On Tuesday 31 March 2009 23:16:10 Shasi Thati wrote:
Hi,
I have a question regarding the openssl speed command. When I use this
command to test the crypto offload engine performance what is the
right command to use?
Is it
openssl speed -evp aes-128-cbc -engine xx -elapsed
or
openssl speed -evp aes-128-cbc -engine xx
I have seen examples with both of them on the internet and I get
different results with each of them. What exactly does elapsed
option add here?
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
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