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    SubjectRe: fair group scheduler not so fair?
    Srivatsa Vaddagiri wrote:

    > That seems to be pretty difficult to achieve with the per-cpu runqueue
    > and smpnice based load balancing approach we have now.

    Okay, thanks.

    >>Initially I tried a simple setup with three hogs all in the default "sys"
    >>group. Over multiple retries using 10-sec intervals, sometimes it gave
    >>roughly 67% for each task, other times it settled into a 100/50/50 split
    >>that remained stable over time.

    > Was this with imbalance_pct set to 105? Does it make any difference if
    > you change imbalance_pct to say 102?

    It was set to 105 initially. I later reproduced the problem with 102.
    For example, the following was with 102, with three tasks created in the
    sys class. Based on the runtime, pid 2499 has been getting a cpu all to
    itself for over a minute.

    2499 cfriesen 20 0 3800 392 336 R 99.8 0.0 1:05.85 cat
    2496 cfriesen 20 0 3800 392 336 R 50.0 0.0 0:32.95 cat
    2498 cfriesen 20 0 3800 392 336 R 50.0 0.0 0:32.97 cat

    The next run was much better, with sub-second fairness after a minute.

    2505 cfriesen 20 0 3800 392 336 R 68.2 0.0 1:00.32 cat
    2506 cfriesen 20 0 3800 392 336 R 66.9 0.0 0:59.85 cat
    2503 cfriesen 20 0 3800 392 336 R 64.2 0.0 1:00.21 cat

    The lack of predictability is disturbing, as it implies some sensitivity
    to the specific test conditions.

    >>With three groups, one task in each, I tried both 10 and 60 second
    >>intervals. The longer interval looked better but was still up to 0.8% off:
    >
    >
    > I honestly don't know if we can do better than 0.8%! In any case, I'd
    > expect that it would require more drastic changes.

    No problem. It's still far superior than the SMP performance of CKRM,
    which is what we're currently using (although heavily modified).

    Chris



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