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Thread: SMP or mutiple instances (or both)

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    SMP or mutiple instances (or both)

    System is an AMD Athlon dual core, running Fedora Core 6 64 bit.

    At the moment I am running the FAH6 beta version in two separate directories with a different machine number so that each instance is running on a different core. Each instance runs with a high nice value (19) so they have low priority, and the CPU cores are loaded to about 100%.

    At the same time I can also run an instance with the SMP option and it seems to run OK - so I am processing 3 WUs simultaneously. If I stop the two processes that are running without smp, the core utilisation drops marginally - about 87% each core.

    Obviously there are a finite number of processeor cysles available, so is it more efficient to run one instance with SMP, or two instances with out - one on each core? (Or run 3 instances as I have been)
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    Re: SMP or mutiple instances (or both)

    I would expect that SMP would be the best option on its own. It is the best in terms of points per day you will only be slowing down the SMP client by running the other instances of the CPU client.

    Strange that it doesn't fully utilise the CPU though...

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    Re: SMP or mutiple instances (or both)

    1 core: uniproc
    >1 core: 1 smp instance per 4 cores

    smp f@h is locked to 4-way parallelism. due to context switches (4 processes fighting for execution on 2 cores, and the inherent problems with keeping things in cache as a result) and mpi latencies (1 process sits there waiting for data from another process) you'll never hit 100% utilization on a dual core.

    check your load average, it'll probably be 3.1ish

    and remember, these overheads are WORSE on windows (it just hides them better)

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    Re: SMP or mutiple instances (or both)

    Load average is just over 4 (with one uniprocess and 1 smp + the other stuff on the system)

    With just the SMP process (and the other stuff as above) it is about 3.5

    So it looks like two instances of a uniprocess is the better option.
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    Re: SMP or mutiple instances (or both)

    nope. you'll get MUCH higher PPD with the SMP.

    even with the lower load average, the most you'll do in terms of PPD with 2*uniproc is 600ish - versus 1700ish with SMP
    Last edited by directhex; 24-01-2008 at 10:56 PM.

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    Re: SMP or mutiple instances (or both)

    Ah - Is that because the SMP WUs carry more points, so the value of each WU is greater, even though the eficiency of preocessing is marginally lower?
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    Re: SMP or mutiple instances (or both)

    Quote Originally Posted by peterb View Post
    Ah - Is that because the SMP WUs carry more points, so the value of each WU is greater, even though the eficiency of preocessing is marginally lower?
    precisely

    GROMACS work is very hardware dependent - i.e. it's only possible to calculate certain things on certain kit. some research REQUIRES parallel GROMACS - and as a result, due to the relative rarity of the hardware, is rewarded more

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    Re: SMP or mutiple instances (or both)

    Thanks for that - I'm now looking for a break point so I can stop the two uni-processes at the end of the current WUs - it sames a shame to waste what has been done! However it keeps connecting even though I have set "Ask before connecting" to yes!
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