linux-kernel - Re: [PATCH 10/10] ksm: change ksm nice level to be 5

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Message-ID: <20090720121450.GG24157@balbir.in.ibm.com>
Date:	Mon, 20 Jul 2009 17:44:50 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Izik Eidus <ieidus@...hat.com>
Cc:	akpm@...ux-foundation.org, hugh.dickins@...cali.co.uk,
	aarcange@...hat.com, chrisw@...hat.com, avi@...hat.com,
	riel@...hat.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	nickpiggin@...oo.com.au
Subject: Re: [PATCH 10/10] ksm: change ksm nice level to be 5

* Izik Eidus <ieidus@...hat.com> [2009-07-20 14:48:04]:

> Balbir Singh wrote:
>> * Izik Eidus <ieidus@...hat.com> [2009-07-17 20:30:50]:
>>
>>   
>>> From: Izik Eidus <ieidus@...hat.com>
>>>
>>> ksm should try not to disturb other tasks as much as possible.
>>>
>>> Signed-off-by: Izik Eidus <ieidus@...hat.com>
>>> ---
>>>  mm/ksm.c |    2 +-
>>>  1 files changed, 1 insertions(+), 1 deletions(-)
>>>
>>> diff --git a/mm/ksm.c b/mm/ksm.c
>>> index 75d7802..4afe345 100644
>>> --- a/mm/ksm.c
>>> +++ b/mm/ksm.c
>>> @@ -1270,7 +1270,7 @@ static void ksm_do_scan(unsigned int scan_npages)
>>>
>>>  static int ksm_scan_thread(void *nothing)
>>>  {
>>> -	set_user_nice(current, 0);
>>> +	set_user_nice(current, 5);
>>>     
>>
>> Is the 5 arbitrary? Why not +19? What is the intention of this change
>> - to run when no other task is ready to run?
>>   
>
> Hey Balbir,
>
> I thought about giving it the lowest priority of nice before I did this  
> patch, but then I came into understanding that it isn't right,
> Although ksm should not distrub other tasks while they are running, it  
> does need to run while they are running,
> most of the use cases for ksm is to find identical pages in real time  
> while they are changing in the application, so giving it the lowest  
> priority doesn't seems right to me,
>
> But my understanding of how the nice prioritys are working is just my  
> intuition, so if you know better and think that for the use case i  
> described above other nice priority is better fit, tell me and I wont  
> have any problem to change.
>
>


I was just checking to see why 5? May be it might be a good idea to
document or at-least have rules on what priorities kernel threads can
take? 

>>   
>>>  	while (!kthread_should_stop()) {
>>>  		if (ksm_run & KSM_RUN_MERGE) {
>>>     
>>
>>   
>

-- 
	Balbir
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