linux-kernel - Re: [PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN

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Date:	Tue, 10 Feb 2015 19:32:47 +0000
From:	Eric Wong <normalperson@...t.net>
To:	Jason Baron <jbaron@...mai.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Linux API <linux-api@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Davide Libenzi <davidel@...ilserver.org>,
	Michael Kerrisk-manpages <mtk.manpages@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN

Jason Baron <jbaron@...mai.com> wrote:
> On 02/09/2015 11:49 PM, Eric Wong wrote:
> > Do you have a userland use case to share?
> 
> I've been trying to describe the use case, maybe I haven't been doing a good
> job :(

Sorry, I meant if you had any public code.

Anyways, I've restarted work on another project which I'll hopefully be
able to share in a few weeks which might be a good public candidate for
epoll performance testing.

> > Did you try my suggestion of using a dedicated thread (or thread pool)
> > which does nothing but loop on accept() + EPOLL_CTL_ADD?
> >
> > Those dedicated threads could do its own round-robin in userland to pick
> > a different epollfd to call EPOLL_CTL_ADD on.
> 
> Thanks for your suggestion! I'm not actively working on the user-space
> code here, but I will pass it along.
> 
> I would prefer though not to have to context switch the 'accept' thread
> on and off the cpu every time there is a new connection. So the approach
> suggested here essentially moves this dedicated thread (threads), down
> into the kernel and avoids the creation of these threads entirely.

For cmogstored, I stopped using TCP_DEFER_ACCEPT when using the
dedicated thread.  This approach offloads to epoll and ends up giving
similar behavior to what used to be infinite in TCP_DEFER_ACCEPT in
Linux <= 2.6.31
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