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

On Thu, Sep 15, 2016 at 04:40:00PM -0400, Tom Lane wrote:
> Thomas Munro <thomas.munro@enterprisedb.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

Hi,

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.

Regards,
   Marco

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
 PostmasterIsAlive(void)
 {
 #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 */