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@xxxxxxxxxx> 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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