linux-kernel - Re: [PATCH v2 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN

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Date:	Wed, 18 Feb 2015 22:18:08 +0000
From:	Eric Wong <normalperson@...t.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Jason Baron <jbaron@...mai.com>, peterz@...radead.org,
	mingo@...hat.com, viro@...iv.linux.org.uk,
	akpm@...ux-foundation.org, davidel@...ilserver.org,
	mtk.manpages@...il.com, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH v2 2/2] epoll: introduce EPOLLEXCLUSIVE and
 EPOLLROUNDROBIN

Ingo Molnar <mingo@...nel.org> wrote:
> 
> * Ingo Molnar <mingo@...nel.org> wrote:
> 
> > > [...] However, I think the userspace API change is less 
> > > clear since epoll_wait() doesn't currently have an 
> > > 'input' events argument as epoll_ctl() does.
> > 
> > ... but the change would be a bit clearer and somewhat 
> > more flexible: LIFO or FIFO queueing, right?
> > 
> > But having the queueing model as part of the epoll 
> > context is a legitimate approach as well.
> 
> Btw., there's another optimization that the networking code 
> already does when processing incoming packets: waking up a 
> thread on the local CPU, where the wakeup is running.
> 
> Doing the same on epoll would have real scalability 
> advantages where incoming events are IRQ driven and are 
> distributed amongst multiple CPUs.

Right.  One thing in the back of my mind has been to have CPU
affinity for epoll.  Either having everything in an epoll set
favor a certain CPU or even having affinity down to the epitem
level (so concurrent epoll_wait callers end up favoring the
same epitems).

I'm not convinced this series is worth doing without a
comparison against my previous suggestion to use a dedicated
thread which only makes blocking accept4 + EPOLL_CTL_ADD calls.

The majority of epoll events in a typical server should not be
for listen sockets, so I'd rather not bloat existing code paths
for them.  For web servers nowadays, the benefits of maintaining
long-lived connections to avoid handshakes is even more
beneficial with increasing HTTPS and HTTP2 adoption; so
listen socket events should become less common.
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