[Paraview] Issues with PVSB 5.2 and OSMesa support

[Chuck Atkins]

Hi Michel,

Setting KNOB_MAX_WORKER_THREADS to 1 fixed the oversubscription of threads
and the memory consumption issue when the number of mpi processes is equal
to the number of cores requested.

Great!

If I do not allow threading of swr by setting KNOB_MAX_WORKER_THREADS to 1
(basically serial swr vs serial llvmpipe), do you know which performance
gain I could still expect by relying only on a potential better usage of
the AVX instructions?

Short answer: swr is marginally faster because of instruction set
optimizations but way faster because of better thread scaling.

Long answer: With Mesa 11.x and 12.x, our experimental data showed similar
single threaded performance between the two, however, swr in 13.x
(currently deployed with 5.2) saw a significant performance increase
between 20% - 100% depending on the workload.  Multithreaded performance,
however, is quite another story.  Swr scales nearly linearly on Haswell
CPUs for both geometry and pixel operations while llvmpipe scales well up
to 16c for pixel operations and is entirely single-threaded for geometry
shaders. Coincidentally, most actions in paraview are geometry based
(volume rendering being the exception).  So, in general, swr will perform
better than llvmpipe by a fair amount and if using it in multi-threading
mode then by a drastic amount.

- Chuck
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