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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 <mingo@elte.hu> wrote:

>
> * Andrew Morton <akpm@linux-foundation.org> 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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