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SubjectRe: 2.5.32 IO performance issues
On Thu, Aug 29 2002, Andrew Morton wrote:
> > We want to allow high mem for block devices other than SCSI direct access
> > devices (TYPE_DISK), such as CD ROM (SDpnt->type TYPE_ROM), WORM devices
> > (TYPE_WORM), and optical disks (TYPE_MOD).
> >
> > So it is better to patch scsi_initialize_merge_fn:
> >
> > --- 1.16/drivers/scsi/scsi_merge.c Fri Jul 5 09:43:00 2002
> > +++ edited/drivers/scsi/scsi_merge.c Thu Aug 29 14:30:12 2002
> > @@ -140,7 +140,7 @@
> > * Enable highmem I/O, if appropriate.
> > */
> > bounce_limit = BLK_BOUNCE_HIGH;
> > - if (SHpnt->highmem_io && (SDpnt->type == TYPE_DISK)) {
> > + if (SHpnt->highmem_io) {
> > if (!PCI_DMA_BUS_IS_PHYS)
> > /* Platforms with virtual-DMA translation
> > * hardware have no practical limit.
> >
>
> That will certainly fix it. But who added the TYPE_DISK check,
> and why???

I guess that block-highmem has been around long enough, that I can use
the term 'historically' at least in the kernel sense :-)

This extra check was added for IDE because each device type driver
(ide-disk, ide-cd, etc) needed to be updated to not assume virtual
mappings of request data was valid. I only did that for ide-disk, since
this is the only one where bounce buffering really hurt performance
wise. So while ide-cd and ide-tape etc could have been updated, I deemed
it uninteresting and not worthwhile.

Now, this was just carried straight into the scsi counter parts,
conveniently, because of laziness. A quick glance at sr shows that it
too can aviod bouncing easily (no changes needed). st may need some
changes, though. So again, for scsi it was a matter of not impacting
existing code in 2.4 too much.

So TYPE_DISK check can be killed in 2.5 if someone does the work of
checking that it is safe. I'm not so sure it will make eg your SCSI
CD-ROM that much faster :-)

--
Jens Axboe

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