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

[Barry Smith]

> 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 <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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