Re: Status of the IO scheduler fixes for 2.4

From: Marcelo Tosatti (
Date: Fri Jul 04 2003 - 15:01:54 EST

On Wed, 2 Jul 2003, Chris Mason wrote:

> On Wed, 2003-07-02 at 18:28, Marcelo Tosatti wrote:
> > On Wed, 2 Jul 2003, Marcelo Tosatti wrote:
> >
> > >
> > > Hello people,
> > >
> > > What is the status of the IO scheduler fixes for increased fairness for
> > > 2.4 ?
> > >
> > > I haven't had time to read and think about everything you guys discussed,
> > > so a brief summary would be very helpful for me.
> > >
> > > Danke
> >
> > Ah, we all want that the fairness issues to be fixed in 2.4.22, right ?
> My current code is attached, it's basically a merge of these 3 patches,
> with modifications based on benchmarks and latency measurements here.
> fix_pausing: From Andrea, it fixes a few corner case races where
> wakeups can be missed in wait_on_buffer, wait_on_page, and
> __get_request_wait.
> elevator-low-latency: From Andrea, it keeps the amount of io on a given
> queue to a reasonable number. This prevents a small number of huge
> requests from introducing large latencies on smaller requests.
> q->full: From Nick, it reduces latency in __get_request_wait by making
> sure new io can't come in and steal requests before old waiters are
> served.
> Those represent the big 3 areas I believe the latencies are coming
> from. The q->full patch can hurt throughput badly as the number of
> writers increases (50% of what 2.4.21 gets for 10 or more concurrent
> streaming writers), but it really seems to help desktop workloads here.


Would you please separate those tree fixes in separate diffs?

For me it seems low latency and fix-pausing patches should be enough for
"better" IO fairness. I might be wrong about that, though.

Lets try this: Include elevator-low-latency in -pre3 (which I'm trying to
release today), then fix pausing in -pre4. If the IO fairness still doesnt
get somewhat better for general use (well get isolated user reports and
benchmarks for both the two patches), then I might consider the q->full
patch (it has throughtput drawbacks and I prefer avoiding a tunable

Sounds good?
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