On Mon, Jun 03, 2019 at 08:42:38PM +0800, Xiaoguang Wang wrote:
> /proc/fs/jbd2/${device}/info only shows whole average statistical
> info about jbd2's life cycle, but it can not show jbd2 info in
> specified time interval and sometimes this capability is very useful
> for trouble shooting. For example, we can not see how rs_locked and
> rs_flushing grows in specified time interval, but these two indexes
> can explain some reasons for app's behaviours.
We actually had something like this, but we removed it in commit
bf6993276f7: "jbd2: Use tracepoints for history file". The idea was
that you can get the same information using the jbd2_run_tracepoints
# echo jbd2_run_stats > /sys/kernel/debug/tracing/set_event
# cat /sys/kernel/debug/tracing/trace_pipe
... which will produce output like this:
jbd2/vdg-8-293 [000] ...2 122.822487: jbd2_run_stats: dev 254,96 tid 4403 wait 0 request_delay 0 running 4 locked 0 flushing 0 logging 7 handle_count 98 blocks 3 blocks_logged 4
jbd2/vdg-8-293 [000] ...2 122.833101: jbd2_run_stats: dev 254,96 tid 4404 wait 0 request_delay 0 running 14 locked 0 flushing 0 logging 4 handle_count 198 blocks 1 blocks_logged 2
jbd2/vdg-8-293 [000] ...2 122.839325: jbd2_run_stats: dev 254,96 tid 4405 wait
With eBPF, we should be able to do something even more user friendly.
BTW, if you are looking to try to optimize jbd2, a good thing to do is
to take a look at jbd2_handle_stats, filtered on ones where the
interval is larger than some cut-off. Ideally, the time between a
handle getting started and stopped should be as small as possible,
because if a transaction is trying to close, an open handle will get
in the way of that, and other CPU's will be stuck waiting for handle
to complete. This means that pre-reading blocks before starting a
handle, etc., is a really good idea. And monitoring jbd2_handle_stats
is a good way to find potential spots to topimize in ext4.
- Ted
