Re:Performance variation in ‘identical’ processors
by userbinator on Sunday January 05, 2020 @11:02PM
(#21966333)
I'm surprised that, even at the same frequency, there is still some pretty large variation. I wonder if that's due to other sources of noise that are often ignored by a lot of people running benchmarks (e.g. background processes, SMM, ME, etc.)
e.g., memory, which presumably have their own temperature characteristics
To my knowledge, and this is based on DDR3 and older; memory frequencies are essentially fixed because the transceivers on both ends need to sample in the middle of a bit cell, and to do that they need to know the clock period, which must not change once it's known. There's a delay-locked-loop (DLL) in the RAM which generates a phase-shifted local reference clock.
If the processors could all be locked to one constant frequency (i.e. all the power/performance "dynamic tuning" stuff disabled) that would help show whether there's other sources of noise. This of course also assumes the clock generators are identical.
e.g., memory, which presumably have their own temperature characteristics
To my knowledge, and this is based on DDR3 and older; memory frequencies are essentially fixed because the transceivers on both ends need to sample in the middle of a bit cell, and to do that they need to know the clock period, which must not change once it's known. There's a delay-locked-loop (DLL) in the RAM which generates a phase-shifted local reference clock.
If the processors could all be locked to one constant frequency (i.e. all the power/performance "dynamic tuning" stuff disabled) that would help show whether there's other sources of noise. This of course also assumes the clock generators are identical.