Bug#789941: Performance regression: slow sequential reads for some block devices (readpage vs readpages) - patched in 3.18+
Package: linux-image-3.16.0-4-amd64
Version: 3.16.7-ckt11-1
Initially discovered inside a QEMU guest, by doing the following:
# hdparm -t /dev/vda
Under the Wheezy 3.2 kernel:
Timing buffered disk reads: 384 MB in 3.01 seconds = 127.50 MB/sec
Under the Jessie 3.16 kernel:
Timing buffered disk reads: 46 MB in 3.07 seconds = 14.97 MB/sec
After some work swapping kernels, I discovered that this behaviour
exists in the 3.16 and 3.17 kernels, but not in 3.2->3.15 or 3.18+
Watching iostat as the I/O is happening indicated that the 3.16/3.17
guests were performing the I/O in 8-sector (512 bytes per sector)
chunks; in the other kernels, the request sizes were 254 sectors instead.
Some further work identified this patch in 3.18:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/diff/?id=447f05bb488bff4282088259b04f47f0f9f76760
diff --git a/fs/block_dev.c b/fs/block_dev.c
index 6d72746..e2f3ad08 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -304,6 +304,12 @@ static int blkdev_readpage(struct file * file,
struct page * page)
return block_read_full_page(page, blkdev_get_block);
}
+static int blkdev_readpages(struct file *file, struct address_space
*mapping,
+ struct list_head *pages, unsigned nr_pages)
+{
+ return mpage_readpages(mapping, pages, nr_pages, blkdev_get_block);
+}
+
static int blkdev_write_begin(struct file *file, struct address_space
*mapping,
loff_t pos, unsigned len, unsigned flags,
struct page **pagep, void **fsdata)
@@ -1622,6 +1628,7 @@ static int blkdev_releasepage(struct page *page,
gfp_t wait)
static const struct address_space_operations def_blk_aops = {
.readpage = blkdev_readpage,
+ .readpages = blkdev_readpages,
.writepage = blkdev_writepage,
.write_begin = blkdev_write_begin,
.write_end = blkdev_write_end,
It applies cleanly to 3.16 and 3.17; with the patch applied, the
larger request sizes are again seen and performance returns to the
previous level.
Could we apply this to the 3.16 kernel in Jessie?
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