Messages in this thread | ![/](/images/icornerl.gif) | | Subject | Re: HT schedulers' performance on single HT processor | From | Nathan Fredrickson <> | Date | Sun, 14 Dec 2003 16:15:40 -0500 |
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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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