linux-kernel - Re: recvmmsg() timeout behavior strangeness [RESEND]

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Message-ID: <CAKgNAkhLDxpBZ0yu+Ab3b1LOw4XZe6MSn03wBANykzDw9C8NFg@mail.gmail.com>
Date:	Sat, 3 May 2014 13:39:37 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Florian Westphal <fw@...len.de>
Cc:	Arnaldo Carvalho de Melo <acme@...radead.org>,
	lkml <linux-kernel@...r.kernel.org>,
	"linux-man@...r.kernel.org" <linux-man@...r.kernel.org>,
	netdev <netdev@...r.kernel.org>,
	Ondřej Bílka <neleai@...nam.cz>,
	Caitlin Bestler <caitlin.bestler@...il.com>,
	Neil Horman <nhorman@...driver.com>,
	Elie De Brauwer <eliedebrauwer@...il.com>,
	David Miller <davem@...emloft.net>,
	Steven Whitehouse <steve@...gwyn.com>,
	Rémi Denis-Courmont 
	<remi.denis-courmont@...ia.com>, Paul Moore <paul@...l-moore.com>,
	Chris Friesen <chris.friesen@...driver.com>
Subject: Re: recvmmsg() timeout behavior strangeness [RESEND]

On Sat, May 3, 2014 at 1:29 PM, Florian Westphal <fw@...len.de> wrote:
> Michael Kerrisk (man-pages) <mtk.manpages@...il.com> wrote:
>> Reinvestigating the problem, I see that I got my description of the
>> behavior slightly wrong, although the fundamental problem remains.
>> Here's my improved formulation:
> [..]
>
>> Since the timeout is only checked after the arrival of each datagram,
>> we can have scenarios like the following:
>>
>> 0. Assume a timeout of 10 (T) seconds, that vlen is 5, and the call
>>    is made at time X
>>
>> 1. First datagram arrives at time X+2.
>>
>> 2. Second datagram arrives at time X+4 secs
>>
>> 3. Third datagram arrives at time X+6 secs
>>
>> 4. No more datagrams arrive.
>>
>> In this case, the call blocks forever. It hardly seems that this could
>> be intended behavior. The problem, of course is that the timeout is
>> checked only after receipt of a datagram.
>
> Isn't that what MSG_WAITFORONE is supposed to solve?

I don't think so. I understand the idea of the timeout to be: get as
many datagrams as you can within a certain interval. MSG_WAITFORONE is
orthogonal to that goal (you can specify MSG_WAITFORONE without an
infiniite timeout, for example).

Also, consider the algorithm above: if no datagrams arrive, the
timeout is in effect ignored.

Thanks,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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