Re: [patch V3] percpu_counter: scalability works

From: Tejun Heo
Date: Tue May 17 2011 - 05:11:13 EST

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.

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