Re: Log connection establishment timings

On 26/02/2025 08:41, Bertrand Drouvot wrote:
> Hi,
>
> On Wed, Feb 26, 2025 at 01:46:19PM +0900, Fujii Masao wrote:
>>
>>
>> On 2025/02/26 6:36, Melanie Plageman wrote:
>>> On Tue, Feb 25, 2025 at 3:23 PM Melanie Plageman
>>> <melanieplageman(at)gmail(dot)com> wrote:
>>>>
>>>> Thanks for doing this! I have implemented your suggestion in attached v3.
>
> Thanks for the new patch version!
>

+1

>> + /* Capture time Postmaster initiates fork for logging */
>> + if (child_type == B_BACKEND)
>> + INSTR_TIME_SET_CURRENT(((BackendStartupData *) startup_data)->fork_time);
>>
>> When log_connections is enabled, walsender connections are also logged.
>> However, with the patch, it seems the connection time for walsenders isn't captured.
>> Is this intentional?
>
> Good point. I'm tempted to say that it should also be, specially because a
> connection done as "psql replication=database" is of "walsender" backend type and
> would not be reported.
>

Agreed.

>> 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)
>

track_connection_delay_timing ? I'm fine with this, but I'm a bit afraid
that it will lead us to an awful lot of GUCs for simple things.

>> + ereport(LOG,
>> + errmsg("backend ready for query. pid=%d. socket=%d. connection establishment times (ms): total: %ld, fork: %ld, authentication: %ld",
>> + MyProcPid,
>> + (int) MyClientSocket->sock,
>>
>> Why expose the socket's file descriptor? I'm not sure how users would use that information.
>>
>>
>> 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?
>

Sounds definitely better to me.

> A few random comments:
>
> === 1
>
> +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).
>
> === 2
>
> +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.
>
> === 3
>
> 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)
>
> === 4
>
> + /* 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?
>
> Regards,
>

--
Guillaume Lelarge
Consultant
https://dalibo.com