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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