Re: [HACKERS] PATCH: Keep one postmaster monitoring pipe per process

From: Marco Pfatschbacher <Marco_Pfatschbacher(at)genua(dot)de>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [HACKERS] PATCH: Keep one postmaster monitoring pipe per process
Date: 2017-12-04 16:55:25
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On Thu, Sep 15, 2016 at 04:40:00PM -0400, Tom Lane wrote:
> Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com> writes:
> > Very interesting. Perhaps that is why NetBSD shows a speedup with the
> > kqueue patch[1] but FreeBSD doesn't. I guess that if I could get the
> > kqueue patch to perform better on large FreeBSD systems, it would also
> > be a solution to this problem.
> I just noticed that kqueue appears to offer a solution to this problem,
> ie one of the things you can wait for is exit of another process (named
> by PID, looks like). If that's portable to all kqueue platforms, then
> integrating a substitute for the postmaster death pipe might push that
> patch over the hump to being a net win.
> regards, tom lane


sorry for the long silence.
I forgot about this after being on vacation.

kqueue does indeed support EVFILT_PROC like you said.
But using this effectively, would need a separate monitoring process,
because we cannot use poll(2) and kevent(2) together within the same loop.

But how about this:
We just use getppid to see whether the process id of our parent
has changed to 1 (init).
Just like calling read on the pipe, that's just one systemcall.

I did not bother to weed the postmaster_alive_fds yet, but
this change works fine for me.


diff --git a/src/backend/storage/ipc/pmsignal.c b/src/backend/storage/ipc/pmsignal.c
index 85db6b21f8..a0596c61fd 100644
--- a/src/backend/storage/ipc/pmsignal.c
+++ b/src/backend/storage/ipc/pmsignal.c
@@ -272,21 +272,16 @@ bool
#ifndef WIN32
- char c;
- ssize_t rc;
+ pid_t ppid;

- rc = read(postmaster_alive_fds[POSTMASTER_FD_WATCH], &c, 1);
- if (rc < 0)
+ ppid = getppid();
+ if (ppid == 1)
- if (errno == EAGAIN || errno == EWOULDBLOCK)
- return true;
- else
- elog(FATAL, "read on postmaster death monitoring pipe failed: %m");
+ elog(FATAL, "postmaster died: our ppid changed to init");
+ return false;
- else if (rc > 0)
- elog(FATAL, "unexpected data in postmaster death monitoring pipe");

- return false;
+ return true;
#else /* WIN32 */
return (WaitForSingleObject(PostmasterHandle, 0) == WAIT_TIMEOUT);
#endif /* WIN32 */

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