Re: Putrid Elevator Behavior 2.4.18/19

Jeff V. Merkey (
Wed, 27 Mar 2002 16:20:10 -0700

On Wed, Mar 27, 2002 at 08:03:25AM +0100, Jens Axboe wrote:
> On Tue, Mar 26 2002, Jeff V. Merkey wrote:
> > >
> > > That's good news.
> > >
> > > Are you still working on the A/B list patch? I'd imagine that it could make
> > > several problems easier to fix in the block layer.
> > >
> >
> > Yes. I am asking Darren Major, who wrote the A/B implementation
> > in NetWare to review the patch before we submit it. It may affect
> > some drivers. We are verifying that the change I instrumented
> > will not break anything.
> I'm curious how you are doing this cleanly in 2.4. There are lots of
> places in the kernel that do direct list management on the queue_head.
> Are you adding two separate hidden lists and splicing content to the
> queue_head?

Correct. I am still reviewing drivers and kernel code to ascertain
whether I am not leaving any holes. I have spliced it as a non-obtrusive
implementation that preserves the existing code with no changes.

Some of the drivers may have problems if they cache the head address
of the current list.

> 2.5 has this done much more cleanly (of course I'm very biased). See the
> deadline I/O scheduler patch I've posted before, stuff like this can be
> done a lot cleaner there. Internal I/O scheduler structures are
> completely hidden from drivers.

2.5 would be nice, but 2.4.X needs it too and this is the kernel we are
using for our development and testing, so we will need it there.


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