linux-kernel - Re: [PATCH RFC 5/6] epoll: Add implementation for epoll_mod

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Message-ID: <20150121111404.GA3804@ad.nay.redhat.com>
Date:	Wed, 21 Jan 2015 19:14:04 +0800
From:	Fam Zheng <famz@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	Andy Lutomirski <luto@...capital.net>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	David Herrmann <dh.herrmann@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	David Drysdale <drysdale@...gle.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Vivek Goyal <vgoyal@...hat.com>,
	Mike Frysinger <vapier@...too.org>,
	"Theodore Ts'o" <tytso@....edu>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Rashika Kheria <rashika.kheria@...il.com>,
	Hugh Dickins <hughd@...gle.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
	Josh Triplett <josh@...htriplett.org>
Subject: Re: [PATCH RFC 5/6] epoll: Add implementation for epoll_mod_wait

On Wed, 01/21 11:37, Paolo Bonzini wrote:
> 
> 
> On 21/01/2015 09:58, Fam Zheng wrote:
> >> > See my comment in the earlier mail. If you split this into two 
> >> > APIs, and epoll_ctl_batch() is guaranteed to execute 'cmds' in order, 
> >> > then the return value of epoll_ctl_batch() could be used to tell
> >> > user space how many commands succeeded. Much simpler!
> > Yes it is much simpler. However the reason to add batching in the first place is
> > to make epoll faster, by reducing syscalls. Splitting makes the result
> > sub-optimal: we still need at least 2 calls instead of 1.  Each one of the three
> > proposed new call *is* a step forward, but I don't think we will have everything
> > solved even by implementing them all. Compromise needed between performance or
> > complexity.
> > 
> > My take for simplicity will be leaving epoll_ctl as-is, and my take for
> > performance will be epoll_pwait1. And I don't really like putting my time on
> > epoll_ctl_batch, thinking it as a ambivalent compromise in between.
> 
> I agree with Michael actually.  The big change is going from O(n)
> epoll_ctl calls to O(1), and epoll_ctl_batch achieves that just fine.
> Changing 2 syscalls to 1 is the icing on the cake, but we're talking of
> a fraction of a microsecond.
> 

Maybe I'm missing something, but in common cases, the set of fds for epoll_wait
doesn't change that radically from one iteration to another, does it?

Fam
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