Messages in this thread | | | Date | Mon, 24 Apr 2006 21:49:10 +0200 | From | Jens Axboe <> | Subject | Re: [PATCH] Direct I/O bio size regression |
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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
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