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

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Date:	Wed, 18 Feb 2015 17:33:00 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Jason Baron <jbaron@...mai.com>
Cc:	peterz@...radead.org, mingo@...hat.com, viro@...iv.linux.org.uk,
	akpm@...ux-foundation.org, normalperson@...t.net,
	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

* Jason Baron <jbaron@...mai.com> wrote:

> > This has two main advantages: firstly it solves the 
> > O(N) (micro-)problem, but it also more evenly 
> > distributes events both between task-lists and within 
> > epoll groups as tasks as well.
> 
> Its solving 2 issues - spurious wakeups, and more even 
> loading of threads. The event distribution is more even 
> between 'epoll groups' with this patch, however, if 
> multiple threads are blocking on a single 'epoll group', 
> this patch does not affect the the event distribution 
> there. [...]

Regarding your last point, are you sure about that?

If we have say 16 epoll threads registered, and if the list 
is static (no register/unregister activity), then the 
wakeup pattern is in strict order of the list: threads 
closer to the list head will be woken more frequently, in a 
wake-once fashion. So if threads do just quick work and go 
back to sleep quickly, then typically only the first 2-3 
threads will get any runtime in practice - the wakeup 
iteration never gets 'deep' into the list.

With the round-robin shuffling of the list, the threads get 
shuffled to the tail on wakeup, which distributes events 
evenly: all 16 epoll threads will accumulate an even 
distribution of runtime, statistically.

Have I misunderstood this somehow?

Thanks,

	Ingo
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