On Tue, Sep 08, 2009 at 07:50:35AM -0400, Ric Wheeler wrote:
quoted text
>>
>> So here's what we do on a non-async commit:
>>
>> This is what we do with an async commit:
>>
>> That's the only difference at this point.  The fatal flaw with async
>> commit from before was this that we weren't writing the commit block
>> in step (2) with a barrier --- and that *was* disastrous, since it
>> meant the equivalent of mounting with barrier=0.
>
> I think that the difference is basically that in the original mode,  
> waiting for stage (2) to finish means that our commit block will never  
> hit the storage before the dependent data is committed. Remember that  
> barriers are actually 2 CACHE_FLUSH_EXT commands - one before the  
> flagged barrier IO is issued and one afterwards.

I didn't realize that doing an ordered write meant that we had a
barrier *before* and *after* the commit block; I didn't realiuze it
was quite that strong.  I thought an ordered write only put a barrier
*after* the commit block.  Looking more closely, you're right, and
that actually explains why I wasn't see that much of a difference with
and without journal_async_write.

The fact that an ordered write puts barriers before and after the
commit means that right now the two scenarios above are in fact
*identical*.

So here's a respin of the fix-async-journal patch that changes what we
do from:

1)  Write the journal data, revoke, and descriptor blocks
2)  Wait for the block I/O layer to signal that all of these blocks
     have been written out --- *without* a barrier
3)  Write the commit block in ordered mode
4)  Wait for the I/O to commit block to be done

To this (in journal_async_commit):

1)  Write the journal data, revoke, and descriptor blocks
2)  Write the commit block (with a checksum) without setting ordered mode
3)  Send an empty barrier bio (so we only send a *single* CACHE_FLUSH_EXT)
4)  Wait for the I/O to in steps (1) and (2) to be done

This *does* show significant improvements:

Using ./fs_mark  -d  /mnt  -s  10240  -n  1000 

W/o journal_async_commit:

FSUse%        Count         Size    Files/sec     App Overhead
     8         1000        10240         30.5            28242

w/ journal_async_commit:

     8         1000        10240         45.8            28620

w/ barrier=0

     8         1000        10240        320.0            27699


Since this patch is a bit more complicated, I'll hold off on making it
be the default for now, but if the testing goes well, I plan to make
it default in the next kernel release, since an increase of 50% of
fs_mark is something I think we all would agree counts as a "clear
performance advantage".  :-)

							- Ted

commit fd67d1cfd73f554bae6c37745222eac2723983c8
Author: Theodore Ts'o <tytso@mit.edu>
Date:   Thu Sep 10 22:34:27 2009 -0400

    ext4: Fix async commit mode to be safe by using a barrier
    
    Previously the journal_async_commit mount option was equivalent to
    using barrier=0 (and just as unsafe).  This patch fixes it so that we
    eliminate the barrier before the commit block (by not using ordered
    mode), and explicitly issuing an empty barrier bio after writing the
    commit block.  Because of the journal checksum, it is safe to do this;
    if the journal blocks are not all written before a power failure, the
    checksum in the commit block will prevent the last transaction from
    being replayed.
    
    Using the fs_mark benchmark, using journal_async_commit shows a 50%
    improvement:
    
    FSUse%        Count         Size    Files/sec     App Overhead
         8         1000        10240         30.5            28242
    
    vs.
    
    FSUse%        Count         Size    Files/sec     App Overhead
         8         1000        10240         45.8            28620
    
    
    Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
index 7b4088b..d6f4763 100644
--- a/fs/jbd2/commit.c
+++ b/fs/jbd2/commit.c
@@ -25,6 +25,7 @@
 #include <linux/writeback.h>
 #include <linux/backing-dev.h>
 #include <linux/bio.h>
+#include <linux/blkdev.h>
 #include <trace/events/jbd2.h>
 
 /*
@@ -83,6 +84,34 @@ nope:
 	__brelse(bh);
 }
 
+static void end_empty_barrier(struct bio *bio, int err)
+{
+	if (err) {
+		if (err == -EOPNOTSUPP)
+			set_bit(BIO_EOPNOTSUPP, &bio->bi_flags);
+		clear_bit(BIO_UPTODATE, &bio->bi_flags);
+	}
+	complete(bio->bi_private);
+}
+
+struct bio *issue_flush(struct block_device *bdev, struct completion *wait)
+{
+
+	struct bio *bio;
+
+	if (!bdev->bd_disk || !bdev->bd_disk->queue)
+		return NULL;
+
+	bio = bio_alloc(GFP_KERNEL, 0);
+	if (!bio)
+		return NULL;
+	bio->bi_end_io = end_empty_barrier;
+	bio->bi_private = wait;
+	bio->bi_bdev = bdev;
+	submit_bio(WRITE_BARRIER, bio);
+	return bio;
+}
+
 /*
  * Done it all: now submit the commit record.  We should have
  * cleaned up our previous buffers by now, so if we are in abort
@@ -133,8 +162,8 @@ static int journal_submit_commit_record(journal_t *journal,
 	bh->b_end_io = journal_end_buffer_io_sync;
 
 	if (journal->j_flags & JBD2_BARRIER &&
-		!JBD2_HAS_INCOMPAT_FEATURE(journal,
-					 JBD2_FEATURE_INCOMPAT_ASYNC_COMMIT)) {
+	    !JBD2_HAS_INCOMPAT_FEATURE(journal,
+				       JBD2_FEATURE_INCOMPAT_ASYNC_COMMIT)) {
 		set_buffer_ordered(bh);
 		barrier_done = 1;
 	}
@@ -352,6 +381,8 @@ void jbd2_journal_commit_transaction(journal_t *journal)
 	transaction_t *commit_transaction;
 	struct journal_head *jh, *new_jh, *descriptor;
 	struct buffer_head **wbuf = journal->j_wbuf;
+	struct bio *bio_flush = NULL;
+	DECLARE_COMPLETION_ONSTACK(wait_flush);
 	int bufs;
 	int flags;
 	int err;
@@ -707,11 +738,13 @@ start_journal_io:
 	/* Done it all: now write the commit record asynchronously. */
 
 	if (JBD2_HAS_INCOMPAT_FEATURE(journal,
-		JBD2_FEATURE_INCOMPAT_ASYNC_COMMIT)) {
+				      JBD2_FEATURE_INCOMPAT_ASYNC_COMMIT)) {
 		err = journal_submit_commit_record(journal, commit_transaction,
 						 &cbh, crc32_sum);
 		if (err)
 			__jbd2_journal_abort_hard(journal);
+		if (journal->j_flags & JBD2_BARRIER)
+			bio_flush = issue_flush(journal->j_dev, &wait_flush);
 	}
 
 	/*
@@ -833,8 +866,13 @@ wait_for_iobuf:
 
 	jbd_debug(3, "JBD: commit phase 5\n");
 
-	if (!JBD2_HAS_INCOMPAT_FEATURE(journal,
-		JBD2_FEATURE_INCOMPAT_ASYNC_COMMIT)) {
+	if (JBD2_HAS_INCOMPAT_FEATURE(journal,
+				      JBD2_FEATURE_INCOMPAT_ASYNC_COMMIT)) {
+		if (bio_flush) {
+			wait_for_completion(&wait_flush);
+			bio_put(bio_flush);
+		}
+	} else {
 		err = journal_submit_commit_record(journal, commit_transaction,
 						&cbh, crc32_sum);
 		if (err)
--
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