Attachment 314576 Details for Bug 438153 – second patch

[patch] second patch

linux-2.6.18-dm-unplug-queues-in-threads.patch (text/plain), 4.11 KB, created by Mikuláš Patočka on 2008-08-19 21:10:17 UTC

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Creator: Mikuláš Patočka

Created: 2008-08-19 21:10:17 UTC

Size: 4.11 KB

patch

obsolete

>Upstream commit: 7ff14a36159d947872870e7a3e9dcaebc46b23eb (2.6.26-rc1)
>
>Avoid 3ms delay in dm-raid and kcopyd.
>
>It is specified that any submitted bio without BIO_RW_SYNC flag may plug the
>queue (i.e. block the requests from being dispatched to the physical device).
>
>The queue is unplugged when the caller calls blk_unplug() function. Usually, thesequence is that someone calls submit_bh to submit IO on a buffer. The IO plugs the queue and waits (to be possibly joined with other adjacent bios). Then, whenthe caller calls wait_on_buffer(), it unplugs the queue and submits the IOs to
>the disk.
>
>This was happenning:
>
>When doing O_SYNC writes, function fsync_buffers_list() submits a list of
>bios to dm_raid1, the bios are added to dm_raid1 write queue and kmirrord is
>woken up.
>
>fsync_buffers_list() calls wait_on_buffer() --- that unplugs the queue, but
>there are no bios on the device queue (they are still in dm_raid1 queue)
>
>wait_on_buffer() starts waiting until the IO is finished.
>
>kmirrord is scheduled, kmirrord takes bios and submits them to the devices.
>
>the submitted bio plugs the harddisk queue, there is no one to unplug it
>(the process that called wait_on_buffer() is already sleeping)
>
>So there is 3ms timeout, after which the queues on the harddisks are
>unplugged, and requests are processed.
>
>This 3ms timeout caused that in certain workloads (O_SYNC, 8kb writes), dm-raid1is 10 times slower than md-raid1.
>
>
>Every time we submit something asynchronously via dm_io, we must unplug the
>queue to actually put the request to the device.
>
>This patch adds unplug call to kmirrord --- while processing requests, it keeps the queue plugged (so that adjacent bios can be merged), when it finishes
>processing all bios, it unplugs the queue to submit the bios.
>
>It also fixes kcopyd, that has the same potential problem. All kcopyd requests
>are submitted with BIO_RW_SYNC.
>
>Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
>
>---
>
>diff -p -u -r linux-2.6.18.x86_64.p2/drivers/md/dm-io.c linux-2.6.18.x86_64/drivers/md/dm-io.c
>--- linux-2.6.18.x86_64.p2/drivers/md/dm-io.c	2008-08-18 14:06:33.000000000 +0200
>+++ linux-2.6.18.x86_64/drivers/md/dm-io.c	2008-08-18 15:58:51.000000000 +0200
>@@ -421,7 +421,7 @@ static int sync_io(struct dm_io_client *
> {
> 	struct io io;
> 
>-	if (num_regions > 1 && rw != WRITE) {
>+	if (num_regions > 1 && (rw & RW_MASK) != WRITE) {
> 		WARN_ON(1);
> 		return -EIO;
> 	}
>@@ -458,7 +458,7 @@ static int async_io(struct dm_io_client 
> {
> 	struct io *io;
> 
>-	if (num_regions > 1 && rw != WRITE) {
>+	if (num_regions > 1 && (rw & RW_MASK) != WRITE) {
> 		WARN_ON(1);
> 		fn(1, context);
> 		return -EIO;
>@@ -555,6 +555,11 @@ static int dp_init(struct dm_io_request 
> 
> /*
>  * New collapsed (a)synchronous interface
>+ *
>+ * If the IO is asynchronous (i.e. it has notify.fn), you must either unplug the
>+ * queue with blk_unplug() some time later or you must set the BIO_RW_SYNC bit in
>+ * io_req->bi_rw. If you fail to do one of these, the IO will be submitted to
>+ * the disk after q->unplug_delay, which defaults to 3ms in blk-settings.c.
>  */
> int dm_io(struct dm_io_request *io_req, unsigned num_regions,
> 	  struct io_region *where, unsigned long *sync_error_bits)
>diff -p -u -r linux-2.6.18.x86_64.p2/drivers/md/dm-raid1.c linux-2.6.18.x86_64/drivers/md/dm-raid1.c
>--- linux-2.6.18.x86_64.p2/drivers/md/dm-raid1.c	2008-08-18 15:57:05.000000000 +0200
>+++ linux-2.6.18.x86_64/drivers/md/dm-raid1.c	2008-08-18 15:58:51.000000000 +0200
>@@ -1312,6 +1312,8 @@ static void do_mirror(struct mirror_set 
> 	do_reads(ms, &reads);
> 	do_writes(ms, &writes);
> 	do_failures(ms, &failures);
>+
>+	dm_table_unplug_all(ms->ti->table);
> }
> 
> static void do_work(void *data)
>diff -p -u -r linux-2.6.18.x86_64.p2/drivers/md/kcopyd.c linux-2.6.18.x86_64/drivers/md/kcopyd.c
>--- linux-2.6.18.x86_64.p2/drivers/md/kcopyd.c	2008-08-18 14:06:33.000000000 +0200
>+++ linux-2.6.18.x86_64/drivers/md/kcopyd.c	2008-08-18 15:58:51.000000000 +0200
>@@ -346,7 +346,7 @@ static int run_io_job(struct kcopyd_job 
> {
> 	int r;
> 	struct dm_io_request io_req = {
>-		.bi_rw = job->rw,
>+		.bi_rw = job->rw | (1 << BIO_RW_SYNC),
> 		.mem.type = DM_IO_PAGE_LIST,
> 		.mem.ptr.pl = job->pages,
> 		.mem.offset = job->offset,

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Attachments on bug 438153: 314575 | 314576