:Howdy, : :Is the TSC timecounter synchronized across multiple cores and/or :processors? A quick search seems to indicate it's not but I'd like to :find a definitive reference on the TSC. : :Thanks, :George At least on AMD X2 boxes they are not synchronized. The absolute values of the TSC counters will be off and they will also drift relative to each other, which can readily be demonstrated with some test IPIs on DragonFly with the TSC synchronization code turned off. In these tests a single IPI is sent from one cpu to another and the absolute value of each cpu's TSC is recorded. The number in the parenthesis is the difference between the two absolute values. The difference should be about the same same when the test is repeated, but as you can see the gap increases in each successive test. index cpu timestamp ID trace 0140bb 0 1083883698618 testlog_pingpong pingpong (720862) 00f619 1 1083882977756 testlog_pingpong pingpong 0140bc 0 1143172876697 testlog_pingpong pingpong (768494) 00f61a 1 1143172108203 testlog_pingpong pingpong 0140bd 0 1164275800252 testlog_pingpong pingpong (785266) 00f61b 1 1164275014986 testlog_pingpong pingpong 0140be 0 1179148341147 testlog_pingpong pingpong (799137) 00f61c 1 1179147542010 testlog_pingpong pingpong So not only are the TSC's not synchronized with each other, but their frequencies are also not locked relative to each other. There will a small amount of drift for each cpu and I'm guessing the dift will also slide around a bit based on temperature. Intel might work differently but I don't have any intel multi-cores handy to run the test on so I don't know. -Matt Matthew Dillon <dillon_at_backplane.com>Received on Thu Apr 10 2008 - 05:27:09 UTC
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