Re: Log connection establishment timings - Mailing list pgsql-hackers

Thanks for the continued review!

On Wed, Feb 26, 2025 at 2:41 AM Bertrand Drouvot
<bertranddrouvot.pg@gmail.com> wrote:
>
> On Wed, Feb 26, 2025 at 01:46:19PM +0900, Fujii Masao wrote:
> >
> > With the current patch, when log_connections is enabled, the connection time is always
> > captured, and which might introduce performance overhead. No? Some users who enable
> > log_connections may not want this extra detail and want to avoid such overhead.
> > So, would it make sense to extend log_connections with a new option like "timing" and
> > log the connection time only when "timing" is specified?
>
> +1, I also think it's a good idea to let users decide if they want the timing
> measurement overhead (and it's common practice with track_io_timing,
> track_wal_io_timing, the newly track_cost_delay_timing for example)

It seems to me like the extra timing collected and the one additional
log message isn't enough overhead to justify its own guc (for now).

> > Including the PID seems unnecessary since it's already available via log_line_prefix with %p?
>
> Yeah, we would get things like:
>
> [1111539] LOG:  connection received: host=[local]
> [1111539] LOG:  connection authenticated: user="postgres" method=trust
(/home/postgres/postgresql/pg_installed/pg18/data/pg_hba.conf:117)
> [1111539] LOG:  connection authorized: user=postgres database=postgres application_name=psql
> [1111539] LOG:  backend ready for query. pid=1111539. socket=9. connection establishment times (ms): total: 2, fork:
0,authentication: 0 
>
> I also wonder if "backend ready for query" is worth it. Maybe something like:
>
> 2025-02-26 06:44:23.265 UTC [1111539] LOG:  connection establishment times (ms): total: 2, fork: 0, authentication: 0
>
> would be good enough?

Yes, thank you. v5 attached in [1] changes the wording as you recommend..

> +typedef struct ConnectionTiming
> +{
> +       instr_time      fork_duration;
> +       instr_time      auth_duration;
> +} ConnectionTiming;
>
> As it's all about instr_time, I wonder if we could use an enum + array instead.
> That's probably just a matter of taste but that sounds more flexible to extend
> (should we want to add more timing in the future).

I think we can change it later if we add many more. For now I prefer
the clarity of accessing members by name. Especially because we don't
have any code yet that loops through all of them or anything like
that.

> +ConnectionTiming conn_timing = {0};
>
> There is no padding in ConnectionTiming and anyway we just access its fields
> so that's ok to initialize that way.

Yes, this properly zero initializes the struct. In fact it shouldn't
be needed since a global like this should be zero initialized. But all
of the globals defined above conn_timing zero initialize themselves,
so I thought I would be consistent with them.

> Add a few words in the log_connections GUC doc? (anyway we will have to if
> Fujii-san idea above about the timing is implemented)

I forgot to do this in v5 attached in [1]. Let me go ahead and do this next.

> +               /* Calculate total fork duration in child backend for logging */
> +               if (child_type == B_BACKEND)
> +               {
> +                       INSTR_TIME_SET_CURRENT(conn_timing.fork_duration);
> +                       INSTR_TIME_SUBTRACT(conn_timing.fork_duration,
> +                                                               ((BackendStartupData *) startup_data)->fork_time);
> +               }
> +
>                 /* Close the postmaster's sockets */
>                 ClosePostmasterPorts(child_type == B_LOGGER);
>
> @@ -618,6 +630,14 @@ SubPostmasterMain(int argc, char *argv[])
>         /* Read in the variables file */
>         read_backend_variables(argv[2], &startup_data, &startup_data_len);
>
> +       /* Calculate total fork duration in child backend for logging */
> +       if (child_type == B_BACKEND)
> +       {
> +               INSTR_TIME_SET_CURRENT(conn_timing.fork_duration);
> +               INSTR_TIME_SUBTRACT(conn_timing.fork_duration,
> +                               ((BackendStartupData *) startup_data)->fork_time);
> +       }
>
> worth to add a helper function to avoid code duplication?

I've added INSTR_TIME_GET_DURATION_SINCE(start_time). Which I like
because it seems generally useful. It does not however cut down on
LOC, so I'm somewhat on the fence.

- Melanie

[1] https://www.postgresql.org/message-id/CAAKRu_Y9sgZAWCiQoHtpwx6Mv28fBGav5ztrWyeSrx%2BB%3DACN6g%40mail.gmail.com