On Tuesday 31 March 2009 23:16:10 Shasi Thati wrote:

It means "elapsed". :-) Ie. how much time elapsed during the benchmark. The normal measurement is cpu usage, which is something less than or equal to the elapsed time - if the benchmark used half the available cpu cycles during the elapsed period (according to scheduler stats, accurate or otherwise), the time given would be half the elapsed time.

The usefulness of using cpu-time (instead of "-elapsed") is to eliminate; (a) skewed statistics due to the system running other tasks while the benchmark was in progress (ie. you're only billed for what you use), and (b) to eliminate time the s/w (and driver) spent waiting for the crypto accelerator to respond to crypto operations. The value of (b) is to interpolate certain theoretical limits. Ie. if 80% of the time is spent waiting on the accelerator, the cpu-time for the benchmark run would be 1/5 of the elapsed time and so the calculated number of crypto ops per second would be 5 times what actually happened in real/elapsed time. If the latency of the accelerator is roughly constant but it can process multiple things at once due to having multiple execution units, then this inflated number is a useful "estimate" of how much you could theoretically process if you had multiple threads/processes keeping the cpu busy rather than waiting. In this example you'd need at least 5 threads to achieve such a performance level. (Which also assumes the accelerator performance would continue to scale up that far.)

Cheers, Geoff