linux-kernel - Re: [PATCH 22/25] Generic dynamic per cpu refcounting

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Message-ID: <20121129215751.GA4102@jl-vm1.vm.bytemark.co.uk>
Date:	Thu, 29 Nov 2012 21:57:52 +0000
From:	Jamie Lokier <jamie@...reable.org>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Benjamin LaHaise <bcrl@...ck.org>,
	linux-kernel@...r.kernel.org, linux-aio@...ck.org,
	linux-fsdevel@...r.kernel.org, zab@...hat.com, jmoyer@...hat.com,
	axboe@...nel.dk, viro@...iv.linux.org.uk
Subject: Re: [PATCH 22/25] Generic dynamic per cpu refcounting

Kent Overstreet wrote:
> On Thu, Nov 29, 2012 at 09:54:47PM +0100, Andi Kleen wrote:
> > > > The regular atomic_t is limited in ways that you are not.
> > > > See my original mail.
> > > 
> > > I don't follow, can you explain?
> > 
> > For most cases the reference count is tied to some object, which are
> > naturally limited by memory size or other physical resources.
> > 
> > But in the assymetric CPU case with your ref count no such limiter
> > exists.
> 
> It's got exactly the same limit as the old code which used the atomic_t
> - we're limited by the number of threads that can be issuing aio
> syscalls at a time.
> 
> The assymetry you're talking about _doesn't matter_, individual cpu
> counters wrapping does not affect what the counters all sum to when we
> go to tear down.
> 
> A coworker at lunch actually pointed out to me that the reason this is
> true is just that modular arithmatic is still associative with addition
> and subtraction.

It's just like jiffies.  Everyone understands jiffies arithmetic I hope.

-- Jamie
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