linux-kernel - Re: [patch] CFS scheduler, -v7

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Date:	Thu, 03 May 2007 12:00:52 -0400
From:	Ting Yang <tingy@...umass.edu>
To:	Ingo Molnar <mingo@...e.hu>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [patch] CFS scheduler, -v7

Hi, Ingo

    I wrote that email in a hurry, therefore might not explain the 
problem clearly. However I do think there is a problem for this part, 
after I carefully read the code again. Now I want to try again :-) 
Hopefully, this time I will do a right job.

Starting from the following code:

+    if (__delta > niced_granularity(rq, curr, granularity))
+        resched_task(curr);

Suppose, "curr" has nice value -10, then curr->load_shift = 15.  
Granularity passed into this function is
fixed 2,000,000 (for CFS -v8). Let's just divide everything by 1,000,000 
for simplicity, say granularity used is 2.

Now, we look at how granularity is rescaled:

+    int load_shift = p->load_shift;
+
+    if (load_shift == SCHED_LOAD_SHIFT)
+        return value;
+
+    return (value << load_shift) >> SCHED_LOAD_SHIFT;

it returns (2  << 15) >> 10 = 2 * 32 = 64, therefore __delta has to be 
larger than 64 so that the current process can be preempted.

Suppose, "curr" executes for 1 tick, an timer interrupts comes. It 
executes about 1,000,000 (roughly speaking, since timer interrupts come 
1000/second). Since we divided everything by 1,000,000, it becomes 1 in 
this discussion. After this execution, how much will "curr" increments 
its fair_key?
It is weighted: 1/32.

then how much time is needed for "curr" to build a 2 * 32 difference on 
fair_key, with every 1 ms it updates fair_key by 1/32 ?    2 * 32 * 32 !
On the other hand, for a task  has nice value 1, the amount work needed 
to preemption is 2 * 1 *1.
If we have only 2 task running, p1 with nice value -10, p2 with nice 
value 0.
           p1 get cup share:  (32 * 32) / (32 * 32 + 1 *1)
           p2 get cpu share:  ( 1* 1) / (32 * 32 + 1 * 1)

I do see a quadratic effect here. Did I missed anything? sorry to bother 
you again, I just want to help :-)

Thanks a lot !

Ting

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