Re: Terminate the idle sessions

On Nov 17, 2020, at 2:07 PM, kuroda(dot)hayato(at)fujitsu(dot)com<mailto:kuroda(dot)hayato(at)fujitsu(dot)com> wrote:

Dear Li, David,

> Additionally, using postgres_fdw within the server doesn't cause issues,
> its using postgres_fdw and the remote server having this setting set to zero that causes a problem.

I didn't know the fact that postgres_fdw can use within the server... Thanks.

I read optimize-setitimer patch, and looks basically good. I put what I understanding,
so please confirm it whether your implementation is correct.
(Maybe I missed some simultaneities, so please review anyone...)

[besic consept]

sigalrm_due_at means the time that interval timer will ring, and sigalrm_delivered means who calls schedule_alarm().
If fin_time of active_timeouts[0] is larger than or equal to sigalrm_due_at,
stop calling setitimer because handle_sig_alarm() will be call sooner.

Agree. The sigalrm_delivered means a timer has been handled by handle_sig_alarm(), so we should call setitimer() in next schedule_alarm().

[when call setitimer]

In the attached patch, setitimer() will be only called the following scenarios:

* when handle_sig_alarm() is called due to the pqsignal
* when a timeout is registered and its fin_time is later than active_timeous[0]
* when disable a timeout
* when handle_sig_alarm() is interrupted and rescheduled(?)

According to comments, handle_sig_alarm() may be interrupted because of the ereport.
I think if handle_sig_alarm() is interrupted before subsutituting sigalrm_due_at to true,
interval timer will be never set. Is it correct, or is my assumption wrong?

I’m not familiar with the system interrupt, however, the sigalrm_due_at is subsutitue between HOLD_INTERRUPTS()
and RESUM_INTERRUPTS(), so I think it cannot be interrupted. The following comments comes from miscadmin.h.

> The HOLD_INTERRUPTS() and RESUME_INTERRUPTS() macros
> allow code to ensure that no cancel or die interrupt will be accepted,
> even if CHECK_FOR_INTERRUPTS() gets called in a subroutine. The interrupt
> will be held off until CHECK_FOR_INTERRUPTS() is done outside any
> HOLD_INTERRUPTS() ... RESUME_INTERRUPTS() section.

Lastly, I found that setitimer is obsolete and should change to another one. According to my man page:

```
POSIX.1-2001, SVr4, 4.4BSD (this call first appeared in 4.2BSD).
POSIX.1-2008 marks getitimer() and setitimer() obsolete,
recommending the use of the POSIX timers API (timer_gettime(2), timer_settime(2), etc.) instead.
```

Do you have an opinion for this? I think it should be changed
if all platform can support timer_settime system call, but this fix affects all timeouts,
so more considerations might be needed.

Not sure! I find that Win32 do not support setitimer(), PostgreSQL emulate setitimer() by creating a persistent thread to handle
the timer setting and notification upon timeout.

So if we want to replace it, I think we should open a new thread to achieve this.

--
Best regards
Japin Li