From mboxrd@z Thu Jan  1 00:00:00 1970
From: Paolo Bonzini <pbonzini-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Subject: Re: [PATCH RFC 5/6] epoll: Add implementation for epoll_mod_wait
Date: Wed, 21 Jan 2015 12:50:44 +0100
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References: <1421747878-30744-1-git-send-email-famz@redhat.com> <1421747878-30744-6-git-send-email-famz@redhat.com> <54BE4F1D.7090807@gmail.com> <20150121045903.GA2858@fam-t430.nay.redhat.com> <54BF5ACE.6030206@gmail.com> <20150121085827.GB23024@ad.nay.redhat.com> <54BF814F.7090703@redhat.com> <20150121111404.GA3804@ad.nay.redhat.com>
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Cc: "Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
	Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>,
	linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Thomas Gleixner <tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>,
	Ingo Molnar <mingo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	"H. Peter Anvin" <hpa-YMNOUZJC4hwAvxtiuMwx3w@public.gmane.org>, x86-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org,
	Alexander Viro <viro-RmSDqhL/yNMiFSDQTTA3OLVCufUGDwFn@public.gmane.org>,
	Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>,
	Kees Cook <keescook-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>,
	David Herrmann <dh.herrmann-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
	Alexei Starovoitov <ast-uqk4Ao+rVK5Wk0Htik3J/w@public.gmane.org>,
	Miklos Szeredi <mszeredi-AlSwsSmVLrQ@public.gmane.org>,
	David Drysdale <drysdale-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>,
	Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	"David S. Miller" <davem-fT/PcQaiUtIeIZ0/mPfg9Q@public.gmane.org>,
	Vivek Goyal <vgoyal-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	Mike Frysinger <vapier-aBrp7R+bbdUdnm+yROfE0A@public.gmane.org>,
	"Theodore Ts'o" <tytso-3s7WtUTddSA@public.gmane.org>,
	Heiko Carstens <heiko.carstens-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org>,
	Rasmus Villemoes <linux-qQsb+v5E8BnlAoU/VqSP6n9LOBIZ5rWg@public.gmane.org>,
	Rashika Kheria <rashika.kheria-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
	Hugh Dickins <hughd-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>,
	Mathieu Desnoyers
To: Fam Zheng <famz-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
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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.

Paolo