Re: The cause of the "VM" performance problem with 2.4.X

[Andrew Morton <akpm@zip.com.au>]

"Van Maren, Kevin" wrote:
> 
> It looks like the same patch as yesterday.  Did you attach the wrong
> patch?

Drat, sorry - it's on the other machine.  Here's a reconstruction:

--- linux-2.4.9/fs/buffer.c	Thu Aug 16 12:23:19 2001
+++ linux-akpm/fs/buffer.c	Thu Aug 23 10:15:41 2001
@@ -199,7 +199,7 @@ static void write_locked_buffers(struct 
  * return without it!
  */
 #define NRSYNC (32)
-static int write_some_buffers(kdev_t dev)
+static int write_some_buffers(kdev_t dev, struct buffer_head **start_bh)
 {
 	struct buffer_head *next;
 	struct buffer_head *array[NRSYNC];
@@ -207,6 +207,12 @@ static int write_some_buffers(kdev_t dev
 	int nr;
 
 	next = lru_list[BUF_DIRTY];
+	if (start_bh && *start_bh) {
+		if ((*start_bh)->b_list == BUF_DIRTY)
+			next = *start_bh;
+		brelse(*start_bh);
+		*start_bh = NULL;
+	}
 	nr = nr_buffers_type[BUF_DIRTY] * 2;
 	count = 0;
 	while (next && --nr >= 0) {
@@ -215,8 +221,11 @@ static int write_some_buffers(kdev_t dev
 
 		if (dev && bh->b_dev != dev)
 			continue;
-		if (test_and_set_bit(BH_Lock, &bh->b_state))
+		if (test_and_set_bit(BH_Lock, &bh->b_state)) {
+			/* Shouldn't be on BUF_DIRTY */
+			__refile_buffer(bh);
 			continue;
+		}
 		if (atomic_set_buffer_clean(bh)) {
 			__refile_buffer(bh);
 			get_bh(bh);
@@ -224,6 +233,10 @@ static int write_some_buffers(kdev_t dev
 			if (count < NRSYNC)
 				continue;
 
+			if (start_bh && next) {
+				get_bh(next);
+				*start_bh = next;
+			}
 			spin_unlock(&lru_list_lock);
 			write_locked_buffers(array, count);
 			return -EAGAIN;
@@ -243,9 +256,11 @@ static int write_some_buffers(kdev_t dev
  */
 static void write_unlocked_buffers(kdev_t dev)
 {
+	struct buffer_head *start_bh = NULL;
 	do {
 		spin_lock(&lru_list_lock);
-	} while (write_some_buffers(dev));
+	} while (write_some_buffers(dev, &start_bh));
+	brelse(start_bh);
 	run_task_queue(&tq_disk);
 }
 
@@ -1116,14 +1131,14 @@ int balance_dirty_state(kdev_t dev)
  */
 void balance_dirty(kdev_t dev)
 {
-	int state = balance_dirty_state(dev);
+	int state = balance_dirty_state(NODEV);
 
 	if (state < 0)
 		return;
 
 	/* If we're getting into imbalance, start write-out */
 	spin_lock(&lru_list_lock);
-	write_some_buffers(dev);
+	write_some_buffers(NODEV, NULL);
 
 	/*
 	 * And if we're _really_ out of balance, wait for
@@ -1132,8 +1147,9 @@ void balance_dirty(kdev_t dev)
 	 * This will throttle heavy writers.
 	 */
 	if (state > 0) {
-		wait_for_some_buffers(dev);
+		write_some_buffers(dev, NULL);
 		wakeup_bdflush();
+		wait_for_some_buffers(dev);
 	}
 }
 
@@ -2607,7 +2623,7 @@ static int sync_old_buffers(void)
 		bh = lru_list[BUF_DIRTY];
 		if (!bh || time_before(jiffies, bh->b_flushtime))
 			break;
-		if (write_some_buffers(NODEV))
+		if (write_some_buffers(NODEV, NULL))
 			continue;
 		return 0;
 	}
@@ -2706,7 +2722,7 @@ int bdflush(void *startup)
 		CHECK_EMERGENCY_SYNC
 
 		spin_lock(&lru_list_lock);
-		if (!write_some_buffers(NODEV) || balance_dirty_state(NODEV) < 0) {
+		if (!write_some_buffers(NODEV, NULL) || balance_dirty_state(NODEV) < 0) {
 			wait_for_some_buffers(NODEV);
 			interruptible_sleep_on(&bdflush_wait);
 		}