Turn off hyperthreading on dual core Atom?

Jerome Herman jherman at dichotomia.fr
Mon Aug 29 21:36:57 UTC 2011

On 29/08/2011 23:15, Brett Glass wrote:
> At 01:55 PM 8/29/2011, Bruce Cran wrote:
>> Actually, the ULE scheduler does know about HyperThreading and the 
>> topology of such CPUs. I don't know what it does with the 
>> information, but it probably works to optimize cache usage etc.
> Alas, during a recent kernel build, I used the -j2 command line option 
> in "make" and watched as the scheduler repeatedly assigned two 
> instances of cc (the most CPU-intensive program) to the same core.
I might be wrong, but that would be the result I expect if I were to 
pass -j2 to a dual core CPU. If I wanted the the compilation to run on 
both core I would use -j3. The good old "number of cores+1".
I think the last compilation "slot" is used to prepare the next 
compile/do trivial compile, so that  the cores dedicated to compile can 
switch from one task to the next faster.

The interesting test to do would be if you do -j3 would ULE assign the 
second compile to HT or to second core.

> During that process, I also watched CPU utilization in top(1). The 
> peak was 46% idle, which means that HTT appeared to be making at most 
> a 4% difference. (If the peak were 50% idle, HTT would be doing 
> nothing at all, because top(1) can't tell that there aren't really 4 
> CPUs.)

H.T varies greatly from one processor to the next, on the Pentium 4 1st 
gen it is a sad joke, on the Atom it is a small help for easy tasks, on 
i7 it is almost as good as a real core (Translation : if you use only 
the HT of a core you will get roughly the same perfs as if you use only 
the direct core)
None the less H.T is just another entry point on the same core, so if HT 
is not used at all you will have 100% CPU power on direct, if direct is 
not used at all you will have 30%-95% cpu power on HT (depending on the 
processor). If both are used at he same time you will get between 
45%+20% (P4 1st gen) to 50%+45% (i7 last gen).
But since raw CPU power is not the only thing that matters in real world 
you can get up to +25% perfs on a I7 with HT enabled.

> --Brett Glass
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