And even those tools just report an average speed, which means the part of the program you care about might be running faster or slower than the rest. The new Haswell-EP chips even have both an additional AVX base and AVX turbo speed, and switch speed for any portion of code that uses full width vectors. As a result, his traditional solutions 2 (read the performance counters) and 3 (write a test loop) can be harder than they sound.
So while the reading the MSR's can help a lot to diagnose the issue and to reduce the error, if you really want reliable measurements, the "turn it off in the BIOS" option is the best choice. But even this can be harder than it sounds, as it seems some BIOS will lie and use it anyway. At least, I have one Dell Haswell machine to which I have root but not physical access, which i7z and turbostat show to be running Turbo Boost although the otherwise reliable on site admin swears he's checked several times that it's turned off in BIOS.
It seems like it should be possible to turn Turbo Boost on or off by _writing_ to the appropriate MSR but I haven't succeeded with this yet. Is there a trick I'm missing? Has anyone else had success doing it this way? It would be nice to be able to turn it off as part of the timing routine, and then turn it back on for normal operation after.
That sounds a bit on the high end, considering that 65 is the highest my system (early i7) goes when fully loaded... but then looking at the temperature graph, it looks like they're really pushing them to the limits and letting thermal throttling take care of the rest - the spec for that particular model is ~80 max.
Since this is a VM, there's always a remote chance that they can trap the MSR reads and return whatever data they want. (Maybe it could even be used as a convert channel...?)
Just checking the Intel manual docs:
"• Thermal Status (bit 0, RO) — This bit indicates whether the digital thermal sensor high-temperature output signal (PROCHOT#) is currently active. Bit 0 = 1 indicates the feature is active. This bit may not be written by software; it reflects the state of the digital thermal sensor.
• Thermal Status Log (bit 1, R/WC0) — This is a sticky bit that indicates the history of the thermal sensor high temperature output signal (PROCHOT#). Bit 1 = 1 if PROCHOT# has been asserted since a previous RESET or the last time software cleared the bit. Software may clear this bit by writing a zero."
I was checking both, and both were zero.
So if we aren't thermal throttling, then I guess it still may be the fans...
But their i7 is probably stuffed in a room with 10000 other machines.
