Right, it should be very impossible for it
to happen in practice. We probably are not lucky enough to see it happen at
system reboot, but putting the pidfile in a non-persistent area across reboot
will help solve the problem there. Thanks,
[mailto:address@hidden On Behalf Of Martin Pala
Sent: Tuesday, June 12, 2012 12:53
To: This is the general mailing
list for monit
Subject: Re: Monit not able to
start after box reboot
Yes that could happen in theory - i have never seen it in practice, as
on normal platform the PID is not recycled immediately, but the PID is assigned
from incremental sequence. The PID gets reused in the future when the sequence
It seems that your platform is aggressive on PID recycling - what
platform and kernel it is?
The solution for the services check on platform that reuses the PIDs so
quickly is to use the pattern based process check which doesn't depend on the
pidfile ("check process myproc matching …")
On Jun 12, 2012, at 5:56 AM, Fei, Yuming wrote:
The same problem exists with the service
pid files too. Further more, it seems that the use of pidfiles in below, as
shown in many examples, does not really work:
check process ntpd with pidfile
start program =
stop program =
Because there is always a chance that the
process dies, and another process starts and obtains the same pid before monit
detects it. Monit may send out an alert if the ppid changes, but it will think
that the process is running and won’t run the start program. This may happen at
system reboot time but can also happen at run time, in which case, the tmpfs
solution may not help.
If this is true, it may explain what I
have also seen: after system reboot, monit does not always start the processes…
Maybe I am missing something here, but
please let me know.
From: Fei, Yuming
Sent: Monday, June 11, 2012 4:59 PM
To: 'This is the general mailing list for
Subject: RE: Monit not able to start after box
Thanks Martin, placing the pidfile on
tmpfs will help. So, is there a way to avoid this problem if placing the
pidfile on disk, or currently it shouldn’t be a disk file since it is
persistent across reboot?
From: address@hidden [mailto:monit-general-bounces+yfei=address@hidden] On
Behalf Of Martin Pala
Sent: Monday, June 11, 2012 4:32 PM
To: This is the general mailing list for
Subject: Re: Monit not able to start after box
If you run the monit binary, it checks whether the daemon is running
already by reading the PID from its pidfile and looking for the given process.
It seems that after your system rebooted, some other process obtained the same
PID, so monit thinks that it is running and doesn't daemonize itself.
The solution could be to place the pidfile on tmpfs (memory based
filesystem), which is not persistent across reboot => the file will
disappear when you reboot the system.
The pidfile location can be set with the "set pidfile"
On Jun 11, 2012, at 11:24 PM, Fei, Yuming wrote:
I have seen a problem that monit is not
able to start after the box reboot. The monit is run from init as daemon.
This problem happens occasionally. When
it does happen, I saw these:
First of all, the .monit.pid file was not
removed after the box shutdown.
Then monit was started from init, but it
wrote “monit daemon at 12327 awakened” into its log, where 12327 is the pid in
.monit.pid. The monit startup process then exited and no monit process run.
Cleaning up the .monit.pid file helped:
restart monit after the cleaning up, then monit came up.
Anyone has experienced this? What could
cause this to happen?
Looking at monit’s source code, I found
(1) the removal function of the pid file
is registered in atexit(), however it may not be called if the process
terminates abnormally. Thus the pid file may not be removed, which is probably
what was seen above.
(2) When monit starts up, it retrieves
the pid value from the pid file, and calls getpgid to check the result. But if
the process with that pid is running as a zombile, the getpgid checking will
pass and monit thinks that the monit daemon is running, and will “awake” that
daemon which is actually a zombie. The result is that monit daemon won’t come
It doesn’t seem that monit tests if there
is a monit zombie process during startup.
Also, if by any chance there is a process
running with the same pid in the .monit.pid, monit will send a signal to it to
“awake” it …, and then it may kill that process.
This is seen in monit 5.1.1, and seems to
be in the latest version 5.4 as well.
CONFIDENTIALITY AND SECURITY NOTICE
The contents of this message and any attachments may be confidential and
proprietary. If you are not an intended recipient, please inform the sender of
the transmission error and delete this message immediately without reading,
distributing or copying the contents.