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    SubjectRe: dynamic-hz
    On Mon, Dec 13, 2004 at 12:22:29PM +0100, Pavel Machek wrote:
    > I tried defining HZ to 10 once, and there are some #if arrays in the
    > kernel that prevented me from doing that.

    I guess you're right and the minimum is HZ=12. I'm pretty sure I could
    go down to 25, perhaps the absolute minium was 12 and not 10.

    There's also some side effect like this by setting strange HZ:

    --- x-ref/net/sched/estimator.c 2003-03-15 03:25:19.000000000 +0100
    +++ x/net/sched/estimator.c 2004-05-31 15:51:42.778909936 +0200
    @@ -71,10 +71,6 @@
    at user level painlessly.

    -#if (HZ%4) != 0
    -#error Bad HZ value.
    #define EST_MAX_INTERVAL 5

    struct qdisc_estimator
    @@ -136,6 +132,9 @@ int qdisc_new_estimator(struct tc_stats
    struct qdisc_estimator *est;
    struct tc_estimator *parm = RTA_DATA(opt);

    + if (unlikely(HZ % 4))
    + return -EINVAL;
    if (RTA_PAYLOAD(opt) < sizeof(*parm))
    return -EINVAL;

    If you boot with an HZ not divisible by 4 you get -EINVAL at runtime
    (instead of a compile failure since we can't check it at compile time
    anymore ;).

    Anyway the major point of the patch is to get HZ switchable from 100 to
    1000, those two values are really the only supported ones. The rest is a
    bonus, and I'm sure at least 50 and 2000 will work flawlessy too.
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