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

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Message-ID: <1305634807.2850.89.camel@edumazet-laptop>
Date:	Tue, 17 May 2011 14:20:07 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Shaohua Li <shaohua.li@...el.com>,
	"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 mardi 17 mai 2011 à 11:50 +0200, Tejun Heo a écrit :

> I'm not asking to make it more accurate but the initial patches from
> Shaohua made the _sum() result to deviate by @batch even when only one
> thread is doing _inc() due to the race window between adding to the
> main counter and resetting the local one.  All I'm asking is closing
> that hole and I'll be completely happy with it.  The lglock does that
> but it's ummm.... not a very nice way to do it.
> 
> Please forget about deviations from concurrent activities.  I don't
> care and nobody should.  All I'm asking is removing that any update
> having the possibility of that unnecessary spike and I don't think
> that would be too hard.
> 

Spikes are expected and have no effect by design.

batch value is chosen so that granularity of the percpu_counter
(batch*num_online_cpus()) is the spike factor, and thats pretty
difficult when number of cpus is high.

In Shaohua workload, 'amount' for a 128Mbyte mapping is 32768, while the
batch value is 48. 48*24 = 1152.
So the percpu s32 being in [-47 .. 47] range would not change the
accuracy of the _sum() function [ if it was eventually called, but its
not ]

No drift in the counter is the only thing we care - and _read() being
not too far away from the _sum() value, in particular if the
percpu_counter is used to check a limit that happens to be low (against
granularity of the percpu_counter : batch*num_online_cpus()).

I claim extra care is not needed. This might give the false impression
to reader/user that percpu_counter object can replace a plain
atomic64_t.

For example, I feel vm_committed_as could be a plain atomic_long_t

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