Re: HT schedulers' performance on single HT processor

[Adam Kropelin <akropel1@rochester.rr.com>]

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...

> 2phys w/HT       1.01  1.01  0.64  0.50  0.48
> 2phys w/HT (w26) 1.02  1.01  0.55  0.49  0.47
> 2phys w/HT (C1)  1.02  1.01  0.53  0.50  0.48

> There was not much benefit from either HT or SMP with j=2.  Maximum
> speedup was not realized until j=3 for one physical processor and j=5
> for 2 physical processors.

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.

> 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.

--Adam