[petsc-users] Obtaining bytes per second

[Jed Brown]

Justin Chang <jychang48 at gmail.com> writes:

> Thank you guys for your responses. If I want to estimate the number of
> bytes that come down, would -memory_info give me that information?

No.  There is no substitute for a performance model.  Hardware
performance counters can tell you something, but without a cache-aware
performance model, it can give you information that is off by an order
of magnitude or more.

> And with this information plus the total number of logged flops, i can get
> the ratio of flop to bytes and hence the (crude estimation of) upper bound
> FLOPS/s based on the reported stream BW?

Flop/s is logged (for PETSc operations) and you can use hardware
performance counters for that (much better than for bandwidth, but still
erroneous due to speculative execution and incomplete/gratuitous
vectorization).  But to compute an arithmetic intensity or find
performance bounds based on memory bandwidth considerations, there is no
substitute for understanding what you are doing.  Sorry.
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