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    SubjectRe: [tbench regression fixes]: digging out smelly deadmen.
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    On Sat, 2008-10-25 at 00:24 -0700, David Miller wrote:
    > From: Mike Galbraith <efault@gmx.de>
    > Date: Sat, 25 Oct 2008 08:53:43 +0200
    >
    > > On Sat, 2008-10-25 at 07:58 +0200, Mike Galbraith wrote:
    > > 2.6.24.7-up
    > > ring-test - 1.100 us/cycle = 909 KHz (gcc-4.1)
    > > ring-test - 1.068 us/cycle = 936 KHz (gcc-4.3)
    > > netperf - 122300.66 rr/s = 244 KHz sb 280 KHz / 140039.03 rr/s
    > > tbench - 341.523 MB/sec
    > >
    > > 2.6.25.17-up
    > > ring-test - 1.163 us/cycle = 859 KHz (gcc-4.1)
    > > ring-test - 1.129 us/cycle = 885 KHz (gcc-4.3)
    > > netperf - 132102.70 rr/s = 264 KHz sb 275 KHz / 137627.30 rr/s
    > > tbench - 361.71 MB/sec
    > >
    > > ..in 25, something happened that dropped my max context switch rate from
    > > ~930 KHz to ~885 KHz. Maybe I'll have better luck trying to find that.
    > > Added to to-do list. Benchmark mysteries I'm going to have to leave
    > > alone, they've kicked my little butt quite thoroughly ;-)
    >
    > But note that tbench performance improved a bit in 2.6.25.

    Yeah, netperf too.

    > In my tests I noticed a similar effect, but from 2.6.23 to 2.6.24,
    > weird.

    23->24 I can understand. In my testing, 23 CFS was not a wonderful
    experience for rapid switchers. 24 is cfs-24.1.

    > Just for the public record here are the numbers I got in my testing.
    > Each entry was run purely on the latest 2.6.X-stable tree for each
    > release. First is the tbench score and then there are 40 numbers
    > which are sparc64 cpu cycle counts of default_wake_function().

    Your numbers seem to ~agree with mine. And yeah, that hrtick is damned
    expensive. I didn't realize _how_ expensive until I trimmed my config
    way way down from distro. Just having highres timers enabled makes a
    very large difference here, even without hrtick enabled, and with the
    overhead of a disabled hrtick removed.

    -Mike



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