Re: HT and idle = poll

["Martin J. Bligh" <mbligh@aracnet.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.

BTW, could someone give a brief summary of why idle=poll is needed for 
oprofile, I'd love to add it do the "documentation for dummies" file I
was writing.

M.