Re: Status of the IO scheduler fixes for 2.4

[Chris Mason <mason@suse.com>]

On Fri, 2003-07-04 at 20:05, Andrea Arcangeli wrote:
> On Fri, Jul 04, 2003 at 05:37:35PM -0400, Chris Mason wrote:
> > I've also attached a patch I've been working on to solve the latencies a
> > different way.  bdflush-progress.diff changes balance_dirty to wait on
> > bdflush instead of trying to start io.  It helps a lot here (both
> > throughput and latency) but hasn't yet been tested on a box with tons of
> > drives.
> 
> that's orthogonal, it changes the write throttling, it doesn't touch the
> blkdev layer like the other patches does. So if it helps it solves a
> different kind of latencies.

It's also almost useless without elevator-low-latency applied ;-)  One
major source of our latencies is a bunch of procs hammering on
__get_request_wait, so bdflush-progess helps by reducing the number of
procs doing io.  It does push some of the latency problem up higher into
balance_dirty, but I believe it is easier to manage there.

bdflush-progress doesn't help at all for non-async workloads.

> However the implementation in theory can run the box oom, since it won't
> limit the dirty buffers anymore. To be safe you need to wait 2
> generations. I doubt in practice it would be easily reproducible though ;).

Hmmm, I think the critical part here is to write some buffers after
marking a buffer dirty.  We don't check the generation number until
after marking the buffer dirty, so if the generation has incremented at
all we've made progress.  What am I missing?

-chris