Re: [PATCH RFC 5/6] epoll: Add implementation for epoll_mod_wait

From: Fam Zheng <famz@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
	Andy Lutomirski <luto@amacapital.net>,
	linux-kernel@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, Alexander Viro <viro@zeniv.linux.org.uk>,
	Andrew Morton <akpm@linux-foundation.org>,
	Kees Cook <keescook@chromium.org>,
	David Herrmann <dh.herrmann@gmail.com>,
	Alexei Starovoitov <ast@plumgrid.com>,
	Miklos Szeredi <mszeredi@suse.cz>,
	David Drysdale <drysdale@google.com>,
	Oleg Nesterov <oleg@redhat.com>,
	"David S. Miller" <davem@davemloft.net>,
	Vivek Goyal <vgoyal@redhat.com>,
	Mike Frysinger <vapier@gentoo.org>,
	"Theodore Ts'o" <tytso@mit.edu>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Rashika Kheria <rashika.kheria@gmail.com>,
	Hugh Dickins <hughd@google.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org,
	Josh Triplett <josh@joshtriplett.org>
Subject: Re: [PATCH RFC 5/6] epoll: Add implementation for epoll_mod_wait
Date: Wed, 21 Jan 2015 19:14:04 +0800	[thread overview]
Message-ID: <20150121111404.GA3804@ad.nay.redhat.com> (raw)
In-Reply-To: <54BF814F.7090703@redhat.com>

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