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Subject: Re: [patch V3] percpu_counter: scalability works
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Shaohua Li <shaohua.li@intel.com>
Cc: Tejun Heo <tj@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "cl@linux.com" <cl@linux.com>, "npiggin@kernel.dk" <npiggin@kernel.dk>
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Date: Mon, 16 May 2011 16:22:16 +0200
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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.