Re: [SLE] For or against ..Hyperthreading.

From: Filipe Joel Almeida (fijo.lists_at_netcabo.pt)
Date: 08/31/03

  • Next message: Richard Bos: "Re: [SLE] repost: Just installed apt4rpm on SuSE 8.2 and having problems" 
    To: suse-linux-e@suse.com
Date: Sun, 31 Aug 2003 21:52:22 +0100

    On Sun, 2003-08-31 at 15:45, Adalberto Castelo wrote:
    > There are two contexts you want to analyze this in: scientific
    > applications and non-scientific apps. SA are usually compute intensive
    > algorithms that take a task and divide it in smaller parts -- the term
    > SIMD, tho it usually refers to computer architectures, could also be used
    > here (Single Instruction stream, Multiple Data streams). So, in a common
    > parallel SA, all pieces are executing the same instructions over and over
    > again, but over diff parts of the data. This is by far the most common
    > kind of high demand sci app you'll see: weather models, finite element
    > analysis, protein folding, pattern discovery, etc. There are other types
    > of algorithms (I described the divide-and-conquer type), but they are far
    > less common.
    >
    > NonSA is a context where you have lots of different apps doing different
    > things on different data (most common) or one app doing different things
    > over the same data.
    >
    > Now, to answer your question: if you'll run on a SA context, forget about
    > HT and go get a real SMP machine. HT will slow you down. Reason: HT is
    > just a mechanism that facilitates parallel access to the computing units
    > inside the processor (integer units, floating point units, etc.). The
    > clock speed is not affected (theoretically). So, if all your threads are
    > trying to use THE SAME units, there'll be contention, and delay while a
    > thread wants for a unit to clear. These delays involve overheads, that
    > will make the app slower than in a single thread mode.
    >
    > If you'll run on the NonSA context, then HT will perhaps help you (true
    > SMP will always be better tho). Reason: mix of apps (integer based,
    > floating point based, etc.) may be using different processor units,
    > effectively allowing HT to seem like two different processors. It will of
    > course depend on the mis of apps. My gut feeling is that it would help.
    >
    > I hope that helps.
    >
    > Adalberto

    Well Adalberto, that was definitively a pretty good explanation!

    But personally I'm also curious about something else. I have asked this
    lots of times, one of them in this list, but noone has been able to give
    me a good answer.

    What are the major benefits of a 64 bit processor over a 32 bit
    processor?

    Let's say for mail or Database server... would there be significant
    benefits from having a Dual Opteron over a Dual Xeon, and stuff like
    that?

    Can anyone please shed some light on this subject? 

    -- 
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