linux-ext4 - [PATCH v2 10/22] jbd2: add todo for a fast commit performance optimization

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Message-Id: <20201106035911.1942128-11-harshadshirwadkar@gmail.com>
Date:   Thu,  5 Nov 2020 19:58:59 -0800
From:   Harshad Shirwadkar <harshadshirwadkar@...il.com>
To:     linux-ext4@...r.kernel.org
Cc:     tytso@....edu, Harshad Shirwadkar <harshadshirwadkar@...il.com>,
        Jan Kara <jack@...e.cz>
Subject: [PATCH v2 10/22] jbd2: add todo for a fast commit  performance optimization

Fast commit performance can be optimized if commit thread doesn't wait
for ongoing fast commits to complete until the transaction enters
T_FLUSH state. Document this optimization.

Suggested-by: Jan Kara <jack@...e.cz>
Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@...il.com>
---
 fs/jbd2/commit.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
index ec516490cb35..b121d7d434c6 100644
--- a/fs/jbd2/commit.c
+++ b/fs/jbd2/commit.c
@@ -450,6 +450,15 @@ void jbd2_journal_commit_transaction(journal_t *journal)
 		schedule();
 		write_lock(&journal->j_state_lock);
 		finish_wait(&journal->j_fc_wait, &wait);
+		/*
+		 * TODO: by blocking fast commits here, we are increasing
+		 * fsync() latency slightly. Strictly speaking, we don't need
+		 * to block fast commits until the transaction enters T_FLUSH
+		 * state. So an optimization is possible where we block new fast
+		 * commits here and wait for existing ones to complete
+		 * just before we enter T_FLUSH. That way, the existing fast
+		 * commits and this full commit can proceed parallely.
+		 */
 	}
 	write_unlock(&journal->j_state_lock);
 
-- 
2.29.1.341.ge80a0c044ae-goog