linux-kernel - Re: fair group scheduler not so fair?

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Message-ID: <20080523074431.GJ3780@linux.vnet.ibm.com>
Date:	Fri, 23 May 2008 13:14:31 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	"Chris Friesen" <cfriesen@...tel.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Li, Tong N" <tong.n.li@...el.com>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, pj@....com
Subject: Re: fair group scheduler not so fair?

On Thu, May 22, 2008 at 06:17:37PM -0600, Chris Friesen wrote:
> Peter Zijlstra wrote:
>
>> Given the following:
>>       root
>>      / | \
>>    _A_ 1  2
>>   /| |\
>>  3 4 5 B
>>       / \
>>      6   7
>>      CPU0            CPU1
>>      root            root
>>      /  \            /  \
>>     A    1          A    2
>>    / \             / \
>>   4   B           3   5
>>      / \
>>     6   7
>
> How do you move specific groups to different cpus.  Is this simply using 
> cpusets?

No. Moving groups to different cpus is just a group-aware extension to
move_tasks() that is invoked as part of regular load balance operation.
move_tasks()->sched_fair_class.load_balance() has been modified to
understand how much various task-groups at various levels (ex: A at level 1,
B at level 2 etc) contribute to cpu load. It moves tasks between cpus
using this knowledge.

For ex: if we were to consider all tasks shown above to be in same cpu,
CPU0, this is how it would look:

	CPU0		CPU1
       root		root
      / | \
     A  1  2
   /| |\
  3 4 5 B
       / \
      6   7

Then cpu0 load = weight of A + weight of 1 + weight of 2
	       = 1024 + 1024 + 1024 = 3072

while cpu1 load = 0

load to be moved to cut down this imbalance = 3072/2 = 1536

move_tasks() running on CPU1 would try to pull iteratively tasks such
that total weight moved is <= 1536.

	Task moved		Total Weight moved
	---------		------------
	    2			     1024
	    3			     1024 + 256 = 1280
	    5			     1280 + 256 = 1536

resulting in:

      CPU0            CPU1
      root            root
      /  \            /  \
     A    1          A    2
    / \             / \
   4   B           3   5
      / \
     6   7

>> Numerical examples given the above scenario, assuming every body's
>> weight is 1024:
>
>>  s_(0,A) = s_(1,A) = 512
>
> Just to make sure I understand what's going on...this is half of 1024 
> because it shows up on both cpus?

not exactly ..as Peter put it:

  s_(i,g) = W_g * rw_(i,g) / \Sum_j rw_(j,g)

In this case, 

  s_(0,A) = W_A * rw_(0, A) / \Sum_j rw_(j, A)

W_A = shares given to A by admin = 1024

rw_(0,A) = Weight of 4 + Weight of B = 1024 + 1024 = 2048
rw_(1,A) = Weight of 3 + Weight of 5 = 1024 + 1024 = 2048
\Sum_j rw_(j, A) = 4096

So,

  s_(0,A) = 1024 *2048 / 4096 = 512


>>  s_(0,B) = 1024, s_(1,B) = 0
>
> This gets the full 1024 because it's only on one cpu.

Not exactly. rw_(0, B) = \Sum_j rw_(j, B) and that's why s_(0,B) = 1024

>>  rw_(0,A) = rw(1,A) = 2048
>>  rw_(0,B) = 2048, rw_(1,B) = 0
>
> How do we get 2048?  Shouldn't this be 1024?

Hope this is clarified from above.

>>  h_load_(0,A) = h_load_(1,A) = 512
>>  h_load_(0,B) = 256, h_load(1,B) = 0
>
> At this point the numbers make sense, but I'm not sure how the formula for 
> h_load_ works given that I'm not sure what's going on for rw_.

-- 
Regards,
vatsa
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