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Date: Tue, 12 Jul 2011 23:19:06 +0800
From: Tao Ma <tm@....ma>
To: Vivek Goyal <vgoyal@...hat.com>
CC: linux-ext4@...r.kernel.org, stable@...nel.org,
Jan Kara <jack@...e.cz>, Jeff Moyer <jmoyer@...hat.com>,
Corrado Zoccolo <czoccolo@...il.com>,
Jens Axboe <jaxboe@...ionio.com>
Subject: Re: [PATCH] jbd/2[stable only]: Use WRITE_SYNC_PLUG in journal_commit_transaction.
On 07/12/2011 08:30 PM, Vivek Goyal wrote:
> On Tue, Jul 12, 2011 at 06:43:51PM +0800, Tao Ma wrote:
>> From: Tao Ma <boyu.mt@...bao.com>
>>
>> In commit 749ef9f8423, we use WRITE_SYNC instead of WRITE in
>> journal_commit_transaction. It causes a much heavy burden for
>> the disk as now the seqenctial write can't be merged(see the blktrace below).
>
> Tao Ma,
>
> Few queries.
>
> - What's the workload you are using for this test.
A very simple one.
mkfs.ext4 -b 2048 /dev/sdx 10000000
sync
mount -t ext4 -o delalloc /dev/sdx /mnt/ext4
dd if=/dev/zero of=/mnt/ext4/a bs=1024K count=1
and run blktrace immediately after the 'dd'. When jbd2 begins to work,
you will get the blktrace output you want.
>
> - Do you see any performance improvement by switching to WRITE_SYNC_PLUG.
I haven't done much tests yet. But I guess if there are many heavy sync
workload, we should suffer from some latency if we dispatch these
sequential write one by one. As I have said, Jens added plug/unplug in
39, and now these sequential write are dispatched in a one request. Run
the same test cases with 3.0-rcX, you will get the same result.
>
> - Why writes are not being merged? Because request got dispatched
> immediately? Do you have logs for insertion of requests also.
You can get it from the above test case.
>
> - WRITE_SYNC_PLUG will plug the queue and expects explicity unplug. Who
> is doing unplug in this case?
See the comments I removed, "we rely on sync_buffer() doing the unplug
for us". I removed them cause we all use pluged write now.
>
> - I am not sure in how many cases we are expecting to submit multiple
> sequential write here.
All the journal write will cause a sequential write to be split to many
requests here. So it would mean too much for metadata heavy test I think.
Thanks
Tao
>
> Thanks
> Vivek
>
>>
>> Given the description of that commit 749ef9f8423, the reason why
>> we use WRITE_SYNC is that it wants to use REQ_NOIDLE and WRITE_SYNC_PLUG
>> also has that flag, so use WRITE_SYNC_PLUG instead. From blktrace,
>> we can get that:
>>
>> without the patch:
>> 8,0 6 18 0.016058423 3342 D W 461101317 + 4 [jbd2/sda11-8]
>> 8,0 6 19 0.016065473 3342 D W 461101321 + 4 [jbd2/sda11-8]
>> 8,0 6 20 0.016070751 3342 D W 461101325 + 4 [jbd2/sda11-8]
>> 8,0 6 21 0.016076180 3342 D W 461101329 + 4 [jbd2/sda11-8]
>> 8,0 6 22 0.016081255 3342 D W 461101333 + 4 [jbd2/sda11-8]
>> 8,0 6 23 0.016085963 3342 D W 461101337 + 4 [jbd2/sda11-8]
>> 8,0 6 24 0.016182048 0 C W 461101317 + 4 [0]
>> 8,0 6 25 0.016190820 0 C W 461101325 + 4 [0]
>> 8,0 6 26 0.016193927 0 C W 461101321 + 4 [0]
>> 8,0 6 27 0.016196532 0 C W 461101333 + 4 [0]
>> 8,0 6 28 0.016199180 0 C W 461101337 + 4 [0]
>> 8,0 6 29 0.016206180 0 C W 461101329 + 4 [0]
>>
>> with this patch:
>> 8,0 4 23 4.320315739 3129 D W 461101317 + 24 [jbd2/sda11-8]
>> 8,0 4 24 4.320364518 0 C W 461101317 + 24 [0]
>>
>> This only affects .37 and .38 since Jens' new plug patches are included
>> in .39 and the bug is removed as a side effect. But I think it is needed
>> anyway for the stable. And RHEL6 needs this also I guess.
>>
>> Cc: stable@...nel.org # 2.6.37 and 2.6.38
>> Cc: Jan Kara <jack@...e.cz>
>> Cc: Vivek Goyal <vgoyal@...hat.com>
>> Cc: Jeff Moyer <jmoyer@...hat.com>
>> Cc: Corrado Zoccolo <czoccolo@...il.com>
>> Cc: Jens Axboe <jaxboe@...ionio.com>
>> Signed-off-by: Tao Ma <boyu.mt@...bao.com>
>> ---
>> fs/jbd/commit.c | 9 +--------
>> fs/jbd2/commit.c | 9 +--------
>> 2 files changed, 2 insertions(+), 16 deletions(-)
>>
>> diff --git a/fs/jbd/commit.c b/fs/jbd/commit.c
>> index 34a4861..6d13df5 100644
>> --- a/fs/jbd/commit.c
>> +++ b/fs/jbd/commit.c
>> @@ -294,7 +294,7 @@ void journal_commit_transaction(journal_t *journal)
>> int first_tag = 0;
>> int tag_flag;
>> int i;
>> - int write_op = WRITE_SYNC;
>> + int write_op = WRITE_SYNC_PLUG;
>>
>> /*
>> * First job: lock down the current transaction and wait for
>> @@ -327,13 +327,6 @@ void journal_commit_transaction(journal_t *journal)
>> spin_lock(&journal->j_state_lock);
>> commit_transaction->t_state = T_LOCKED;
>>
>> - /*
>> - * Use plugged writes here, since we want to submit several before
>> - * we unplug the device. We don't do explicit unplugging in here,
>> - * instead we rely on sync_buffer() doing the unplug for us.
>> - */
>> - if (commit_transaction->t_synchronous_commit)
>> - write_op = WRITE_SYNC_PLUG;
>> spin_lock(&commit_transaction->t_handle_lock);
>> while (commit_transaction->t_updates) {
>> DEFINE_WAIT(wait);
>> diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
>> index f3ad159..fc3840f 100644
>> --- a/fs/jbd2/commit.c
>> +++ b/fs/jbd2/commit.c
>> @@ -329,7 +329,7 @@ void jbd2_journal_commit_transaction(journal_t *journal)
>> int tag_bytes = journal_tag_bytes(journal);
>> struct buffer_head *cbh = NULL; /* For transactional checksums */
>> __u32 crc32_sum = ~0;
>> - int write_op = WRITE_SYNC;
>> + int write_op = WRITE_SYNC_PLUG;
>>
>> /*
>> * First job: lock down the current transaction and wait for
>> @@ -363,13 +363,6 @@ void jbd2_journal_commit_transaction(journal_t *journal)
>> write_lock(&journal->j_state_lock);
>> commit_transaction->t_state = T_LOCKED;
>>
>> - /*
>> - * Use plugged writes here, since we want to submit several before
>> - * we unplug the device. We don't do explicit unplugging in here,
>> - * instead we rely on sync_buffer() doing the unplug for us.
>> - */
>> - if (commit_transaction->t_synchronous_commit)
>> - write_op = WRITE_SYNC_PLUG;
>> trace_jbd2_commit_locking(journal, commit_transaction);
>> stats.run.rs_wait = commit_transaction->t_max_wait;
>> stats.run.rs_locked = jiffies;
>> --
>> 1.7.4
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