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

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Date:	Fri, 04 Apr 2008 16:59:27 +0200
From:	Nadia Derbey <Nadia.Derbey@...l.net>
To:	Mike Galbraith <efault@....de>
Cc:	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>
Subject: Re: Scalability requirements for sysv ipc (+namespaces broken with
 SEM_UNDO)

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




View attachment "output.25_rc3_mm1.ref.8" of type "text/x-troff-man" (5846 bytes)

View attachment "plot.25_rc3_mm1.ref.8" of type "text/x-troff-man" (74 bytes)

View attachment "output.25_rc3_mm1.ridr.8" of type "text/x-troff-man" (5851 bytes)

View attachment "plot.25_rc3_mm1.ridr.8" of type "text/x-troff-man" (80 bytes)