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Build times for OpenFOAM 2.0.x code with Ubuntu 10.10 with its gcc 4.4.5

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Build times for OpenFOAM 2.0.x code with Ubuntu 10.10 with its gcc 4.4.5

Posted August 20, 2011 at 11:41 by wyldckat
Updated August 27, 2011 at 13:36 by wyldckat (Added a few notes)

This post is gathers information about OpenFOAM 2.0.x build times, using the gcc version that comes with Ubuntu 10.10, namely 4.4.5.

_____________ This was initially posted here: http://www.openfoam.com/mantisbt/view.php?id=256 _____________

Since I was curious about using N+1 processes, I've built OpenFOAM 3 times on my machine that has 8GB of DDR2 800MHz and one AMD 1055T X6, using Ubuntu 10.10 with its original gcc version:
  • 1st build with 6 processes;
  • 2nd build with 7 processes;
  • 3rd build with 6 processes again, to account for file cache on RAM.

Which resulted in the following timings:
  1. Code:
    10273.99user 372.12system 36:14.82elapsed 489%CPU (0avgtext+0avgdata 2001584maxresident)k
    227072inputs+4583896outputs (452major+127455000minor)pagefaults 0swaps
  2. Code:
    10274.53user 368.04system 36:10.27elapsed 490%CPU (0avgtext+0avgdata 2001552maxresident)k
    56inputs+4584104outputs (0major+127470259minor)pagefaults 0swaps
  3. Code:
    10209.99user 366.84system 35:49.39elapsed 492%CPU (0avgtext+0avgdata 2001536maxresident)k
    200inputs+4583880outputs (0major+127473965minor)pagefaults 0swaps
Conclusion: at least on my machine, it does not justify using N+1.

Additional timings for comparison:
  • 1 core:
    Code:
    8213.30user 298.34system 2:24:07elapsed 98%CPU (0avgtext+0avgdata 2001648maxresident)k
    218880inputs+4585496outputs (343major+127443737minor)pagefaults 0swaps
  • 2 cores:
    Code:
    8437.93user 321.37system 1:16:39elapsed 190%CPU (0avgtext+0avgdata 2001584maxresident)k
    16inputs+4584432outputs (0major+127456817minor)pagefaults 0swaps
  • 3 cores:
    Code:
    8990.99user 363.41system 56:40.62elapsed 275%CPU (0avgtext+0avgdata 2001600maxresident)k
    792inputs+4584168outputs (0major+127467703minor)pagefaults 0swaps
  • 4 cores:
    Code:
    9963.10user 382.46system 48:36.42elapsed 354%CPU (0avgtext+0avgdata 2001584maxresident)k
    8inputs+4584064outputs (0major+127476961minor)pagefaults 0swaps
  • 5 cores:
    Code:
    10105.92user 385.89system 40:57.82elapsed 426%CPU (0avgtext+0avgdata 2001424maxresident)k
    176inputs+4583944outputs (2major+127468657minor)pagefaults 0swaps
_____________ End of copy-paste-organize ________________

More conclusions:
  • Linking is mostly done in single core, therefore it's normal that the performance drops when more CPUs are added to the computation pool.
  • The second line in each timing output doesn't reflect much about what really happened.
  • Neither does the maxresident value report much about the real total memory used during build time.
Other benchmarks that might be interesting:
  • Using more than one machine.
  • Monitoring the maximum memory used with each set of cores. I already know that 4 cores need around 1.5 to 1.6 GB of RAM, but I don't know how much is needed for 6 cores.
  • Using limited RAM, to reduce file cache, which might then improve timings for using N+1.
  • Building inside a virtual machine on this very same real machine, to compare performance.
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Comments

  1. Old Comment
    So no "-j N+1" myth... I believe the story should deserve a "Busted" by MythBusters

    T
    permalink
    Posted September 3, 2011 at 10:42 by 7islands 7islands is offline
  2. Old Comment
    Hi Takuya!

    Actually, I do believe it seriously depends on the speed at which the files are accessed and/or accessible. And since modern machines + Linux OS use file cache on RAM abundantly, the necessity for N+1 is reduced to naught.

    But indeed: experiments must be conducted to reach some sort of proof, even if it can be disproved in the future with other experiments

    Best regards,
    Bruno
    permalink
    Posted September 12, 2011 at 14:59 by wyldckat wyldckat is offline
 

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