[petsc-users] CPU speed or DRAM speed bottlenecks ?

[C B]

Barry,

Thank you so much for your quick reply and insight.

Are there any tools/simple ways to determine how much time is lost in cache
misses / etc, please direct me to any resources to learn about this.

Thanks again!


On Thu, Dec 3, 2020 at 4:09 PM Barry Smith <bsmith at petsc.dev> wrote:

>
>
> On Dec 3, 2020, at 2:25 PM, C B <cebau.mail at gmail.com> wrote:
>
> Resorting to your expertise in software performance:
>
> Subject: Looking for a crude assessment of CPU speed or DRAM speed
> bottlenecks in shared memory multi-core PCs
>
> On a typical PC with one Xeon CPU (8 cores),  a serial code runs a case in
> say 10 hours of Wall time, and on the same computer 4 instances of the same
> code running simultaneously (the same case) take essentially the same Wall
> time, 10 hrs or a marginal increase such as 10hrs 30 mins.   There is no
> I/O, lots of free physical RAM, each core running an instance shows ~ 100%
> utilization.
>
> Q1: What could we conclude about this hardware-software-case combination
> in terms of being CPU bound, memory bandwidth bound, etc ?
>
>    It does not appear to be memory bandwidth bound.  Presumably the 4
> cases will each be utilizing the same memory bandwidth as one case so I
> think one can conclude that the 1 case is using at most 25 percent of the
> memory bandwidth.
>
>
> Q2: Can we say that this hardware-software-case combination is not DRAM
> bound, and that it “may be amenable” to a good speedup running multiple
> threads in the same shared memory environment ?
>
>    I think this is good a way to say it, "since it is not DRAM bound it
> may be amendable to good speedup running multiple threads", it may also be
> amendable to MPI parallelism. There are other factors that affect parallel
> performance besides memory bandwidth without more information these are
> unknown".
>
>   Barry
>
>
>
> I did look into the shared memory benchmark
> http://www.cs.virginia.edu/stream  but I could not draw any conclusions.
>
> If this is a trivial question, please point me to a good resource to learn.
>
> Thanks!
>
>
>
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