netdev - Re: [PATCH 0/1] RFC: poll/select performance on datagram sockets

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Date:	Fri, 29 Oct 2010 13:08:34 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	Alban Crequy <alban.crequy@...labora.co.uk>,
	"David S. Miller" <davem@...emloft.net>,
	Stephen Hemminger <shemminger@...tta.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Alexey Dobriyan <adobriyan@...il.com>, netdev@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Pauli Nieminen <pauli.nieminen@...labora.co.uk>,
	Rainer Weikusat <rweikusat@...gmbh.com>
Subject: Re: [PATCH 0/1] RFC: poll/select performance on datagram sockets

On Fri, 29 Oct 2010, Eric Dumazet wrote:

> Le vendredi 29 octobre 2010 à 19:18 +0100, Alban Crequy a écrit :
> > Hi,
> > 
> > When a process calls the poll or select, the kernel calls (struct
> > file_operations)->poll on every file descriptor and returns a mask of
> > events which are ready. If the process is only interested by POLLIN
> > events, the mask is still computed for POLLOUT and it can be expensive.
> > For example, on Unix datagram sockets, a process running poll() with
> > POLLIN will wakes-up when the remote end call read(). This is a
> > performance regression introduced when fixing another bug by
> > 3c73419c09a5ef73d56472dbfdade9e311496e9b and
> > ec0d215f9420564fc8286dcf93d2d068bb53a07e.
> > 
> > The attached program illustrates the problem. It compares the
> > performance of sending/receiving data on an Unix datagram socket and
> > select(). When the datagram sockets are not connected, the performance
> > problem is not triggered, but when they are connected it becomes a lot
> > slower. On my computer, I have the following time:
> > 
> > Connected datagram sockets: >4 seconds
> > Non-connected datagram sockets: <1 second
> > 
> > The patch attached in the next email fixes the performance problem: it
> > becomes <1 second for both cases. I am not suggesting the patch for
> > inclusion; I would like to change the prototype of (struct
> > file_operations)->poll instead of adding ->poll2. But there is a lot of
> > poll functions to change (grep tells me 337 functions).
> > 
> > Any opinions?
> 
> My opinion would be to use epoll() for this kind of workload.

Yeah, epoll does check for event hints coming with the callback wakeup, 
and avoid waking up epoll_wait() waiters, for non matching events.
Most of the devices we care about, have been modified to report the event 
mask with the wakeup call.


- Davide