linux-kernel - Re: [PATCH] improve jbd fsync batching

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Message-ID: <20081103202442.GA25736@unused.rdu.redhat.com>
Date:	Mon, 3 Nov 2008 15:24:43 -0500
From:	Josef Bacik <jbacik@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Josef Bacik <jbacik@...hat.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	rwheeler@...hat.com
Subject: Re: [PATCH] improve jbd fsync batching

On Mon, Nov 03, 2008 at 12:27:29PM -0800, Andrew Morton wrote:
> On Tue, 28 Oct 2008 16:16:15 -0400
> Josef Bacik <jbacik@...hat.com> wrote:
> 
> > Hello,
> > 
> > This is a rework of the patch I did a few months ago, taking into account some
> > comments from Andrew and using the new schedule_hrtimeout function (thanks
> > Arjan!).
> > 
> > There is a flaw with the way jbd handles fsync batching.  If we fsync() a file
> > and we were not the last person to run fsync() on this fs then we automatically
> > sleep for 1 jiffie in order to wait for new writers to join into the transaction
> > before forcing the commit.  The problem with this is that with really fast
> > storage (ie a Clariion) the time it takes to commit a transaction to disk is way
> > faster than 1 jiffie in most cases, so sleeping means waiting longer with
> > nothing to do than if we just committed the transaction and kept going.  Ric
> > Wheeler noticed this when using fs_mark with more than 1 thread, the throughput
> > would plummet as he added more threads.
> > 
> > ...
> >
> > ...
> >  
> > @@ -49,6 +50,7 @@ get_transaction(journal_t *journal, transaction_t *transaction)
> >  {
> >  	transaction->t_journal = journal;
> >  	transaction->t_state = T_RUNNING;
> > +	transaction->t_start_time = ktime_get();
> >  	transaction->t_tid = journal->j_transaction_sequence++;
> >  	transaction->t_expires = jiffies + journal->j_commit_interval;
> >  	spin_lock_init(&transaction->t_handle_lock);
> > @@ -1371,7 +1373,7 @@ int journal_stop(handle_t *handle)
> >  {
> >  	transaction_t *transaction = handle->h_transaction;
> >  	journal_t *journal = transaction->t_journal;
> > -	int old_handle_count, err;
> > +	int err;
> >  	pid_t pid;
> >  
> >  	J_ASSERT(journal_current_handle() == handle);
> > @@ -1407,11 +1409,26 @@ int journal_stop(handle_t *handle)
> >  	 */
> >  	pid = current->pid;
> >  	if (handle->h_sync && journal->j_last_sync_writer != pid) {
> 
> It would be nice to have a comment here explaining the overall design. 
> it's a bit opaque working that out from the raw implementation.
> 
> > +		u64 commit_time, trans_time;
> > +
> >  		journal->j_last_sync_writer = pid;
> > -		do {
> > -			old_handle_count = transaction->t_handle_count;
> > -			schedule_timeout_uninterruptible(1);
> > -		} while (old_handle_count != transaction->t_handle_count);
> > +
> > +		spin_lock(&journal->j_state_lock);
> > +		commit_time = journal->j_average_commit_time;
> > +		spin_unlock(&journal->j_state_lock);
> 
> OK, the lock is needed on 32-bit machines, I guess.
> 
> 
> > +		trans_time = ktime_to_ns(ktime_sub(ktime_get(),
> > +						   transaction->t_start_time));
> > +
> > +		commit_time = min_t(u64, commit_time,
> > +				    1000*jiffies_to_usecs(1));
> 
> OK.  The multiplication of an unsigned by 1000 could overflow, but only
> if HZ is less than 0.25.  I don't think we need worry about that ;)
> 
> 
> > +		if (trans_time < commit_time) {
> > +			ktime_t expires = ktime_add_ns(ktime_get(),
> > +						       commit_time);
> > +			set_current_state(TASK_UNINTERRUPTIBLE);
> > +			schedule_hrtimeout(&expires, HRTIMER_MODE_ABS);
> 
> We should have schedule_hrtimeout_uninterruptible(), but we don't.
> 
> > +		}
> >  	}
> >  
> >  	current->journal_info = NULL;
> > diff --git a/include/linux/jbd.h b/include/linux/jbd.h
> > index 346e2b8..d842230 100644
> > --- a/include/linux/jbd.h
> > +++ b/include/linux/jbd.h
> > @@ -543,6 +543,11 @@ struct transaction_s
> >  	unsigned long		t_expires;
> >  
> >  	/*
> > +	 * When this transaction started, in nanoseconds [no locking]
> > +	 */
> > +	ktime_t			t_start_time;
> > +
> > +	/*
> >  	 * How many handles used this transaction? [t_handle_lock]
> >  	 */
> >  	int t_handle_count;
> > @@ -800,6 +805,8 @@ struct journal_s
> >  
> >  	pid_t			j_last_sync_writer;
> >  
> > +	u64			j_average_commit_time;
> 
> Every field in that structure is carefully documented (except for
> j_last_sync_writer - what vandal did that?)
> 
> please fix.

I see you already pulled this into -mm, would you like me to repost with the
same changelog and the patch updated, or just reply to this with the updated
patch?  Thanks,

Josef
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