linux-kernel - Re: Scalability requirements for sysv ipc (+namespaces broken with SEM

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Date:	Fri, 04 Apr 2008 17:03:25 +0200
From:	Nadia Derbey <Nadia.Derbey@...l.net>
To:	Nadia Derbey <Nadia.Derbey@...l.net>
Cc:	Mike Galbraith <efault@....de>,
	Manfred Spraul <manfred@...orfullife.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	paulmck@...ux.vnet.ibm.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Pavel Emelianov <xemul@...nvz.org>,
	NADIA DERBEY <Nadia.Derbey@...l.net>
Subject: Re: Scalability requirements for sysv ipc (+namespaces broken with
 SEM_UNDO)

Nadia Derbey wrote:
> Mike Galbraith wrote:
> 
>> On Sun, 2008-03-30 at 16:12 +0200, Manfred Spraul wrote:
>>
>>> Mike Galbraith wrote:
>>>
>>>> On Sat, 2008-03-22 at 20:08 +0100, Manfred Spraul wrote:
>>>>
>>>>  
>>>>
>>>>> just the normal performance of 2.6.25-rc3 is abyssimal, 55 to 60% 
>>>>> slower than 2.6.18.8:
>>>>>    
>>>>
>>>>
>>>> After manually reverting 3e148c79938aa39035669c1cfa3ff60722134535,
>>>> 2.6.25.git scaled linearly
>>>
>>>
>>> We can't just revert that patch: with IDR, a global lock is mandatory 
>>> :-(
>>> We must either revert the whole idea of using IDR or live with the 
>>> reduced scalability.
>>
>>
>>
>> Yeah, I looked at the problem, but didn't know what the heck to do about
>> it, so just grabbed my axe to verify/quantify.
>>
>>
>>> Actually, there are further bugs: the undo structures are not 
>>> namespace-aware, thus semop with SEM_UNDO, unshare, create new array 
>>> with same id, but more semaphores, another semop with SEM_UNDO will 
>>> corrupt kernel memory :-(
>>> I'll try to clean up the bugs first, then I'll look at the 
>>> scalability again.
>>
>>
>>
>> Great!
>>
>>     -Mike
>>
>>
>>
>>
> 
> I could get better results with the following solution:
> wrote an RCU-based idr api (layers allocation is managed similarly to 
> the radix-tree one)
> 
> Using it in the ipc code makes me get rid of the read lock taken in 
> ipc_lock() (the one introduced in 
> 3e148c79938aa39035669c1cfa3ff60722134535).
> 
> You'll find the results in attachment (kernel is 2.6.25-rc3-mm1).
> output.25_rc3_mm1.ref.8  --> pmsg output for the 2.6.25-rc3-mm1
> plot.25_rc3_mm1.ref.8    --> previous file results for use by gnuplot
> output.25_rc3_mm1.ridr.8 --> pmsg output for the 2.6.25-rc3-mm1
>                              + rcu-based idrs
> plot.25_rc3_mm1.ridr.8   --> previous file results for use by gnuplot
> 
> 
> I think I should be able to send a patch next week. It is presently an 
> uggly code: I copied idr.c and idr.h into ridr.c and ridr.h to go fast, 
> so didn't do any code factorization.
> 
> Regards
> Nadia
> 
> 
> 

Sorry forgot the command:

for i in 1 2 3 4 5 6 7 8;do ./pmsg $i 5;done > output.25_rc3_mm1.ref.8


Regards,
Nadia

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