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@suse.cz>
Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Link: https://lore.kernel.org/r/20201106035911.1942128-11-harshadshirwadkar@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
index ec516490cb35f84064738eb6d3f1addacf014347..b121d7d434c67510a58c7d962610abc68f3423cc 100644 (file)
--- 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);