complex' topologies. That is when we have an epoll descriptor
> that doesn't nest with other epoll descriptors, we don't need the
> global 'epmutex' either. Any chance you can re-run with this? Its a bit
> hacky, but we can clean it up if it makes sense.
>
> Thanks,
>
> -Jason
>
That is working GREAT.  It is scaling to 16 jobs quite well.
I will have to grab a larger machine( to see what the new scaling curve 
will be.
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