Re: Regression seen for patch "sched:dont decrease idle sleep avg"

[Con Kolivas <kernel@kolivas.org>]

Tim Chen writes:

> Con,
>
> As a result of the patch "sched:dont decrease idle sleep avg" 
> introduced after 2.6.15, there is a 4% drop in Volanomark 
> throughput on our Itanium test machine.  
> Probably the following happened:
> Compared to previous code, this patch slightly increases 
> the the priority boost when a job is woken up.
> This adds priority spread and variations to the wait time of jobs
> on run queue if we have a lot of similar jobs in the system.
>
> See patch:
> http://www.kernel.org/git/?
> p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=e72ff0bb2c163eb13014ba113701bd42dab382fe 

Lovely

This patch corrects a bug in the original code which unintentionally dropped 
the priority of tasks that were idle but were already high priority on other 
merits. It doesn't further increase the priority. The 4% almost certainly is 
due to the lack of any locking in the threading model used by the java 
virtual machine on volanomark and it being pure luck that penalising 
particularly idle tasks previously improved the wakeup timing of basically 
yielding dependant threads. This patch did fix bugs related to interactive 
yet idle tasks like consoles misbehaving. The fact that the presence of that 
particular bug improved a multithreaded benchmark that uses such a threading 
model is pure chance and (obviously) not design. I wouldn't like to see this 
bug reintroduced on the basis of this benchmark result.

---
-ck

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