Re: [PATCH] Direct I/O bio size regression

[Jens Axboe <axboe@suse.de>]

On Mon, Apr 24 2006, Al Boldi wrote:
> David Chinner wrote:
> > On Mon, Apr 24, 2006 at 11:05:08AM +0200, Jens Axboe wrote:
> > > On Mon, Apr 24 2006, Jens Axboe wrote:
> > > > > Index: 2.6.x-xfs-new/fs/bio.c
> > > > > ===================================================================
> > > > > --- 2.6.x-xfs-new.orig/fs/bio.c   2006-02-06 11:57:50.000000000
> > > > > +1100 +++ 2.6.x-xfs-new/fs/bio.c        2006-04-24
> > > > > 15:46:16.849484424 +1000 @@ -304,7 +304,7 @@ int
> > > > > bio_get_nr_vecs(struct block_device request_queue_t *q =
> > > > > bdev_get_queue(bdev);
> > > > >   int nr_pages;
> > > > >
> > > > > - nr_pages = ((q->max_sectors << 9) + PAGE_SIZE - 1) >> PAGE_SHIFT;
> > > > > + nr_pages = ((q->max_hw_sectors << 9) + PAGE_SIZE - 1) >>
> > > > > PAGE_SHIFT; if (nr_pages > q->max_phys_segments)
> > > > >           nr_pages = q->max_phys_segments;
> > > > >   if (nr_pages > q->max_hw_segments)
> > > > > @@ -446,7 +446,7 @@ int bio_add_page(struct bio *bio, struct
> > > > >            unsigned int offset)
> > > > >  {
> > > > >   struct request_queue *q = bdev_get_queue(bio->bi_bdev);
> > > > > - return __bio_add_page(q, bio, page, len, offset, q->max_sectors);
> > > > > + return __bio_add_page(q, bio, page, len, offset,
> > > > > q->max_hw_sectors); }
> > > > >
> > > > >  struct bio_map_data {
> > > >
> > > > Clearly correct, I'll make sure this gets merged right away.
> > >
> > > Spoke too soon... The last part is actually on purpose, to prevent
> > > really huge requests as part of normal file system IO.
> >
> > I don't understand why this was considered necessary. It
> > doesn't appear to be explained in any of the code so can you
> > explain the problem that large filesystem I/Os pose to the block
> > layer? We _need_ to be able to drive really huge requests from the
> > filesystem down to the disks, especially for direct I/O.....
> > FWIW, we've just got XFS to the point where we could issue large
> > I/Os (up to 8MB on 16k pages) with a default configuration kernel
> > and filesystem using md+dm on an Altix. That makes an artificial
> > 512KB filesystem I/O size limit a pretty major step backwards in
> > terms of performance for default configs.....
> >
> > > That's why we
> > > have a bio_add_pc_page(). The first hunk may cause things to not work
> > > optimally then if we don't apply the last hunk.
> >
> > bio_add_pc_page() requires a request queue to be passed to it.  It's
> > called only from scsi layers in the context of mapping pages into a
> > bio from sg_io(). The comment for bio_add_pc_page() says for use
> > with REQ_PC queues only, and that appears to only be used by ide-cd
> > cdroms. Is that comment correct?
> >
> > Also, it seems to me that using bio_add_pc_page() in a filesystem
> > or in the generic direct i/o code seems like a gross layering
> > violation to me because they are supposed to know nothing about
> > request queues.
> >
> > > The best approach is probably to tune max_sectors on the system itself.
> > > That's why it is exposed, after all.
> >
> > You mean /sys/block/sd*/max_sector_kb?
> 
> On my system max_hw_sectors_kb is fixed at 1024, and max_sectors_kb defaults 
> to 512, which leads to terribly fluctuating thruput.
> 
> Setting max_sectors_kb = max_hw_sectors_kb makes things even worse.
> 
> Tuning max_sectors_kb to ~192 only stabilizes this situation.

That sounds pretty strange. Do you have a test case?

-- 
Jens Axboe