huge fsync latencies for a small file on ext4

I am observing huge fsync latencies for a small file under the below test scenario -

process A -
Issue async write of 4GB using dd command (say large_file) on /data mounted with ext4:
dd if=/dev/zero of=/data/testfile bs=1M count=4096

process B -
In parallel another process wrote a small 4KB data to another file (say, small_file) and has issued fsync on this file.

Problem -
The fsync() on 4KB file, is taking upto ~30sec (worst case latency). This is tested on an eMMC based device.

Observations -
This happens when the small_file and large_file both are part of the same committing transaction or when the small_file is part of the running transaction
while large_file is part of the committing transaction.

During the commit of a transaction which includes large_file, the jbd2 thread
does journal_finish_inode_data_buffers() by calling filemap_fdatawait_keep_errors() on the file's inode address space. While this is
happening, if the writeback thread is running in parallel for the large_file, then
filemap_fdatawait_keep_errors() could potentially run in a loop of all the pages (upto 4GB of data) and also wait for all the file's data to be written to the disk in the current transaction context itself. At the time

of calling journal_finish_inode_data_buffers(), the file size is of only 150MB.
and by the time filemap_fdatawait_keep_errors() returns, the file size is 4GB
and the page index also points to 4GB file offset in __filemap_fdatawait_range(), indicating that is has scanned and waited for writeback
all the pages upto 4GB and not just 150MB.

Ideally, I think the jbd2 thread should have waited for only the amount of data
it has submitted as part of the current transaction and not to wait for the on-going pages that are getting tagged for writeback in parallel in another context.
So along these lines, I have tried to use the inode's size at the time of calling
journal_finish_inode_data_buffers() as below -

fs/jbd2/commit.c +2 -2

index 2eb55c3..e86cf67 100644
@@ -261,8 +261,8 @@ static int journal_finish_inode_data_buffers(journal_t
 *journal,
                        continue;
                jinode->i_flags |= JI_COMMIT_RUNNING;
                spin_unlock(&journal->j_list_lock);
-               err = filemap_fdatawait_keep_errors(
-                               jinode->i_vfs_inode->i_mapping);
+               err =
 filemap_fdatawait_range(jinode->i_vfs_inode->i_mapping,
+                               0,
 i_size_read(jinode->i_vfs_inode->i_mapping->host));

                if (!ret)
                        ret = err;
                spin_lock(&journal->j_list_lock);

With this, the fsync latencies for small_file have reduced significantly. It took upto max ~5sec (worst case latency).

Although this is seen in a test code, this could potentially impact the phone's performance if any application or main UI thread in Android issues fsync() in foreground while a large data transfer is going on in another context.

Request you to share your thoughts and comments on this issue and the fix suggested above.

Thanks,
Sahitya.

--
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.

Hi!

Quoted 32 lines · On Tue 19-02-19 15:50:23, [email protected] wrote:

I am observing huge fsync latencies for a small file under the below test scenario -

process A -
Issue async write of 4GB using dd command (say large_file) on /data mounted with ext4:
dd if=/dev/zero of=/data/testfile bs=1M count=4096

process B -
In parallel another process wrote a small 4KB data to another file (say, small_file) and has issued fsync on this file.

Problem -
The fsync() on 4KB file, is taking upto ~30sec (worst case latency). This is tested on an eMMC based device.

Observations -
This happens when the small_file and large_file both are part of the same committing transaction or when the small_file is part of the running transaction
while large_file is part of the committing transaction.

During the commit of a transaction which includes large_file, the jbd2 thread
does journal_finish_inode_data_buffers() by calling filemap_fdatawait_keep_errors() on the file's inode address space. While this is
happening, if the writeback thread is running in parallel for the large_file, then
filemap_fdatawait_keep_errors() could potentially run in a loop of all the pages (upto 4GB of data) and also wait for all the file's data to be written to the disk in the current transaction context itself. At the time

Quoted 8 lines

of calling journal_finish_inode_data_buffers(), the file size is of only 150MB.
and by the time filemap_fdatawait_keep_errors() returns, the file size is 4GB
and the page index also points to 4GB file offset in __filemap_fdatawait_range(), indicating that is has scanned and waited for writeback
all the pages upto 4GB and not just 150MB.

Thanks for the detailed analysis! I'm somewhat surprised that the flusher is able to submit new batch of pages for writeback faster than __filemap_fdatawait_range() is scanning the radix tree but it is certainly a possibility.

Quoted 8 lines

Ideally, I think the jbd2 thread should have waited for only the amount of data
it has submitted as part of the current transaction and not to wait for the on-going pages that are getting tagged for writeback in parallel in another context.
So along these lines, I have tried to use the inode's size at the time of calling
journal_finish_inode_data_buffers() as below -

One has to be really careful when using i_size like this. By the time the transaction is committing, i_size could have been reduced from the value at the time page writeback was issued. And that change will be journalled only in the following transaction. So if the system crashes in the wrong moment, user could see uninitialized blocks between new_size and old_size after journal replay. So I don't think your patch is really correct.

Ted has outlined a plan how to get rid of data=ordered limitations [1] and thus also this problem. It is quite some work but you're certainly welcome to help out :)

Honza

[1] https://www.spinics.net/lists/linux-ext4/msg64175.html

Quoted 39 lines

fs/jbd2/commit.c +2 -2

index 2eb55c3..e86cf67 100644
@@ -261,8 +261,8 @@ static int journal_finish_inode_data_buffers(journal_t
 *journal,
                        continue;
                jinode->i_flags |= JI_COMMIT_RUNNING;
                spin_unlock(&journal->j_list_lock);
-               err = filemap_fdatawait_keep_errors(
-                               jinode->i_vfs_inode->i_mapping);
+               err =
 filemap_fdatawait_range(jinode->i_vfs_inode->i_mapping,
+                               0,
 i_size_read(jinode->i_vfs_inode->i_mapping->host));

                if (!ret)
                        ret = err;
                spin_lock(&journal->j_list_lock);

With this, the fsync latencies for small_file have reduced significantly. It took upto max ~5sec (worst case latency).

Although this is seen in a test code, this could potentially impact the phone's performance if any application or main UI thread in Android issues fsync() in foreground while a large data transfer is going on in another context.

Request you to share your thoughts and comments on this issue and the fix suggested above.

Thanks,
Sahitya.

--
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.

Signature · 2 lines

Jan Kara <[email protected]>
SUSE Labs, CR

Quoted 7 lines · Jan Kara

One has to be really careful when using i_size like this. By the time the transaction is committing, i_size could have been reduced from the value at the time page writeback was issued. And that change will be journalled only in the following transaction. So if the system crashes in the wrong moment, user could see uninitialized blocks between new_size and old_size after journal replay. So I don't think your patch is really correct.

Thanks Jan for the clarification on the patch. I agree with your comments.

From that discussion, I think the problem that it is discussing is w.r.t journal thread waiting for on-going active transaction updates to be done and thus causing commit latencies.

And I think the proposal is to do not
hold any handle while extents are being mapped in ext4_map_blocks() but defer it till IO is completely done.

And with the new proposal since the inode will be added to transaction->t_inode_list only after the IO is completed, there will be no longer the need to do journal_finish_inode_data_buffers() in the journal context and thus this problem also will not be observed? Is my understanding correct, please clarify.

Quoted 6 lines

Ted has outlined a plan how to get rid of data=ordered limitations [1] and thus also this problem. It is quite some work but you're certainly welcome to help out :)

[1] https://www.spinics.net/lists/linux-ext4/msg64175.html