linux-kernel - Re: nice and hyperthreading on atom

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Message-ID: <1220725479644@dmwebmail.dmwebmail.chezphil.org>
Date:	Sat, 06 Sep 2008 19:24:39 +0100
From:	"Phil Endecott" <phil_wueww_endecott@...zphil.org>
To:	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: nice and hyperthreading on atom

Arjan van de Ven writes:
>> Phil Endecott wrote:
>> Dear Experts,
>>
>> I have an ASUS Eee with an Atom processor, which has hyperthreading 
>> enabled.  If I have two processes, one nice and the other normal, they 
>> each get 50% of the CPU time.  Of course this is what you'd expect if 
>> the scheduler didn't understand that the two virtual processors are not 
>> really independent.  I'd like to fix it.
>
> but you cannot imfluence the cpu's scheduling of the instructions.
> 
> As an OS one COULD decide to just not schedule the nice task at all,
> but then, especially on atom where HT has a high efficiency, your cpu
> is mostly idle ...

Here's how I imagine it: say I have one regular task and one "nice -9" 
task.  On a conventional uniprocessor system they would get about 90% 
and 10% of the CPU respectively.  On the hyperthreadng system they 
currently get equal shares; except that the CPU is more efficient with 
two threads running, so you could perhaps say that they get 60% each or 
something like that.  But 60% is still less than 90%, and I don't want 
my foreground interactive task being slowed down that much by this 
niced task.  So I envisage the system spending 20% of its time running 
both tasks and the remaining 80% of the time running just the 
higher-priority task.  That way, I get half of 20% = 10% spent on the 
nice task and half of 20% plus 80% = 90% spent on the foreground task.  
(Or maybe something like 12% + 92%, allowing for the hyperthreading efficiency.)

Here's a link to Con Kolivas' post where he described something like 
this back in 2004:

   http://thread.gmane.org/gmane.linux.kernel/178090/focus=178882

Phil.

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