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    SubjectRe: Hyperthreading performance oddities
    Hi Andi,

    On Fri, Mar 07, 2008 at 08:20:32PM +0100, Andi Kleen wrote:
    > Chris Snook <> writes:
    > >
    > > Turning on hyperthreading effectively halves the amount of cache
    > > available for each logical CPU when both are doing work, which can do
    > > more harm than good.
    > When the two cores are in the same address space (as in being two
    > threads of the same process) L1 cache will be shared on P4. I think
    > for the other cases the cache management is also a little more
    > sophisticated than a simple split, depending on which HT generation
    > you're talking about (Intel had at least 4 generations out, each with
    > improvements over the earlier ones)

    Oh that's quite interesting to know.

    > BTW your argument would be in theory true also for multi core with
    > shared L2 or L3, but even there the CPUs tend to be more sophisticated.
    > e.g. Core2 has a mechanism called "adaptive cache" which allows one
    > Core to use significantly more of the L2 in some cases.
    > > Number-crunching applications that utilize the
    > > cache effectively generally don't benefit from hyperthreading,
    > > particularly floating-point-intensive ones.
    > That sounds like a far too broad over generalization to me.
    > -Andi (who personally always liked HT)

    Well, in my experience, except for compiling, HT has always caused
    massive slowdowns, especially on network-intensive applications.
    Basically, network perf took a 20-30% hit, while compiling took
    20-30% boost. But I must admit that I never tried HT on anything
    more recent than a P4, maybe things have changed since.


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