Re: [patch] CFS scheduler, -v7

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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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