Re: Time sliced CFQ io scheduler

Andrew Morton
2004-12-08T02:26:19Z

Andrea Arcangeli <andrea@suse.de> wrote:
>
> On Tue, Dec 07, 2004 at 06:00:33PM -0800, Andrew Morton wrote:
> > untuned SCSI benchmark results without realising that. If a distro is
> > always selecting CFQ then they've probably gone and deoptimised all their
> > IDE users.
>
> The enterprise distro definitely shouldn't use "as" by default: database
> apps _must_ not use AS, they've to use either CFQ or deadline. CFQ is
> definitely the best for enterprise distros. This is a tangible result,
> SCSI/IDE doesn't matter at all (and keep in mind they use O_DIRECT a
> lot, so such 64kib Jens found would be a showstopper for a enterprise
> release, slelecting something different than "as" is a _must_ for
> enterprise distro).

That's a missing hint in the direct-io code. This fixes it up:

--- 25/fs/direct-io.c~a 2004-12-07 18:12:25.491602512 -0800
+++ 25-akpm/fs/direct-io.c 2004-12-07 18:13:13.661279608 -0800
@@ -1161,6 +1161,8 @@ __blockdev_direct_IO(int rw, struct kioc
struct dio *dio;
int reader_with_isem = (rw == READ && dio_lock_type == DIO_OWN_LOCKING);

+ current->flags |= PF_SYNCWRITE;
+
if (bdev)
bdev_blkbits = blksize_bits(bdev_hardsect_size(bdev));

@@ -1244,6 +1246,7 @@ __blockdev_direct_IO(int rw, struct kioc
out:
if (reader_with_isem)
up(&inode->i_sem);
+ current->flags &= ~PF_SYNCWRITE;
return retval;
}
EXPORT_SYMBOL(__blockdev_direct_IO);
_

> ...
>
> If you believe AS is going to perform better than CFQ on the database
> enterprise usage, we just need to prove it in practice after the round
> of fixes, then changing the default back to "as" it'll be an additional
> one liner on top of the blocker direct-io bug.

I don't think AS will ever meet the performance of CFQ or deadline for the
seeky database loads, unfortunately. We busted a gut over that and were
never able to get better than 90% or so.