Re: Is Parallel Programming Hard, And, If So, What Can You Do About It?

On 5/20/2025 6:30 PM, George Neuner wrote:

On Tue, 20 May 2025 22:11:41 +0000, mitchalsup@aol.com (MitchAlsup1)
wrote:
 

On Tue, 20 May 2025 20:06:13 +0000, Stefan Monnier wrote:
>

Personally, I rarely use multi-threading, and when I do, it is usually
in
the form of using mutex locks over shared buffers.
You lock the mutex if needed to copy data from one thread to another; or
when doing a task that depends on the data being consistent.

>
FWIW, I think these kinds of things usually fall in the scope of
concurrency rather than parallelism.

>
When I run 20-copies of a FEM CFD application, each uni-process::
am I running concurrently ?? or in parallel ?? or both ??

 Processes on the same core are concurrent - processes on different
cores are parallel.

Only if the cores and/or "hardware threads" do not interfere with one another? Fwiw, an example of an embarrassingly parallel algorithm is computing the Mandelbrot set. Actually, this reminds me of the "alias" problem with Intel hyper threading in the past.

 

Also note: I need to use the "affinity" service under taskMangler
so that only 4 processes share a core, performance goes up by roughly
20%.
>
The CFD simulations typically run for 10 hours (each, wall clock time),
and every second; dump out a dozen KB of compressed data (each). The
disk load is low enough I can do other "stuff" on the computer as
long as I affinitize the heavy loads to 5 of the 6 cores.
>

>
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         Stefan