Messages in this thread | | | Date | Mon, 6 Oct 2014 03:49:44 -0700 (PDT) | From | David Lang <> | Subject | Re: Why do we still have 32 bit counters? Interrupt counters overflow within 50 days |
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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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