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    SubjectRe: HT schedulers' performance on single HT processor
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    On Sun, 2003-12-14 at 15:35, Adam Kropelin wrote:
    > On Sun, Dec 14, 2003 at 02:49:24PM -0500, Nathan Fredrickson wrote:
    > > Same table as above normalized to the j=1 uniproc case to make
    > > comparisons easier. Lower is still better.
    > >
    > > j = 1 2 3 4 8
    > > 1phys (uniproc) 1.00 1.00 1.00 1.00 1.00
    > > 1phys w/HT 1.02 1.02 0.87 0.87 0.87
    > > 1phys w/HT (w26) 1.02 1.02 0.87 0.87 0.88
    > > 1phys w/HT (C1) 1.03 1.02 0.88 0.88 0.88
    > > 2phys 1.00 1.00 0.53 0.53 0.53
    > ^^^^^ ^^^^
    >
    > Ummm...
    >
    > This is mighty suspicious. With -j2 did you check to see that there
    > were indeed two parallel gcc's running? Since -test6 I've found that
    > -j2 only results in a single gcc instance. I've seen this on both an
    > old hacked-up RH 7.3 installation and a brand new RH 9 + updates
    > installation.

    I just checked and you're right, the number of compilers that actually
    run is j-1, for all j>1. I assume this is a problem with the parallel
    build process, but it does not invalidate these results for comparing
    the scheduler performance with different patches.
    >
    > > This suggests that j should be set to at least the number of logical
    > > processors + 1.
    >
    > Since -test6 I've found this to be the case for kernel builds, yes. But
    > I don't think it has anything to do with the scheduler or HT vs SMP
    > platforms.

    The 1-3% performance loss when HT is enabled for -j1 is still very real.

    Nathan



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