Re: [RFC patch 15/15] LTTng timestamp x86

Christoph Lameter
2008-10-17T19:39:04Z

Luck, Tony wrote:
> Even one line bouncing between cpus can be a performamce disaster.
> You'll probably hit a serious wall somewhere between 8 and 16
> cpus (ia64 has code that looks a lot like this in the gettimeofday()
> path because it does not synchronize cpu cycle counters ... some

The code exist by necessity because some systems do not have synchronized ITCs
and one would not have time go backward. The cmpxchg there is usually switched
off. Its horrible in terms of scaling to large numbers of processor and also
horrible in terms of clock accuracy.