Charm++ parallel programming system

[Nikhil Jain <nikhil.jain AT acm.org>]

Hello Scott,

This is indeed strange and unexpected. Everything you are doing seems the way it should be done, yet the results are unexpected. Is this code a small benchmark or something we can get access to and explore?

Thanks
Nikhil

Scott Field

March 21, 2016 at 18:17

Hi,

   On blue waters using an smp-enabled charm++ build, my application typically runs 2 to 5 times slower (as judged by the cpu time [computed as walltime x workers]) than the corresponding non-smp build. For example:

http://www.dam.brown.edu/people/sfield/charmpp_figs/bw_smp_slow/BW_ScalarWave1D_OneNodeComparison.png

I'm at a loss to explain (or fix!) this, but I have a few pieces of evidence to share:

1) On other machines, and using a verbs-* build of charm, I see negligible difference in cpu time between smp and non-smp versions.

 

2) According to Projections, my user defined entry methods run up to 6 times slower compared to non-smp builds (averaged over time and cores)

3) Some cores are idle up to 50% of the time, compared the non-smp version where almost no cores are idle (although non-smp shows significantly higher overhead, which I cannot explain either). See:

http://www.dam.brown.edu/people/sfield/charmpp_figs/bw_smp_slow/nonsmp_bw_timeline.png

http://www.dam.brown.edu/people/sfield/charmpp_figs/bw_smp_slow/smp_bw_timeline.png

4) The smp slowdown is robust to how I launch the executable, although there is a bit of variability. Some examples are given at the end of this email. 

An instructive comparison is a non-smp executable using 1 process per floating unit for 16 total (cpu time of 71 us per algorithm iteration) vs any of the other smp jobs where the threads are restricted to run on the floating point units. These CPU times are 143 us, 166 us, 174 us and 310 us. Comparing jobs using all 32 (logical) cores the times are 105 us vs 500 us.

Just to reiterate, this only appears to be an issue on blue waters. Has anyone else experiences this? Any suggestions? My charm++ builds are

./build charm++ gemini_gni-crayxe smp hugepages persistent --with-production -std=c++11

./build charm++ gemini_gni-crayxe hugepages persistent --with-production -std=c++11

Best,

Scott

# NON-SMP #

aprun -N 1 -n 1 ./pgm

 with CPU time / (points*step) (us) in us = 57.079748 

aprun -N 16 -n 16 -j 1 ./pgm

 with CPU time / (points*step) (us) in us = 71.344819 

aprun -N 32 -n 32 ./pgm

 with CPU time / (points*step) (us) in us = 105.911947 

# SMP #

aprun -N 1 -n 1 -d 31 ./pgm-smp +ppn 1 +pemap 2 +commap 0

 with CPU time / (points*step) (us) in us = 90.569283 

aprun -N 1 -n 1 -d 31 ./pgm-smp +ppn 7 +pemap 2,4,6,8,10,12,14 +commap 0

 with CPU time / (points*step) (us) in us = 166.620733 

aprun -N 1 -n 1 -d 31 ./pgm-smp +ppn 15 +pemap 2,4,6,8,10,12,14,16,18,20,22,24,26,28,30 +commap 0

 with CPU time / (points*step) (us) in us = 310.771909 

aprun -N 1 -n 1 -d 31 ./pgm-smp +ppn 30 +pemap 2-31 +commap 0

 with CPU time / (points*step) (us) in us = 529.651506 

aprun -N 2 -n 2 -d 16 ./pgm-smp +ppn 7 +pemap 2,4,6,8,10,12,14,18,20,22,24,26,28,30 +commap 0,16

 with CPU time / (points*step) (us) in us = 174.455473 

aprun -N 4 -n 4 -d 8 ./pgm-smp +ppn 3 +pemap 2,4,6,10,12,14,18,20,22,26,28,30 +commap 0,8,16,24

 with CPU time / (points*step) (us) in us = 143.428617

--
Nikhil Jain, Ph.D.
nikhil.jain AT acm.org, http://charm.cs.uiuc.edu/~nikhil