From: Josef Bacik Subject: Re: [PATCH] improve jbd fsync batching Date: Mon, 3 Nov 2008 15:24:43 -0500 Message-ID: <20081103202442.GA25736@unused.rdu.redhat.com> References: <20081028201614.GA21600@unused.rdu.redhat.com> <20081103122729.60582692.akpm@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Josef Bacik , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, rwheeler@redhat.com To: Andrew Morton Return-path: Received: from mx2.redhat.com ([66.187.237.31]:39115 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752675AbYKCUhf (ORCPT ); Mon, 3 Nov 2008 15:37:35 -0500 Content-Disposition: inline In-Reply-To: <20081103122729.60582692.akpm@linux-foundation.org> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Mon, Nov 03, 2008 at 12:27:29PM -0800, Andrew Morton wrote: > On Tue, 28 Oct 2008 16:16:15 -0400 > Josef Bacik 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