netdev - Re: [PATCH v9 net-next 5/7] net: simple poll/select low latency socket poll

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Date:	Wed, 05 Jun 2013 06:56:47 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Eliezer Tamir <eliezer.tamir@...ux.intel.com>
Cc:	David Miller <davem@...emloft.net>, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org,
	Jesse Brandeburg <jesse.brandeburg@...el.com>,
	Don Skidmore <donald.c.skidmore@...el.com>,
	e1000-devel@...ts.sourceforge.net,
	Willem de Bruijn <willemb@...gle.com>,
	Ben Hutchings <bhutchings@...arflare.com>,
	Andi Kleen <andi@...stfloor.org>, HPA <hpa@...or.com>,
	Eilon Greenstien <eilong@...adcom.com>,
	Or Gerlitz <or.gerlitz@...il.com>,
	Amir Vadai <amirv@...lanox.com>,
	Eliezer Tamir <eliezer@...ir.org.il>
Subject: Re: [PATCH v9 net-next 5/7] net: simple poll/select low latency
 socket poll

On Wed, 2013-06-05 at 16:41 +0300, Eliezer Tamir wrote:
> On 05/06/2013 16:30, Eric Dumazet wrote:
> > On Wed, 2013-06-05 at 13:34 +0300, Eliezer Tamir wrote:
> >> A very naive select/poll busy-poll support.
> >> Add busy-polling to sock_poll().
> >> When poll/select have nothing to report, call the low-level
> >> sock_poll() again until we are out of time or we find something.
> >> Right now we poll every socket once, this is suboptimal
> >> but improves latency when the number of sockets polled is not large.
> >>
> >> Signed-off-by: Alexander Duyck <alexander.h.duyck@...el.com>
> >> Signed-off-by: Jesse Brandeburg <jesse.brandeburg@...el.com>
> >> Tested-by: Willem de Bruijn <willemb@...gle.com>
> >> Signed-off-by: Eliezer Tamir <eliezer.tamir@...ux.intel.com>
> >> ---
> >
> > I am a bit uneasy with this one, because an applicatio polling() on one
> > thousand file descriptors using select()/poll(), will call sk_poll_ll()
> > one thousand times.
> 
> But we call sk_poll_ll() with nonblock set, so it will only test once
> for each socket and not loop.
> 
> I think this is not as bad as it sounds.
> We still honor the time limit on how long to poll.
> 
> When we busy-wait on a single socket we call sk_poll_ll() repeatedly
> until we timeout or we have something to report.
> 
> Here on the other hand, we sk_poll_ll() once for each file, so we loop 
> on the files. We moved the loop from inside sk_poll_ll to select/poll.
> 
> I also plan on improving this this in the next stage.
> 
> The plan is to give control on whether sk_poll_ll is called to
> select/poll/epoll, so the caller has even more control.

This looks quite easy, by adding in include/uapi/asm-generic/poll.h

#define POLL_LL 0x8000

And do the sk_poll_ll() call only if flag is set.

I do not think we have to support select(), as its legacy interface, and
people wanting ll should really use epoll() or poll().



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