[petsc-dev] MatMult on Summit

[Jed Brown]

"Smith, Barry F." <bsmith at mcs.anl.gov> writes:

>> On Sep 21, 2019, at 11:43 PM, Jed Brown <jed at jedbrown.org> wrote:
>> 
>> "Smith, Barry F." <bsmith at mcs.anl.gov> writes:
>> 
>>>  Jed,
>>> 
>>>  What does latency as a function of message size mean?   It is in the plots
>> 
>> It's just the wall-clock time to ping-pong a message of that size.  All
>> the small sizes take the same amount of time (i.e., the latency), then
>> transition to being network bandwidth limited for large sizes.
>
>    Thanks, this is fine for the small size. But he has the graph up to
>    size 1000000 and the plotted values change for larger sizes, surely
>    for 1000000 the time is a combination of latency and bandwidth?
>    Isn't calling it latency a misnomer, or do people use this
>    inconsistent terminology when doing ping-pongs?

Latency of an operation is just how long from when you initiate it until
it completes.  Latency in a performance model, such as LogP, is additive
with other factors (like bandwidth and compute throughput).