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

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Message-ID: <20110517091102.GE20624@htj.dyndns.org>
Date:	Tue, 17 May 2011 11:11:02 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
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

Hello, Eric, Shaohua.

On Tue, May 17, 2011 at 11:01:01AM +0200, Eric Dumazet wrote:
> Just convince him that percpu_counter by itself cannot bring a max
> deviation guarantee. No percpu_counter user cares at all. If they do,
> then percpu_counter choice for their implementation is probably wrong.
> 
> [ We dont provide yet a percpu_counter_add_return() function ]

I haven't gone through this thread yet but will do so later today, but
let me clarify the whole deviation thing.

1. I don't care reasonable (can't think of a better word at the
   moment) level of deviation.  Under high level of concurrency, the
   exact value isn't even well defined - nobody can tell operations
   happened in what order anyway.

2. But I _do_ object to _sum() has the possibility of deviating by
   multiples of @batch even with very low level of activity.

I'm completely fine with #1.  I'm not that crazy but I don't really
want to take #2 - that makes the whole _sum() interface almost
pointless.  Also, I don't want to add big honking lglock to just avoid
#2 unless it can be shown that the same effect can't be achieved in
saner manner and I'm highly skeptical that would happen.

Thank you.

-- 
tejun
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