On Thu, Mar 31 2005, Jens Axboe wrote:
> On Wed, Mar 30 2005, Ian Pratt wrote:
> > > I'll check the xen block driver to see if there's anything
> > > else that sticks out.
> > >
> > > Jens Axboe
> > Jens, I'd really appreciate this.
> > The blkfront/blkback drivers have rather evolved over time, and I don't
> > think any of the core team fully understand the block-layer differences
> > between 2.4 and 2.6.
> > There's also some junk left in there from when the backend was in Xen
> > itself back in the days of 1.2, though Vincent has prepared a patch to
> > clean this up and also make 'refreshing' of vbd's work (for size
> > changes), and also allow the blkfront driver to import whole disks
> > rather than paritions. We had this functionality on 2.4, but lost it in
> > the move to 2.6.
> > My bet is that it's the 2.6 backend that is where the true perofrmance
> > bug lies. Using a 2.6 domU blkfront talking to a 2.4 dom0 blkback seems
> > to give good performance under a wide variety of circumstances. Using a
> > 2.6 dom0 is far more pernickety. I agree with Andrew that I suspect it's
> > the work queue changes are biting us when we don't have many outstanding
> > requests.
> You never schedule the queues you submit the io against for the 2.6
> kernel, you only have a tq_disk run for 2.4 kernels. This basically puts
> you at the mercy of the timeout unplugging, which is really suboptimal
> unless you can keep the io queue of the target busy at all times.
> You need to either mark the last bio going to that device as BIO_SYNC,
> or do a blk_run_queue() on the target queue after having submitted all
> io in this batch for it.
Here is a temporary work-around, this should bring you close to 100%
performance at the cost of some extra unplugs. Uncompiled.
--- blkback.c~ 2005-03-31 09:06:16.000000000 +0200
+++ blkback.c 2005-03-31 09:09:27.000000000 +0200
@@ -481,7 +481,6 @@
for ( i = 0; i < nr_psegs; i++ )
struct bio *bio;
- struct bio_vec *bv;
bio = bio_alloc(GFP_ATOMIC, 1);
if ( unlikely(bio == NULL) )
@@ -494,17 +493,12 @@
bio->bi_private = pending_req;
bio->bi_end_io = end_block_io_op;
bio->bi_sector = phys_seg[i].sector_number;
- bio->bi_rw = operation;
- bv = bio_iovec_idx(bio, 0);
- bv->bv_page = virt_to_page(MMAP_VADDR(pending_idx, i));
- bv->bv_len = phys_seg[i].nr_sects << 9;
- bv->bv_offset = phys_seg[i].buffer & ~PAGE_MASK;
+ bio_add_page(bio, virt_to_page(MMAP_VADDR(pending_idx, i)),
+ phys_seg[i].nr_sects << 9,
+ phys_seg[i].buffer & ~PAGE_MASK);
- bio->bi_size = bv->bv_len;
- submit_bio(operation, bio);
+ submit_bio(operation | (1 << BIO_RW_SYNC), bio);
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