linux-kernel - Re: [PATCH Resend] epoll: add EPOLLEXCLUSIVE support

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Date:	Wed, 28 Mar 2012 12:21:08 -0400
From:	Jason Baron <jbaron@...hat.com>
To:	richard -rw- weinberger <richard.weinberger@...il.com>
Cc:	Hagen Paul Pfeifer <hagen@...u.net>, torvalds@...ux-foundation.org,
	LKML <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Lucas De Marchi <lucas.demarchi@...fusion.mobi>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [PATCH Resend] epoll: add EPOLLEXCLUSIVE support

On Wed, Mar 28, 2012 at 04:09:24PM +0200, richard -rw- weinberger wrote:
> On Wed, Mar 28, 2012 at 3:57 PM, Hagen Paul Pfeifer <hagen@...u.net> wrote:
> > High performance server sometimes create one listening socket (e.g. port
> > 80), create a epoll file descriptor and add the socket. Afterwards
> > create SC_NPROCESSORS_ONLN threads and wait for events. This often
> > result in a thundering herd problem because all CPUs are scheduled.
> >
> > This patch add an additional flag to epoll_ctl(2) called EPOLLEXCLUSIVE.
> > If a descriptor is added with this flag only one CPU is scheduled in.
> >
> > Signed-off-by: Hagen Paul Pfeifer <hagen@...u.net>
> > ---
> > Dave rejected the patch and said not network specific. Because there
> > is no epoll maintainer this time directly.
> 
> CC'ing maintainers for you...
> Please use scripts/get_maintainer.pl.
> 

Hmmm...Looking at ep_poll() it does an '__add_wait_queue_exclusive()'.
So, I *think* epoll_wait() should do what you want, if you are waiting
on the same epfd in all the threads.

I think the case you are describing is where each thread does its own
ep_create(), and then a subsequent epoll_wait() on the fd from the
create?

So, I *think* you can get what you want without adding this flag.

Thanks,

-Jason
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