Re: RFC on io-stalls patch

[Jens Axboe]

On Fri, Jul 11 2003, Chris Mason wrote:
> 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.

Correct

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

I fail to see the logic in that. Reads are now treated fairly wrt
writes, but it would be really easy to let writes consume the entire
capacity of the queue (be it all the requests, or just going oversized).

I think the oversized logic is flawed right now, and should only apply
to writes. Always let reads get through. And don't let writes consume
the last 1/8th of the requests, or something like that at least. I'll
try and do a patch for pre4.

-- 
Jens Axboe