linux-kernel - Re: Why do we still have 32 bit counters? Interrupt counters overflow within 50 days

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Message-ID: <alpine.DEB.2.02.1410060347300.26324@nftneq.ynat.uz>
Date:	Mon, 6 Oct 2014 03:49:44 -0700 (PDT)
From:	David Lang <david@...g.hm>
To:	Thomas Gleixner <tglx@...utronix.de>
cc:	Christoph Lameter <cl@...ux.com>,
	Richard Cochran <richardcochran@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Why do we still have 32 bit counters? Interrupt counters overflow
 within 50 days

On Sun, 5 Oct 2014, Thomas Gleixner wrote:

>>
>> 64 bit counters on 32 bit machines are not an easy thing and could be
>
> Whats so hard about 64bit counters on 32bit machines?
>
>> expensive to handle in particular because these counters are used in
>> performance critical hotpaths.
>
> The expensive overhead is a single "adcl" instruction.

As I understand it, since the 64 bit math cannot be made atomic, it requires 
protecting the counter with a lock so that it can't be read while half updated. 
Aquiring a lock on every update is an expensive thing to do. It's not something 
people like to see in a fast path, especially for something of as low an 
importance as the counters.

David Lang
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