netdev - Re: behavior of recvmmsg() on blocking sockets

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Date:	Mon, 29 Mar 2010 11:48:22 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	Brandon Black <blblack@...il.com>
CC:	Arnaldo Carvalho de Melo <acme@...hat.com>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: behavior of recvmmsg() on blocking sockets

On 03/29/2010 11:24 AM, Brandon Black wrote:
> On Mon, Mar 29, 2010 at 11:18 AM, Chris Friesen <cfriesen@...tel.com> wrote:
>>
>> prev = current time
>> loop forever
>>        cur = current time
>>        timeout = max_latency - (cur - prev)
>>        recvmmsg(timeout)
>>        process all received messages
>>        prev = cur
>>
>>
>> Basically you determine the max latency you're willing to wait for a
>> packet to be handled, then subtract the amount of time you spent
>> processing messages from that and pass it into the recvmmsg() call as
>> the timeout.  That way no messages will be delayed for longer than the
>> max latency. (Not considering scheduling delays.)
> 
> With a blocking socket, you'd also need to set SO_RCVTIMEO on the
> underlying socket to some value that makes sense and is below your max
> latency, because recvmmsg()'s timeout argument only applies in-between
> underlying recvmsg() calls, not during them.

Hmm...that's a good point.  For some reason I had been under the
impression that the timeout affected the underlying recvmsg() calls as
well.  It think it would make more sense for the kernel to abort a
blocking recvmsg() call once the timeout expires.

As for spending a lot of time spinning if there are gaps in the input
stream...in the cases where the time-based usage makes sense the normal
situation is that there are a lot of packets coming in.  A 10gig
ethernet pipe can theoretically receive something like 19 packets per
usec.  Doesn't take much of a delay before you probably have packets
waiting.

Chris
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