Commit dcd19507 authored by Mikulas Patocka's avatar Mikulas Patocka Committed by Mike Snitzer
Browse files

dm writecache: improve performance of large linear writes on SSDs 



When dm-writecache is used with SSD as a cache device, it would submit a
separate bio for each written block. The I/Os would be merged by the disk
scheduler, but this merging degrades performance.

Improve dm-writecache performance by submitting larger bios - this is
possible as long as there is consecutive free space on the cache
device.

Benchmark (arm64 with 64k page size, using /dev/ram0 as a cache device):

fio --bs=512k --iodepth=32 --size=400M --direct=1 \
    --filename=/dev/mapper/cache --rw=randwrite --numjobs=1 --name=test

block	old	new
size	MiB/s	MiB/s
---------------------
512	181	700
1k	347	1256
2k	644	2020
4k	1183	2759
8k	1852	3333
16k	2469	3509
32k	2974	3670
64k	3404	3810

Signed-off-by: default avatarMikulas Patocka <mpatocka@redhat.com>
Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
parent be240ff5

drivers/md/dm-writecache.c

+25 −4
Original line number Diff line number Diff line
@@ -625,7 +625,7 @@ static void writecache_add_to_freelist(struct dm_writecache *wc, struct wc_entry 
	wc->freelist_size++;

}



static struct wc_entry *writecache_pop_from_freelist(struct dm_writecache *wc)

static struct wc_entry *writecache_pop_from_freelist(struct dm_writecache *wc, sector_t expected_sector)

{

	struct wc_entry *e;
@@ -634,6 +634,8 @@ static struct wc_entry *writecache_pop_from_freelist(struct dm_writecache *wc) 
		if (unlikely(!wc->current_free))

			return NULL;

		e = wc->current_free;

		if (expected_sector != (sector_t)-1 && unlikely(cache_sector(wc, e) != expected_sector))

			return NULL;

		next = rb_next(&e->rb_node);

		rb_erase(&e->rb_node, &wc->freetree);

		if (unlikely(!next))
@@ -643,6 +645,8 @@ static struct wc_entry *writecache_pop_from_freelist(struct dm_writecache *wc) 
		if (unlikely(list_empty(&wc->freelist)))

			return NULL;

		e = container_of(wc->freelist.next, struct wc_entry, lru);

		if (expected_sector != (sector_t)-1 && unlikely(cache_sector(wc, e) != expected_sector))

			return NULL;

		list_del(&e->lru);

	}

	wc->freelist_size--;
@@ -1193,7 +1197,7 @@ read_next_block: 
					goto bio_copy;

				}

			}

			e = writecache_pop_from_freelist(wc);

			e = writecache_pop_from_freelist(wc, (sector_t)-1);

			if (unlikely(!e)) {

				writecache_wait_on_freelist(wc);

				continue;
@@ -1205,9 +1209,26 @@ bio_copy: 
			if (WC_MODE_PMEM(wc)) {

				bio_copy_block(wc, bio, memory_data(wc, e));

			} else {

				dm_accept_partial_bio(bio, wc->block_size >> SECTOR_SHIFT);

				unsigned bio_size = wc->block_size;

				sector_t start_cache_sec = cache_sector(wc, e);

				sector_t current_cache_sec = start_cache_sec + (bio_size >> SECTOR_SHIFT);



				while (bio_size < bio->bi_iter.bi_size) {

					struct wc_entry *f = writecache_pop_from_freelist(wc, current_cache_sec);

					if (!f)

						break;

					write_original_sector_seq_count(wc, f, bio->bi_iter.bi_sector +

									(bio_size >> SECTOR_SHIFT), wc->seq_count);

					writecache_insert_entry(wc, f);

					wc->uncommitted_blocks++;

					bio_size += wc->block_size;

					current_cache_sec += wc->block_size >> SECTOR_SHIFT;

				}



				bio_set_dev(bio, wc->ssd_dev->bdev);

				bio->bi_iter.bi_sector = cache_sector(wc, e);

				bio->bi_iter.bi_sector = start_cache_sec;

				dm_accept_partial_bio(bio, bio_size >> SECTOR_SHIFT);



				if (unlikely(wc->uncommitted_blocks >= wc->autocommit_blocks)) {

					wc->uncommitted_blocks = 0;

					queue_work(wc->writeback_wq, &wc->flush_work);

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