Messages in this thread | | | From | (Alan Cox) | Subject | Re: Linux timekeeping plans | Date | Sun, 6 Dec 1998 16:39:32 +0000 (GMT) |
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> But I don't understand the cross-CPU migration issue.
The CPU's on an X86 SMP box run a MESI cache. That means each cache line has 4 states - those states dont include a 'shared-modified' state. So when you have two people reading the same line and one writes it then the other reads it you get
[M=modified E=exclusive S=shared I=invalid]
S S E I M I (data now written) S S (data now readable)
The move from modified to shared especially is quite slow as it requires the cache line is written to main memory and then read by the other CPU. That will disturb any measurements because in CPU clock source counts thats a long time. You probably need to measure this as well as measuring the TSC difference some how to get a better view
Alan
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