Re: RFC on io-stalls patch

[Chris Mason <mason@suse.com>]

On Thu, 2003-07-10 at 09:57, Jens Axboe wrote:
> On Tue, Jul 08 2003, Marcelo Tosatti wrote:
> > 
> > Hello people,
> > 
> > To get better IO interactivity and to fix potential SMP IO hangs (due to
> > missed wakeups) we, (Chris Mason integrated Andrea's work) added
> > "io-stalls-10" patch in 2.4.22-pre3.
> > 
> > The "low-latency" patch (which is part of io-stalls-10) seemed to be a
> > good approach to increase IO fairness. Some people (Alan, AFAIK) are a bit
> > concerned about that, though.
> > 
> > Could you guys, Stephen, Andrew and maybe Viro (if interested :)) which
> > havent been part of the discussions around the IO stalls issue take a look
> > at the patch, please?
> > 
> > It seems safe and a good approach to me, but might not be. Or have small
> > "glitches".
> 
> Well, I have one naive question. What prevents writes from eating the
> entire request pool now? In the 2.2 and earlier days, we reserved the
> last 3rd of the requests to writes. 2.4.1 and later used a split request
> list to make that same guarentee.
> 
> I only did a quick read of the patch so maybe I'm missing the new
> mechanism for this. Are we simply relying on fair (FIFO) request
> allocation and oversized queue to do its job alone?

Seems that way.  With the 2.4.21 code, a read might easily get a
request, but then spend forever waiting for a huge queue of merged
writes to get to disk.

I believe the new way provides better overall read performance in the
presence of lots of writes.

-chris