Re: HT and idle = poll

[torvalds@transmeta.com (Linus Torvalds)]

In article <200303052318.04647.habanero@us.ibm.com>,
Andrew Theurer  <habanero@us.ibm.com> wrote:
>The test:  kernbench (average of  kernel compiles5) with -j2 on a 2 physical/4 
>logical P4 system.  This is on 2.5.64-HTschedB3:
>
>idle != poll: Elapsed: 136.692s User: 249.846s System: 30.596s CPU: 204.8%
>idle  = poll: Elapsed: 161.868s User: 295.738s System: 32.966s CPU: 202.6%
>
>A 15.5% increase in compile times.
>
>So, don't use idle=poll with HT when you know your workload has idle time!  I 
>have not tried oprofile, but it stands to reason that this would be a 
>problem.  There's no point in using idle=poll with oprofile and HT anyway, as 
>the cpu utilization is totally wrong with HT to begin with (more on that 
>later).
>
>Presumably a logical cpu polling while idle uses too many cpu resources 
>unnecessarily and significantly affects the performance of its sibling. 

Btw, I think this is exactly what the new HT prescott instructions are
for: instead of having busy loops polling for a change in memory (be it
a spinlock or a "need_resched" flag), new HT CPU's will support a
"mwait" instruction. 

But yes, at least for now, I really don't think you should really _ever_
use "idle=poll" on HT-enabled hardware. The idle CPU's will just suck
cycles from the real work.

		Linus