packed storage

Open discussion for MAGMA

packed storage

Postby ryanjparker » Tue Apr 23, 2013 1:14 am

I'm new to MAGMA and am curious why there are no packed storage options (for example, a dpptrf packed Cholesky factorization).

Is this just something yet to be implemented, or are algorithms for these structures are not well suited for GPUs?

The non-packed routines work great so far... I'm just easily hitting my 1GB GPU memory size.

Thanks for any insight!
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Re: packed storage

Postby mgates3 » Tue Apr 23, 2013 8:03 pm

Packed storage yields poor performance, even on CPUs, because the complicated indexing prevents efficient use of cache and memory bandwidth. Just compare the performance on the CPU:

Code: Select all
./testing_spotrf -N 4000 -l -L
  N     CPU GFlop/s (sec)   GPU GFlop/s (sec)   ||R_magma - R_lapack||_F / ||R_lapack||_F
========================================================
 4000     26.55 (   0.80)       --- (  ---)  ---     # spotrf
 4000      1.38 (  15.47)       --- (  ---)  ---     # spptrf (packed)


An alternative is RFP (rectangular full packed) format. This works better than packed storage on CPUs. We haven't explored it on GPUs. See:
http://www.netlib.org/lapack/lawnspdf/lawn199.pdf
-mark
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