Message-ID: <1370440607.24311.251.camel@edumazet-glaptop>
Subject: Re: [PATCH v9 net-next 5/7] net: simple poll/select low latency
 socket poll
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Eliezer Tamir <eliezer.tamir@linux.intel.com>
Cc: David Miller <davem@davemloft.net>, linux-kernel@vger.kernel.org,
        netdev@vger.kernel.org, Jesse Brandeburg <jesse.brandeburg@intel.com>,
        Don Skidmore <donald.c.skidmore@intel.com>,
        e1000-devel@lists.sourceforge.net,
        Willem de Bruijn <willemb@google.com>,
        Ben Hutchings <bhutchings@solarflare.com>,
        Andi Kleen <andi@firstfloor.org>, HPA <hpa@zytor.com>,
        Eilon Greenstien <eilong@broadcom.com>,
        Or Gerlitz <or.gerlitz@gmail.com>, Amir Vadai <amirv@mellanox.com>,
        Eliezer Tamir <eliezer@tamir.org.il>
Date: Wed, 05 Jun 2013 06:56:47 -0700
In-Reply-To: <51AF401C.2030506@linux.intel.com>
References: <20130605103400.11172.49099.stgit@ladj378.jer.intel.com>
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	 <1370439054.24311.248.camel@edumazet-glaptop>
	 <51AF401C.2030506@linux.intel.com>
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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@intel.com>
> >> Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
> >> Tested-by: Willem de Bruijn <willemb@google.com>
> >> Signed-off-by: Eliezer Tamir <eliezer.tamir@linux.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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