linux-kernel - Re: [RFC -mm 0/6] sysv ipc: scale msgmnb with the number of cpus

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Message-ID: <18529.58173.415727.712894@frecb006361.adech.frec.bull.fr>
Date:	Wed, 25 Jun 2008 08:18:37 +0200
From:	Solofo.Ramangalahy@...l.net
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	Solofo.Ramangalahy@...l.net, linux-kernel@...r.kernel.org,
	Matt Helsley <matthltc@...ibm.com>,
	Mingming Cao <cmm@...ibm.com>,
	Nadia Derbey <Nadia.Derbey@...l.net>,
	Yasunori Goto <y-goto@...fujitsu.com>
Subject: Re: [RFC -mm 0/6] sysv ipc: scale msgmnb with the number of cpus

Manfred Spraul writes:
 > > Humm... now this make me think that you did not change the MSGMNB
 > > value when you changed MSGMNI and MSGMAX.
 > > Maybe that was on purpose?
 > >
 > >   
 > I was afraid that it might break user space applications that queue a 
 > few kb of messages.

Ok, the choice of a maximum value of 65536 which is already is use for
several months/years was made partly for the same concern.
Beside, as the values are not enforced, we should be relatively safe.

Searching the archives, I also found usage of a value "around a MB".

 > That's also the reason for
 > >                 if (msgsz + msq->q_cbytes <= msq->q_qbytes &&
 > >                                 1 + msq->q_qnum <= msq->q_qbytes) {
 > >                         break;
 > >                 }
 > It's possible to send 0-byte messages even if the  message queue is full 
 > [except that you can't send more than MSGMNB messages].

Thanks for this information. I should add that I checked that no
regression was introduced with ltp-full-20080531, but I did not look
more closely (e.g. coverage of this part of the code).

-- 
solofo
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