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    SubjectRe: #define HZ 1024 -- negative effects?
    When you change the #define HZ setting in param.h, what effect does that
    have on the CLOCKS_PER_SEC? Are you really going to get a different amount
    of slice time or is the is there another kernel source file (timex.h) that
    just puts you back anyway?

    ----- Original Message -----
    From: "Mike Galbraith" <>
    To: "linux-kernel" <>
    Sent: Friday, April 27, 2001 6:04 AM
    Subject: Re: #define HZ 1024 -- negative effects?

    > > > I have not tried it, but I would think that setting HZ to 1024
    > > > should make a big improvement in responsiveness.
    > > >
    > > > Currently, the time slice allocated to a standard Linux
    > > > process is 5*HZ, or 50ms when HZ is 100. That means that you
    > > > will notice keystrokes being echoed slowly in X when you have
    > > > just one or two running processes,
    > >
    > > Rubbish. Whenever a higher-priority thread than the current
    > > thread becomes runnable the current thread will get preempted,
    > > regardless of whether its timeslices is over or not.
    > (hmm.. noone mentioned this, and it doesn't look like anyone is
    > going to volunteer to be my proxy [see ionut's .sig]. oh well)
    > What about SCHED_YIELD and allocating during vm stress times?
    > Say you have only two tasks. One is the gui and is allocating,
    > the other is a pure compute task. The compute task doesn't do
    > anything which will cause preemtion except use up it's slice.
    > The gui may yield the cpu but the compute job never will.
    > (The gui won't _become_ runnable if that matters. It's marked
    > as running, has yielded it's remaining slice and went to sleep..
    > with it's eyes open;)
    > Since increasing HZ reduces timeslice, the maximum amount of time
    > that you can yield is also decreased. In the above case, isn't
    > it true that changing HZ from 100 to 1000 decreases sleep time
    > for the yielder from 50ms to 5ms if the compute task is at the
    > start of it's slice when the gui yields?
    > It seems likely that even if you're running a normal mix of tasks,
    > that the gui, big fat oinker that the things tend to be, will yield
    > much more often than the slimmer tasks it's competing with for cpu
    > because it's likely allocating/yielding much more often.
    > It follows that increasing HZ must decrease latency for the gui if
    > there's any vm stress.. and that's the time that gui responsivness
    > complaints usually refer to. Throughput for yielding tasks should
    > also increase with a larger HZ value because the number of yields
    > is constant (tied to the number of allocations) but the amount of
    > cpu time lost per yield is smaller.
    > Correct?
    > (if big fat tasks _don't_ generally allocate more than slim tasks,
    > my refering to ionuts .sig was most unfortunate. i hope it's safe
    > to assume that you can't become that obese without eating a lot;)
    > -Mike
    > -
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