Re: [ext3][kernels >= 2.6.20.7 at least] KDE going comatose when FS is under heavy write load (massive starvation)

Mike Galbraith
2007-05-02T07:36:52Z

On Wed, 2007-05-02 at 08:53 +0200, Jens Axboe wrote:
> On Fri, Apr 27 2007, Linus Torvalds wrote:
> > So I do believe that we could probably do something about the IO
> > scheduling _too_:
> >
> > - break up large write requests (yeah, it will make for worse IO
> > throughput, but if make it configurable, and especially with
> > controllers that don't have insane overheads per command, the
> > difference between 128kB requests and 16MB requests is probably not
> > really even noticeable - SCSI things with large per-command overheads
> > are just stupid)
> >
> > Generating huge requests will automatically mean that they are
> > "unbreakable" from an IO scheduler perspective, so it's bad for latency
> > for other reqeusts once they've started.
>
> Overlooked this one initially... We actually don't generate huge
> requests, exactly because of that. Even if the device can do large
> requests (most SATA disks today can do 32meg), we default to 512kB as
> the largest one that we will build due to file system requests. It's
> trivial to reduce that limit, see /sys/block/<dev>/queue/max_sectors_kb.
> That controls the maximum per-request size.

For the record, I haven't been able to stall KDE for ages with
data=writeback.

-Mike