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parallel I/O

uml-devel Nicolas Boullis 2 participants 6 messages Sep 12, 2008 – Oct 6, 2008 User-mode Linux (UML)

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Nicolas Boullis

author
#0 lore raw

Hi,

I've been using uml on production servers for months, and have been quite happy with it, except for I/O performance.

I was using the user-mode-linux package from Debian Etch, based on 2.6.18, and compiled with CONFIG_BLK_DEV_UBD_SYNC. Disabling this option of course helps performance, but I feel that it's somewhat unsafe if the host crashes... Moreover, I don't think it helps read performance.

As far as I can see, if one wants both safety and performance, the I/O must not be serialized. So I decided to give it a try.

The first step was to run a per-device I/O thread. This improves slightly the performance with several UBD devices: I/O on one device do not block I/O on another device. Moreover, that helps to implement parallelized I/O.

Then I managed to run several parallel threads per device. As far as I am concernced, that much improves the performance. But currently, my code is more a dirty proof-of-concept than a clean patch.

Would you be interested by my work?

Cheers,

Nicolas Boullis
École Centrale Paris

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author
#1 lore raw

Hi,

For what it's worth, here is my quick-n-dirty proof-of-concept patch that enables parallel I/O. Note that it is based on 2.6.24.

If people are interested, I can try to make a clean patch out of it.

Cheers,

Nicolas

arch/um/drivers/ubd_kern.c renamed from arch/um/drivers/ubd_kern.c.orig +127 -10
@@ -65,7 +65,7 @@
 	unsigned long length;
 	char *buffer;
 	int sectorsize;
-	unsigned long sector_mask;
+	unsigned long sector_mask[8];
 	unsigned long long cow_offset;
 	unsigned long bitmap_words[2];
 	int error;

@@ -172,6 +172,9 @@
 	struct scatterlist sg[MAX_SG];
 	struct request *request;
 	int start_sg, end_sg;
+	int pipe_fd[2];
+	int io_pid;
+	unsigned long stack;
 };
 
 #define DEFAULT_COW { \

@@ -196,6 +199,9 @@
 	.request =		NULL, \
 	.start_sg =		0, \
 	.end_sg =		0, \
+	.pipe_fd =		{-1, -1}, \
+	.io_pid =		-1, \
+	.stack =		0, \
 }
 
 /* Protected by ubd_lock */

@@ -473,7 +479,7 @@
 static void do_ubd_request(struct request_queue * q);
 
 /* Only changed by ubd_init, which is an initcall. */
-int thread_fd = -1;
+int common_thread_fd = -1;
 
 static void ubd_end_request(struct request *req, int bytes, int uptodate)
 {

@@ -513,7 +519,7 @@
 	int n;
 
 	while(1){
-		n = os_read_file(thread_fd, &req,
+		n = os_read_file(common_thread_fd, &req,
 				 sizeof(struct io_thread_req *));
 		if(n != sizeof(req)){
 			if(n == -EAGAIN)

@@ -529,7 +535,7 @@
 			ubd_finish(rq, rq->hard_nr_sectors << 9);
 		kfree(req);
 	}
-	reactivate_fd(thread_fd, UBD_IRQ);
+	reactivate_fd(common_thread_fd, UBD_IRQ);
 
 	list_for_each_safe(list, next_ele, &restart){
 		ubd = container_of(list, struct ubd, restart);

@@ -565,6 +571,8 @@
 	return os_file_size(file, size_out);
 }
 
+extern pid_t waitpid(pid_t pid, int *status, int options);
+
 static void ubd_close_dev(struct ubd *ubd_dev)
 {
 	os_close_file(ubd_dev->fd);

@@ -576,6 +584,8 @@
 	ubd_dev->cow.bitmap = NULL;
 }
 
+int per_device_io_thread(void *arg);
+
 static int ubd_open_dev(struct ubd *ubd_dev)
 {
 	struct openflags flags;

@@ -636,6 +646,8 @@
 		if(err < 0) goto error;
 		ubd_dev->cow.fd = err;
 	}
+
+
 	return 0;
  error:
 	os_close_file(ubd_dev->fd);

@@ -938,7 +950,7 @@
 	}
 	stack = alloc_stack(0, 0);
 	io_pid = start_io_thread(stack + PAGE_SIZE - sizeof(void *),
-				 &thread_fd);
+				 &common_thread_fd);
 	if(io_pid < 0){
 		printk(KERN_ERR
 		       "ubd : Failed to start I/O thread (errno = %d) - "

@@ -946,7 +958,7 @@
 		io_pid = -1;
 		return 0;
 	}
-	err = um_request_irq(UBD_IRQ, thread_fd, IRQ_READ, ubd_intr,
+	err = um_request_irq(UBD_IRQ, common_thread_fd, IRQ_READ, ubd_intr,
 			     IRQF_DISABLED, "ubd", ubd_devs);
 	if(err != 0)
 		printk(KERN_ERR "um_request_irq failed - errno = %d\n", -err);

@@ -962,12 +974,23 @@
 	int err = 0;
 
 	if(ubd_dev->count == 0){
+		void *sp;
+
 		err = ubd_open_dev(ubd_dev);
 		if(err){
 			printk(KERN_ERR "%s: Can't open \"%s\": errno = %d\n",
 			       disk->disk_name, ubd_dev->file, -err);
 			goto out;
 		}
+
+		pipe(ubd_dev->pipe_fd);
+		os_set_fd_block(ubd_dev->pipe_fd[1], 0);
+
+		ubd_dev->stack = alloc_stack(0, 0);
+		sp = (void *)(ubd_dev->stack + PAGE_SIZE - sizeof(void *));
+		ubd_dev->io_pid = clone(per_device_io_thread, sp, CLONE_VM, (void *)ubd_dev);
+
+		printk("Launched I/O thread.\n");
 	}
 	ubd_dev->count++;
 	set_disk_ro(disk, !ubd_dev->openflags.w);

@@ -987,12 +1010,21 @@
 	struct gendisk *disk = inode->i_bdev->bd_disk;
 	struct ubd *ubd_dev = disk->private_data;
 
-	if(--ubd_dev->count == 0)
+	if(--ubd_dev->count == 0) {
+		printk("Trying to stop I/O thread.\n");
+		os_close_file(ubd_dev->pipe_fd[1]);
+		ubd_dev->pipe_fd[1] = -1;
+		waitpid(ubd_dev->io_pid, NULL, __WCLONE);
+		ubd_dev->io_pid = -1;
+		free_stack(ubd_dev->stack, 0);
+		ubd_dev->stack = 0;
+
 		ubd_close_dev(ubd_dev);
+	}
 	return 0;
 }
 
-static void cowify_bitmap(__u64 io_offset, int length, unsigned long *cow_mask,
+static void cowify_bitmap(__u64 io_offset, int length, unsigned long (*cow_mask)[],
 			  __u64 *cow_offset, unsigned long *bitmap,
 			  __u64 bitmap_offset, unsigned long *bitmap_words,
 			  __u64 bitmap_len)

@@ -1059,6 +1091,7 @@
 {
 	struct gendisk *disk = req->rq_disk;
 	struct ubd *ubd_dev = disk->private_data;
+	int i;
 
 	io_req->req = req;
 	io_req->fds[0] = (ubd_dev->cow.file != NULL) ? ubd_dev->cow.fd :

@@ -1068,7 +1101,8 @@
 	io_req->offset = offset;
 	io_req->length = len;
 	io_req->error = 0;
-	io_req->sector_mask = 0;
+	for (i = 0; i < 8; i++)
+		io_req->sector_mask[i] = 0;
 
 	io_req->op = (rq_data_dir(req) == READ) ? UBD_READ : UBD_WRITE;
 	io_req->offsets[0] = 0;

@@ -1120,7 +1154,7 @@
 					sg->offset, sg->length, sg_page(sg));
 
 			last_sectors = sg->length >> 9;
-			n = os_write_file(thread_fd, &io_req,
+			n = os_write_file(dev->pipe_fd[1], &io_req,
 					  sizeof(struct io_thread_req *));
 			if(n != sizeof(struct io_thread_req *)){
 				if(n != -EAGAIN)

@@ -1482,3 +1516,86 @@
 
 	return 0;
 }
+
+extern int open(const char *pathname, int flags);
+extern int dup2(int oldfd, int newfd);
+
+void reopen(int fd)
+{
+	char name[64];
+	int tmp_fd;
+
+	sprintf(name, "/proc/self/fd/%d", fd);
+	tmp_fd = open(name, O_RDWR | O_SYNC);
+	os_close_file(fd);
+	dup2(tmp_fd, fd);
+}
+
+int io_subthread(void *arg)
+{
+	struct ubd *ubd_dev = (struct ubd *)arg;
+	struct io_thread_req *req;
+	int n;
+
+	if (ubd_dev->fd >= 0)
+		reopen(ubd_dev->fd);
+
+	if (ubd_dev->cow.fd >= 0)
+		reopen(ubd_dev->cow.fd);
+
+	while(1){
+		n = os_read_file(ubd_dev->pipe_fd[0], &req,
+				 sizeof(struct io_thread_req *));
+		if(n != sizeof(struct io_thread_req *)){
+			if(n < 0)
+				printk("io_subthread - read failed, fd = %d, "
+				       "err = %d\n", ubd_dev->pipe_fd[0], -n);
+			else if (n == 0)
+				break;
+			else {
+				printk("io_subthread - short read, fd = %d, "
+				       "length = %d\n", ubd_dev->pipe_fd[0], n);
+			}
+			continue;
+		}
+		io_count++;
+		do_io(req);
+		n = os_write_file(kernel_fd, &req,
+				  sizeof(struct io_thread_req *));
+		if(n != sizeof(struct io_thread_req *))
+			printk("io_subthread - write failed, fd = %d, err = %d\n",
+			       kernel_fd, -n);
+	}
+
+	ubd_close_dev(ubd_dev);
+
+	return 0;
+}
+
+int per_device_io_thread(void *arg)
+{
+	struct ubd *ubd_dev = (struct ubd *)arg;
+	unsigned long stack[16];
+	int i;
+
+	printk("I/O thread has started.\n");
+
+	ignore_sigwinch_sig();
+
+	os_close_file(ubd_dev->pipe_fd[1]);
+
+	for (i = 0; i < 16; i++) {
+		void *sp;
+		stack[i] = alloc_stack(0, 0);
+		sp = (void *)(stack[i] + PAGE_SIZE - sizeof(void *));
+		clone(io_subthread, sp, CLONE_VM, (void *)ubd_dev);
+	}
+
+	while (waitpid(-1, NULL, __WCLONE) != -1);
+	for (i = 0; i < 16; i++)
+		free_stack(stack[i], 0);
+
+	printk("I/O thread has finished.\n");
+
+	return 0;
+}

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author
#2 lore raw

Hi,

Quoted 5 lines · I wrote:

For what it's worth, here is my quick-n-dirty proof-of-concept patch that enables parallel I/O. Note that it is based on 2.6.24.

If people are interested, I can try to make a clean patch out of it.

For what it's worth, I fixed a few bugs in my patch. Here is the new patch. It's still "quick'n'dirty", but it works better, and works for me.

Cheers,

Nicolas

arch/um/drivers/ubd_kern.c renamed from arch/um/drivers/ubd_kern.c.orig +129 -10
@@ -65,7 +65,7 @@
 	unsigned long length;
 	char *buffer;
 	int sectorsize;
-	unsigned long sector_mask;
+	unsigned long sector_mask[8];
 	unsigned long long cow_offset;
 	unsigned long bitmap_words[2];
 	int error;

@@ -172,6 +172,9 @@
 	struct scatterlist sg[MAX_SG];
 	struct request *request;
 	int start_sg, end_sg;
+	int pipe_fd[2];
+	int io_pid;
+	unsigned long stacks[17];
 };
 
 #define DEFAULT_COW { \

@@ -196,6 +199,9 @@
 	.request =		NULL, \
 	.start_sg =		0, \
 	.end_sg =		0, \
+	.pipe_fd =		{-1, -1}, \
+	.io_pid =		-1, \
+	.stacks =		{ [0 ... 16] = 0 }, \
 }
 
 /* Protected by ubd_lock */

@@ -473,7 +479,7 @@
 static void do_ubd_request(struct request_queue * q);
 
 /* Only changed by ubd_init, which is an initcall. */
-int thread_fd = -1;
+int common_thread_fd = -1;
 
 static void ubd_end_request(struct request *req, int bytes, int uptodate)
 {

@@ -513,7 +519,7 @@
 	int n;
 
 	while(1){
-		n = os_read_file(thread_fd, &req,
+		n = os_read_file(common_thread_fd, &req,
 				 sizeof(struct io_thread_req *));
 		if(n != sizeof(req)){
 			if(n == -EAGAIN)

@@ -529,7 +535,7 @@
 			ubd_finish(rq, rq->hard_nr_sectors << 9);
 		kfree(req);
 	}
-	reactivate_fd(thread_fd, UBD_IRQ);
+	reactivate_fd(common_thread_fd, UBD_IRQ);
 
 	list_for_each_safe(list, next_ele, &restart){
 		ubd = container_of(list, struct ubd, restart);

@@ -565,6 +571,8 @@
 	return os_file_size(file, size_out);
 }
 
+extern pid_t waitpid(pid_t pid, int *status, int options);
+
 static void ubd_close_dev(struct ubd *ubd_dev)
 {
 	os_close_file(ubd_dev->fd);

@@ -576,6 +584,8 @@
 	ubd_dev->cow.bitmap = NULL;
 }
 
+int per_device_io_thread(void *arg);
+
 static int ubd_open_dev(struct ubd *ubd_dev)
 {
 	struct openflags flags;

@@ -636,6 +646,8 @@
 		if(err < 0) goto error;
 		ubd_dev->cow.fd = err;
 	}
+
+
 	return 0;
  error:
 	os_close_file(ubd_dev->fd);

@@ -938,7 +950,7 @@
 	}
 	stack = alloc_stack(0, 0);
 	io_pid = start_io_thread(stack + PAGE_SIZE - sizeof(void *),
-				 &thread_fd);
+				 &common_thread_fd);
 	if(io_pid < 0){
 		printk(KERN_ERR
 		       "ubd : Failed to start I/O thread (errno = %d) - "

@@ -946,7 +958,7 @@
 		io_pid = -1;
 		return 0;
 	}
-	err = um_request_irq(UBD_IRQ, thread_fd, IRQ_READ, ubd_intr,
+	err = um_request_irq(UBD_IRQ, common_thread_fd, IRQ_READ, ubd_intr,
 			     IRQF_DISABLED, "ubd", ubd_devs);
 	if(err != 0)
 		printk(KERN_ERR "um_request_irq failed - errno = %d\n", -err);

@@ -962,12 +974,27 @@
 	int err = 0;
 
 	if(ubd_dev->count == 0){
+		void *sp;
+		int i;
+
 		err = ubd_open_dev(ubd_dev);
 		if(err){
 			printk(KERN_ERR "%s: Can't open \"%s\": errno = %d\n",
 			       disk->disk_name, ubd_dev->file, -err);
 			goto out;
 		}
+
+		pipe(ubd_dev->pipe_fd);
+
+		for (i = 0; i <= 16; i++)
+			ubd_dev->stacks[i] = alloc_stack(0, 0);
+		sp = (void *)(ubd_dev->stacks[0] + PAGE_SIZE - sizeof(void *));
+		ubd_dev->io_pid = clone(per_device_io_thread, sp, CLONE_VM, (void *)ubd_dev);
+		os_close_file(ubd_dev->pipe_fd[0]);
+		os_set_exec_close(ubd_dev->pipe_fd[1], 1);
+		os_set_fd_block(ubd_dev->pipe_fd[1], 0);
+
+		printk("Launched I/O thread for %s.\n", disk->disk_name);
 	}
 	ubd_dev->count++;
 	set_disk_ro(disk, !ubd_dev->openflags.w);

@@ -987,12 +1014,26 @@
 	struct gendisk *disk = inode->i_bdev->bd_disk;
 	struct ubd *ubd_dev = disk->private_data;
 
-	if(--ubd_dev->count == 0)
+	if(--ubd_dev->count == 0) {
+		int i;
+		printk("Trying to stop I/O thread for %s... ", disk->disk_name);
+		os_close_file(ubd_dev->pipe_fd[1]);
+		waitpid(ubd_dev->io_pid, NULL, __WCLONE);
+		printk("done.\n");
+		ubd_dev->pipe_fd[0] = -1;
+		ubd_dev->pipe_fd[1] = -1;
+		ubd_dev->io_pid = -1;
+		for (i = 0; i <= 16; i++) {
+			free_stack(ubd_dev->stacks[i], 0);
+			ubd_dev->stacks[i] = 0;
+		}
+
 		ubd_close_dev(ubd_dev);
+	}
 	return 0;
 }
 
-static void cowify_bitmap(__u64 io_offset, int length, unsigned long *cow_mask,
+static void cowify_bitmap(__u64 io_offset, int length, unsigned long (*cow_mask)[],
 			  __u64 *cow_offset, unsigned long *bitmap,
 			  __u64 bitmap_offset, unsigned long *bitmap_words,
 			  __u64 bitmap_len)

@@ -1059,6 +1100,7 @@
 {
 	struct gendisk *disk = req->rq_disk;
 	struct ubd *ubd_dev = disk->private_data;
+	int i;
 
 	io_req->req = req;
 	io_req->fds[0] = (ubd_dev->cow.file != NULL) ? ubd_dev->cow.fd :

@@ -1068,7 +1110,8 @@
 	io_req->offset = offset;
 	io_req->length = len;
 	io_req->error = 0;
-	io_req->sector_mask = 0;
+	for (i = 0; i < 8; i++)
+		io_req->sector_mask[i] = 0;
 
 	io_req->op = (rq_data_dir(req) == READ) ? UBD_READ : UBD_WRITE;
 	io_req->offsets[0] = 0;

@@ -1120,7 +1163,7 @@
 					sg->offset, sg->length, sg_page(sg));
 
 			last_sectors = sg->length >> 9;
-			n = os_write_file(thread_fd, &io_req,
+			n = os_write_file(dev->pipe_fd[1], &io_req,
 					  sizeof(struct io_thread_req *));
 			if(n != sizeof(struct io_thread_req *)){
 				if(n != -EAGAIN)

@@ -1482,3 +1525,79 @@
 
 	return 0;
 }
+
+extern int open(const char *pathname, int flags);
+extern int dup2(int oldfd, int newfd);
+
+void reopen(int fd)
+{
+	char name[64];
+	int tmp_fd;
+
+	sprintf(name, "/proc/self/fd/%d", fd);
+	tmp_fd = open(name, O_RDWR | O_SYNC);
+	os_close_file(fd);
+	dup2(tmp_fd, fd);
+	os_close_file(tmp_fd);
+}
+
+int io_subthread(void *arg)
+{
+	struct ubd *ubd_dev = (struct ubd *)arg;
+	struct io_thread_req *req;
+	int n;
+
+	if (ubd_dev->fd >= 0)
+		reopen(ubd_dev->fd);
+
+	if (ubd_dev->cow.fd >= 0)
+		reopen(ubd_dev->cow.fd);
+
+	while(1){
+		n = os_read_file(ubd_dev->pipe_fd[0], &req,
+				 sizeof(struct io_thread_req *));
+		if(n != sizeof(struct io_thread_req *)){
+			if(n < 0)
+				printk("io_subthread - read failed, fd = %d, "
+				       "err = %d\n", ubd_dev->pipe_fd[0], -n);
+			else if (n == 0)
+				break;
+			else {
+				printk("io_subthread - short read, fd = %d, "
+				       "length = %d\n", ubd_dev->pipe_fd[0], n);
+			}
+			continue;
+		}
+		io_count++;
+		do_io(req);
+		n = os_write_file(kernel_fd, &req,
+				  sizeof(struct io_thread_req *));
+		if(n != sizeof(struct io_thread_req *))
+			printk("io_subthread - write failed, fd = %d, err = %d\n",
+			       kernel_fd, -n);
+	}
+
+	ubd_close_dev(ubd_dev);
+
+	return 0;
+}
+
+int per_device_io_thread(void *arg)
+{
+	struct ubd *ubd_dev = (struct ubd *)arg;
+	int i;
+
+	ignore_sigwinch_sig();
+
+	os_close_file(ubd_dev->pipe_fd[1]);
+
+	for (i = 0; i < 16; i++) {
+		void *sp;
+		sp = (void *)(ubd_dev->stacks[i+1] + PAGE_SIZE - sizeof(void *));
+		clone(io_subthread, sp, CLONE_VM, (void *)ubd_dev);
+	}
+
+	while (waitpid(-1, NULL, __WCLONE) != -1);
+
+	return 0;
+}

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This SF.Net email is sponsored by the Moblin Your Move Developer's challenge Build the coolest Linux based applications with Moblin SDK & win great prizes Grand prize is a trip for two to an Open Source event anywhere in the world
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#3 lore raw
Quoted 9 lines · Nicolas Boullis
Quoted 5 lines · I wrote:

For what it's worth, here is my quick-n-dirty proof-of-concept patch that enables parallel I/O. Note that it is based on 2.6.24.

If people are interested, I can try to make a clean patch out of it.

For what it's worth, I fixed a few bugs in my patch. Here is the new patch. It's still "quick'n'dirty", but it works better, and works for me.

Sorry about the lack of responsiveness.

I've had such a patch for quite a while. I never stuck it in mainline because I couldn't find any common use cases where it made a noticable difference.

Does your patch help noticably with anything reasonable common?

Jeff

Signature · 11 lines
Work email - jdike at linux dot intel dot com

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Build the coolest Linux based applications with Moblin SDK & win great prizes
Grand prize is a trip for two to an Open Source event anywhere in the world
http://moblin-contest.org/redirect.php?banner_id=100&url=/
_______________________________________________
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author
#4 lore raw

Hello,

Quoting Jeff Dike <[email protected]>:

Quoted 1 line

Sorry about the lack of responsiveness.

No problem, I'm glad to read you now.

Quoted 5 lines

I've had such a patch for quite a while. I never stuck it in mainline because I couldn't find any common use cases where it made a noticable difference.

Does your patch help noticably with anything reasonable common?

It did help significantly for me.
On a reasonably new host, using as UBD devices logical volumes built on top of a FC SAN, building a 10GB ext3 filesystem took something like 50 seconds before my patch, and something like 3 seconds after it. (I can check the exact values if you wish.)
Note that this is using CONFIG_UBD_SYNC (or whatever it is called); without it it gets even faster (but I guess parallel I/O make no difference).

My feeling is that it might improve performance when reads and writes are performed simultaneously, but I have not made any try.

As for using CONFIG_UBD_SYNC, my feeling is that it's much safer if the host might crash (or experience power outages, or...): as far as I understand it, journaled filesystems need to know when data is actually secured on the discs.

Hope this helps,

Nicolas Boullis
Ecole Centrale Paris

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Hi,

Quoted 4 lines · Nicolas Boullis

On a reasonably new host, using as UBD devices logical volumes built on top of a FC SAN, building a 10GB ext3 filesystem took something like 50 seconds before my patch, and something like 3 seconds after it. (I can check the exact values if you wish.)

Hmmm... my memory was making thing much better than they really are. It's around 38 seconds with 2.6.18 (before the "batch I/O requests" change); it's several minutes with unpatched 2.6.24, and it's around 7 seconds with patched 2.6.24...

To be more honnest, my patch contains a part similar to Steve VanDeBogart's "ubd does multiple io's when one will suffice" patch. Using 2.6.24 with his patch but still serialized I/O, the time is around 23 seconds.

Hence, my patch (with 16 threads per device) make things around 3 times faster.

Cheers,

Nicolas Boullis
École Centrale Paris

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