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

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Date:	Mon, 09 Feb 2015 12:18:04 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Jason Baron <jbaron@...mai.com>, peterz@...radead.org,
	mingo@...hat.com, viro@...iv.linux.org.uk
CC:	akpm@...ux-foundation.org, normalperson@...t.net,
	davidel@...ilserver.org, mtk.manpages@...il.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN

On 02/09/2015 12:06 PM, Jason Baron wrote:
> Epoll file descriptors that are added to a shared wakeup source are always
> added in a non-exclusive manner. That means that when we have multiple epoll
> fds attached to a shared wakeup source they are all woken up. This can
> lead to excessive cpu usage and uneven load distribution.
>
> This patch introduces two new 'events' flags that are intended to be used
> with EPOLL_CTL_ADD operations. EPOLLEXCLUSIVE, adds the epoll fd to the event
> source in an exclusive manner such that the minimum number of threads are
> woken. EPOLLROUNDROBIN, which depends on EPOLLEXCLUSIVE also being set, can
> also be added to the 'events' flag, such that we round robin around the set
> of waiting threads.
>
> An implementation note is that in the epoll wakeup routine,
> 'ep_poll_callback()', if EPOLLROUNDROBIN is set, we return 1, for a successful
> wakeup, only when there are current waiters. The idea is to use this additional
> heuristic in order minimize wakeup latencies.

I don't understand what this is intended to do.

If an event has EPOLLONESHOT, then this only one thread should be woken 
regardless, right?  If not, isn't that just a bug that should be fixed?

If an event has EPOLLET, then the considerations are similar to 
EPOLLONESHOT, right?

If an event is a normal level-triggered non-one-shot event, then I don't 
understand how a round-robin wakeup makes any sense.  It's 
level-triggered, after all.

--Andy
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