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

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Message-ID: <CALCETrWwuJpFK+38mBxxTQCu7Oig22Nr+mAuO++Y+0CdAhfzkw@mail.gmail.com>
Date:	Thu, 22 Jan 2015 13:12:59 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Fam Zheng <famz@...hat.com>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <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 FS Devel <linux-fsdevel@...r.kernel.org>,
	Linux API <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, Jan 21, 2015 at 3:50 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
>
>
> On 21/01/2015 12:14, Fam Zheng wrote:
>> > 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?
>
> That depends on the application.

In my application, the set of fds almost never changes, but the set of
events I want changes all the time.  The main thing that changes is
whether I care about EPOLLOUT.  If I'm ready to send something, then I
want EPOLLOUT.  If I'm not ready, then I don't want EPOLLOUT.

--Andy
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