Re: RFC on io-stalls patch

[Chris Mason <mason@suse.com>]

On Sun, 2003-07-13 at 05:01, Jens Axboe wrote:
> On Sat, Jul 12 2003, Chris Mason wrote:
> > On Sat, 2003-07-12 at 03:37, Jens Axboe wrote:
> > 
> > > > 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.
> > 
> > If we don't apply oversized checks to reads, what keeps a big streaming
> > reader from starving out all the writes?
> 
> It's just so much easier to full the queue with writes than with reads.
> 

Well, I'd say it's a more common problem to have lots of writes, but it
is pretty easy to fill the queue with reads.

> > The current patch provides a relatively fixed amount of work to get a
> > request, and I don't think we should allow that to be bypassed.  We
> > might want to add a special case for synchronous reads (like bread), via
> > a b_state bit that tells the block layer an immediate unplug is coming
> > soon.  That way the block layer can ignore the oversized checks, grant a
> > request and unplug right away, hopefully lowering the total number of
> > unplugs the synchronous reader has to wait through.
> > 
> > Anyway, if you've got doubts about the current patch, I'd be happy to
> > run a specific benchmark you think will show the bad read
> > characteristics.
> 
> No I don't have anything specific, it just seems like a bad heuristic to
> get rid of. I can try and do some testing tomorrow. I do feel strongly
> that we should at least make sure to reserve a few requests for reads
> exclusively, even if you don't agree with the oversized check. Anything
> else really contradicts all the io testing we have done the past years
> that shows how important it is to get a read in ASAP. And doing that in
> the middle of 2.4.22-pre is a mistake imo, if you don't have numbers to
> show that it doesn't matter for the quick service of reads.

I believe elevator-lowlatency tries to solve the 'get a read in ASAP'
from a different direction, by trying to limit both the time required to
get a request and the time for required for the unplug to run.  Most of
my numbers so far have been timing reads in the face of lots of writes,
where elevator-lowlatency is a big win.

It should make sense to have a reserve of requests for reads, but I
think this should be limited to the synchronous reads.  Still, I haven't
been playing with all of this for very long, so your ideas are much
appreciated.

-chris