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Date:	Mon, 6 Oct 2014 03:49:44 -0700 (PDT)
From:	David Lang <>
To:	Thomas Gleixner <>
cc:	Christoph Lameter <>,
	Richard Cochran <>,
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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