[petsc-users] Obtaining bytes per second

[Jed Brown]

Matthew Knepley <knepley at gmail.com> writes:

> On Thu, May 7, 2015 at 9:23 AM, Justin Chang <jychang48 at gmail.com> wrote:
>
>> So to summarize, if I understand everything, I should do the following:
>>
>> 1) calculate the flop/byte ratio for various problem sizes and solver
>> methods on one process and:

It is impossible to define "arithmetic intensity" (flops/byte) without
selecting a cache model.  And my "cache model", I include register
reuse.  Assuming no reuse at all is not even close to representative
(off by orders of magnitude).

> Should be roughly invariant to problem size.

Except (a) insofar as it depends on cache sizes and (b) the algorithm
characteristics depend on data size (e.g., the cost of supernode
factorization for sparse direct solvers scales superlinearly, so
arithmetic intensity will drift as you increase the problem size).


You can see how we compare "pessimal" versus "perfect" caching here:

http://59A2.org/na/MayBrownLePourhiet-pTatin3d-2014.pdf
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