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

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Date:	Wed, 25 Jun 2008 12:12:17 +0200
From:	Nadia Derbey <Nadia.Derbey@...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>,
	Yasunori Goto <y-goto@...fujitsu.com>
Subject: Re: [RFC -mm 0/6] sysv ipc: scale msgmnb with the number of cpus

Manfred Spraul wrote:
> Solofo.Ramangalahy@...l.net wrote:
> 
>> 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.
> 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].
> 

Manfred,

If I'm not missign anything when reading the code, sending up to MSGMNB 
0-bytes messages would make us enqueue MSGMNB msg_msg structures (this 
is in the worst case where no receiver is waiting for those messages).
==> MSGMNB * 24 bytes (or 48 bytes in 64-bit mode))
==> 384 KB with current MSGMNB value (16K).

But 1,5 MB with a MSGMNB=64K

Even if it is a worst case, it should be considered and may be we should 
refine the formula Solofo has proposed if you think this is not a 
reasonable value.
May be add a dependency on the memory size?

Regards,
Nadia
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