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 11:37:03 +0100
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Cc: linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Thomas Gleixner <tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>,
	Ingo Molnar <mingo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
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	David Herrmann <dh.herrmann-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
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	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>,
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	Rashika Kheria <rashika.kheria-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
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To: Fam Zheng <famz-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	"Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
	Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>
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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.

Paolo