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    SubjectRe: [patch] Give kjournald a IOPRIO_CLASS_RT io priority
    On Mon, 22 Oct 2007 11:40:14 +0200 Ingo Molnar <> wrote:

    > * Andrew Morton <> wrote:
    > > > so lets just goddamn apply this _trivial_ patch. This isnt an
    > > > intrusive 1000 line rewrite that is hard to revert. If it causes any
    > > > bandwidth problems, it will be just as trivial to undo. If we do
    > > > anything else we just stiffle the still young and very much
    > > > under-represented "lets fix latencies that bothers people" movement.
    > > > If anything we need _positive_ discrimination for latency related
    > > > fixes (which treatment this fix does not need at all - all it needs
    > > > is _equal_ footing with the countless bandwidth patches that go into
    > > > the kernel all the time), otherwise it will never take off and
    > > > become as healthy as bandwidth optimizations. Ok?
    > >
    > > I think the situation is that we've asked for some additional
    > > what-can-be-hurt-by-this testing.
    > >
    > > Yes, we could sling it out there and wait for the reports. But often
    > > that's a pretty painful process and regressions can be discovered too
    > > late for us to do anything about them.
    > reverting this oneliner is trivial. Finding bandwidth problems and
    > tracking them down to this oneliner change is relatively easy too.
    > Finding latency problems and fixing them is _not_ trivial.
    > Boot up a Linux desktop and start OOo or firefox, and measure the time
    > it takes to start the app up. 10-20 seconds on a top-of-the-line
    > quad-core 3.2 GHz system - which is a shame. Same box can do in excess
    > of 1GB/sec block IO. Yes, one could blame the apps but in reality most
    > of the blame is mostly on the kernel side. We do not make bloat and
    > latency suckage apparent enough to user-space (due to lack of
    > intelligent instrumentation), we make latencies hard to fix, we have an
    > acceptance bias towards bandwidth fixes (because they are easier to
    > measure and justify) - and that's all what it takes to let such a
    > situation get out of control.
    > and i can bring up the scheduler as an example. CFS broke the bandwidth
    > performance of one particular app and it took only a few days to get it
    > back under control. But it was months to get good latency behavior out
    > of the scheduler. And that is with the help of excellent scheduler
    > instrumentation. In the IO space the latency situation is much, much
    > worse. Really.

    None of which is an argument for simply not bothering to do a bit more
    developer testing before merging.

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