linux-kernel - Re: [patch V3] percpu_counter: scalability works

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Message-ID: <1305555736.2898.46.camel@edumazet-laptop>
Date:	Mon, 16 May 2011 16:22:16 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	Tejun Heo <tj@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"cl@...ux.com" <cl@...ux.com>,
	"npiggin@...nel.dk" <npiggin@...nel.dk>
Subject: Re: [patch V3] percpu_counter: scalability works

Le lundi 16 mai 2011 à 11:35 +0200, Eric Dumazet a écrit :

> Given that vm_committed has one percent resolution need
> (sysctl_overcommit_ratio is expressed with percent resolution), it
> should be used with an appropriate batch value, something like :
> 
> vm_committed_as_batch = max(percpu_counter_batch,
> 			    total_ram_pages/(num_possible_cpus()*100));
> 


Funny thing with vm_committed_as is we dont even read its value with
default vm configuration 

(sysctl_overcommit_memory == OVERCOMMIT_ALWAYS or OVERCOMMIT_GUESS)

[ In this case, we read it only for /proc/meminfo output ]

Ideally, we could dynamically change vm_committed_as_batch when
sysctl_overcommit_memory or other param is changed. This would need a
mechanism to ask all cpus to transfert/clear their local s32 into global
fbc->count [when lowering vm_committed_as_batch]

Another idea would be to use an atomic when manipulating the percpu s32,
so that __percpu_counter_sum() is able to make this operation itself :
At the end of __percpu_counter_sum(), fbc->count would be the final
result, and all s32 would be zero, unless some cpus called _add()
meanwhile.



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