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Subject[rfc patch] optimize o_direct on block device
Date
I've been complaining about O_DIRECT I/O processing being exceedingly
complex and slow since March 2005, see posting below:
http://marc.theaimsgroup.com/?l=linux-kernel&m=111033309732261&w=2

At that time, a patch was written for raw device to demonstrate that
large performance head room is achievable (at ~20% speedup for micro-
benchmark and ~2% for db transaction processing benchmark) with a tight
I/O submission processing loop.

Since raw device is being slowly phased out, I've rewritten the patch
for block device. O_DIRECT on block device is much simpler than O_D
on file system. Part of the reason that direct_io_worker is so complex
is because of O_D on file system, where it needs to perform block
allocation, hole detection, extents file on write, and tons of other
corner cases. The end result is that it takes tons of CPU time to
submit an I/O.

For block device, the block allocation is much simpler and I can write
a really tight double loop to iterate each iovec and each page within
the iovec in order to construct/prepare bio structure and then subsequently
submit it to the block layer.

So here it goes, posted here for comments.

A few notes on the patch:

(1) I need a vector structure similar to pagevec, however, pagevec doesn't
have everything that I need, i.e., an iterator variable. So I create a
new struct pvec. Maybe something can be worked out with pagevec?

(2) there are some inconsistency for synchronous I/O: condition to update
ppos and condition to wait on sync_kiocb is incompatible. Call chain
looks like the following:

do_sync_read
generic_file_aio_read
...
blkdev_direct_IO

do_sync_read will wait for I/O completion only if lower function returned
-EIOCBQUEUED. Updating ppos is done via generic_file_aio_read, but only
if the lower function returned positive value. So I either have to construct
my own wait_on_sync_kiocb, or hack around the ppos update.

(3) I/O length setup in kiocb is inconsistent between normal read vs vector read
or aio_read. One is passed in kiocb->ki_left vs others passing total length
in kiocb->nbytes. I've made them consistent in the read path (note to self:
I need to add the same thing in do_sync_write).



Signed-off-by: Ken Chen <kenneth.w.chen@intel.com>


--- ./fs/block_dev.c.orig 2006-11-29 14:52:20.000000000 -0800
+++ ./fs/block_dev.c 2006-11-29 16:45:36.000000000 -0800
@@ -129,43 +129,147 @@ blkdev_get_block(struct inode *inode, se
return 0;
}

-static int
-blkdev_get_blocks(struct inode *inode, sector_t iblock,
- struct buffer_head *bh, int create)
+int blk_end_aio(struct bio *bio, unsigned int bytes_done, int error)
{
- sector_t end_block = max_block(I_BDEV(inode));
- unsigned long max_blocks = bh->b_size >> inode->i_blkbits;
+ struct kiocb* iocb = bio->bi_private;
+ atomic_t* bio_count = (atomic_t*) &iocb->private;
+ long res;
+
+ if ((bio->bi_rw & 1) == READ)
+ bio_check_pages_dirty(bio);
+ else {
+ bio_release_pages(bio);
+ bio_put(bio);
+ }

- if ((iblock + max_blocks) > end_block) {
- max_blocks = end_block - iblock;
- if ((long)max_blocks <= 0) {
- if (create)
- return -EIO; /* write fully beyond EOF */
- /*
- * It is a read which is fully beyond EOF. We return
- * a !buffer_mapped buffer
- */
- max_blocks = 0;
- }
+ if (error)
+ iocb->ki_left = -EIO;
+
+ if (atomic_dec_and_test(bio_count)) {
+ res = (iocb->ki_left < 0) ? iocb->ki_left : iocb->ki_nbytes;
+ aio_complete(iocb, res, 0);
}

- bh->b_bdev = I_BDEV(inode);
- bh->b_blocknr = iblock;
- bh->b_size = max_blocks << inode->i_blkbits;
- if (max_blocks)
- set_buffer_mapped(bh);
return 0;
}

+#define VEC_SIZE 16
+struct pvec {
+ unsigned short nr;
+ unsigned short idx;
+ struct page *page[VEC_SIZE];
+};
+
+
+struct page *blk_get_page(unsigned long addr, size_t count, int rw,
+ struct pvec *pvec)
+{
+ int ret, nr_pages;
+ if (pvec->idx == pvec->nr) {
+ nr_pages = (addr + count + PAGE_SIZE - 1) / PAGE_SIZE -
+ addr / PAGE_SIZE;
+ nr_pages = min(nr_pages, VEC_SIZE);
+ down_read(&current->mm->mmap_sem);
+ ret = get_user_pages(current, current->mm, addr, nr_pages,
+ rw==READ, 0, pvec->page, NULL);
+ up_read(&current->mm->mmap_sem);
+ if (ret < 0)
+ return ERR_PTR(ret);
+ pvec->nr = ret;
+ pvec->idx = 0;
+ }
+ return pvec->page[pvec->idx++];
+}
+
static ssize_t
blkdev_direct_IO(int rw, struct kiocb *iocb, const struct iovec *iov,
- loff_t offset, unsigned long nr_segs)
+ loff_t pos, unsigned long nr_segs)
{
- struct file *file = iocb->ki_filp;
- struct inode *inode = file->f_mapping->host;
+ struct inode *inode = iocb->ki_filp->f_mapping->host;
+ unsigned blkbits = blksize_bits(bdev_hardsect_size(I_BDEV(inode)));
+ unsigned blocksize_mask = (1<< blkbits) - 1;
+ unsigned long seg, nvec, cur_off, cur_len;
+
+ unsigned long addr;
+ size_t count, nbytes = iocb->ki_nbytes;
+ loff_t size;
+ struct bio * bio;
+ atomic_t *bio_count = (atomic_t *) &iocb->private;
+ struct page *page;
+ struct pvec pvec = {.nr = 0, .idx = 0, };
+
+ size = i_size_read(inode);
+ if (pos + nbytes > size)
+ nbytes = size - pos;
+
+ seg = 0;
+ addr = (unsigned long) iov[0].iov_base;
+ count = iov[0].iov_len;
+ atomic_set(bio_count, 1);
+
+ /* first check the alignment */
+ if (addr & blocksize_mask || count & blocksize_mask ||
+ pos & blocksize_mask)
+ return -EINVAL;
+
+ while (nbytes) {
+ /* roughly estimate number of bio vec needed */
+ nvec = (nbytes + PAGE_SIZE - 1) / PAGE_SIZE;
+ nvec = max(nvec, nr_segs - seg);
+ nvec = min(nvec, (unsigned long) BIO_MAX_PAGES);
+
+ bio = bio_alloc(GFP_KERNEL, nvec);
+ bio->bi_bdev = I_BDEV(inode);
+ bio->bi_end_io = blk_end_aio;
+ bio->bi_private = iocb;
+ bio->bi_sector = pos >> blkbits;
+same_bio:
+ cur_off = addr & ~PAGE_MASK;
+ cur_len = PAGE_SIZE - cur_off;
+ if (count < cur_len)
+ cur_len = count;
+
+ page = blk_get_page(addr, count, rw, &pvec);
+ if (IS_ERR(page)) {
+ bio_release_pages(bio);
+ bio_put(bio);
+ if (atomic_read(bio_count) == 1)
+ return PTR_ERR(page);
+ break;
+ }
+
+ if (bio_add_page(bio, page, cur_len, cur_off)) {
+ pos += cur_len;
+ addr += cur_len;
+ count -= cur_len;
+ nbytes -= cur_len;
+
+ if (count)
+ goto same_bio;
+ if (++seg < nr_segs) {
+ addr = (unsigned long) iov[seg].iov_base;
+ count = iov[seg].iov_len;
+ if (!(addr & blocksize_mask ||
+ count & blocksize_mask))
+ goto same_bio;
+ }
+ }
+
+ /* bio is ready, submit it */
+ if (rw == READ)
+ bio_set_pages_dirty(bio);
+ atomic_inc(bio_count);
+ submit_bio(rw, bio);
+ }
+
+ nbytes = iocb->ki_nbytes = iocb->ki_nbytes - nbytes;
+ iocb->ki_pos += nbytes;
+
+ blk_run_address_space(inode->i_mapping);
+ if (atomic_dec_and_test(bio_count))
+ aio_complete(iocb, nbytes, 0);

- return blockdev_direct_IO_no_locking(rw, iocb, inode, I_BDEV(inode),
- iov, offset, nr_segs, blkdev_get_blocks, NULL);
+ return -EIOCBQUEUED;
}

static int blkdev_writepage(struct page *page, struct writeback_control *wbc)
--- ./fs/read_write.c.orig 2006-11-29 14:46:54.000000000 -0800
+++ ./fs/read_write.c 2006-11-29 14:52:43.000000000 -0800
@@ -235,7 +235,7 @@ ssize_t do_sync_read(struct file *filp,

init_sync_kiocb(&kiocb, filp);
kiocb.ki_pos = *ppos;
- kiocb.ki_left = len;
+ kiocb.ki_nbytes = kiocb.ki_left = len;

for (;;) {
ret = filp->f_op->aio_read(&kiocb, &iov, 1, kiocb.ki_pos);
--- ./fs/bio.c.orig 2006-11-29 14:46:54.000000000 -0800
+++ ./fs/bio.c 2006-11-29 14:52:43.000000000 -0800
@@ -931,7 +931,7 @@ void bio_set_pages_dirty(struct bio *bio
}
}

-static void bio_release_pages(struct bio *bio)
+void bio_release_pages(struct bio *bio)
{
struct bio_vec *bvec = bio->bi_io_vec;
int i;
--- ./include/linux/bio.h.orig 2006-11-29 15:17:27.000000000 -0800
+++ ./include/linux/bio.h 2006-11-29 15:18:48.000000000 -0800
@@ -309,6 +309,7 @@ extern struct bio *bio_map_kern(struct r
gfp_t);
extern void bio_set_pages_dirty(struct bio *bio);
extern void bio_check_pages_dirty(struct bio *bio);
+extern void bio_release_pages(struct bio *bio);
extern struct bio *bio_copy_user(struct request_queue *, unsigned long, unsigned int, int);
extern int bio_uncopy_user(struct bio *);
void zero_fill_bio(struct bio *bio);
-
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