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When should I not kill -9 a process?

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Re: When should I not kill -9 a process?

#11

Post by captainmuon »

The advice to never ever use `kill -9` is too strong. It's fine to use if you know what the program you are killing does.

In my case, processes I have to `kill -9` are

- programs that only read data files (or write to dispensable files)

- programs that I know don't react to SIGTERM (there is no cleanup logic, but still something makes them swallow SIGTERM)

- often then are simple tools (e.g. ls) that become wedged in a system call or in kernel code (when trying to access a bad NFS share)

- in the other cases they are in-house programs that are either badly written, or too complex (the worst offender is CERN's ROOT, if it becomes wedged you have to `kill` and `kill -9` several processes it spawns), or where we don't care enough to fix them

Interestingly, there seem to be some cases where even `kill -9` doesn't help. What I do then is to freeze the process with Ctrl+Z (Ctrl+C doesnt work of course), and then `killall -9 $(jobs -p); fg`.

Actually, I have one program I routinely call with `program; killall -9 $(jobs -p); fg` and end it with Ctrl+Z. Sad, but true.

(Of course, if your process is a database or a GUI tool or something, then all the standard wisdom against `kill -9` applies.)

Re: When should I not kill -9 a process?

#12

Post by stingraycharles »

btmorex wrote: post #4

As if often the case with stackoverflow answers, all of them are wrong in different ways. You should only kill -9 when every other signal the program is likely to respond to has not worked. kill -9 is likely to leave program in a state that requires manual intervention, especially if that program is a database. If you're a developer, before you kill -9 a program send SIGTERM (ie kill without args or kill -15). If the…

As a useful variant of this: I have grown accustomed to killing processes that hang with kill -11 (SIGSEGV). This is essentially the same as -9, with the exception that it creates a coredump.

Saves a lot of hassle with manually attaching GDB.

Re: When should I not kill -9 a process?

#13

Post by staunch »

One example of where you might not want to kill -9 (-KILL) is a web server, like nginx. If you kill -QUIT (-3) nginx it will do a graceful shutdown. Nginx closes its listen sockets while allowing existing clients to finish. Many other daemons have similarly friendly behavior if you give them a chance to shutdown gracefully. They should degrade safely (if not nicely) with -KILL (-9) as well.

Re: When should I not kill -9 a process?

#15

Post by vacri »

justizin wrote: post #7

Over 15 years ago, as a teenager, I taught Linux / UNIX Admin courses, and worked as a consultant advising folks, and in the late 90s I was very adamant that you should never -9 anything unless you know exactly what you are doing. As infrastructures have grown, and I have managed large applications involving tens to hundreds, often over a thousand servers, and I have grown to accept that a power supply can fail and a…

kill -9 should never be worse than pulling the power plug

No, it shouldn't, but just pulling the power plug isn't exactly recommended behaviour, either. There aren't many admins out there who will happily yank the power cord out of their desktops when they want to power it down.

Last night I spent several hours getting a server back into gear after a 'pulled power plug' event. A friend's rack was affected by a (seven-hour!) substation power outage, and it wasn't on a UPS, so it didn't shut down cleanly. Eventually we were able to coax the server to boot again (had to remove all USB devices in the process, including internal ones), and the problem was a corrupted MBR. Make a rescue usb stick, boot into that, finally diagnose the problem, cat a new MBR onto it, and tada, fixed. Let's just say that I don't find the argument "shouldn't be worse than just pulling the plug" to be particularly comforting at the moment :)

Re: When should I not kill -9 a process?

#17

Post by IsTom »

btmorex wrote: post #4

As if often the case with stackoverflow answers, all of them are wrong in different ways. You should only kill -9 when every other signal the program is likely to respond to has not worked. kill -9 is likely to leave program in a state that requires manual intervention, especially if that program is a database. If you're a developer, before you kill -9 a program send SIGTERM (ie kill without args or kill -15). If the…

If a database is not kill -9 resistant then how can it be power-off resistant?

Re: When should I not kill -9 a process?

#18

Post by btmorex »

stingraycharles wrote: post #12
btmorex wrote: post #4

As if often the case with stackoverflow answers, all of them are wrong in different ways. You should only kill -9 when every other signal the program is likely to respond to has not worked. kill -9 is likely to leave program in a state that requires manual intervention, especially if that program is a database. If you're a developer, before you kill -9 a program send SIGTERM (ie kill without args or kill -15). If the…

As a useful variant of this: I have grown accustomed to killing processes that hang with kill -11 (SIGSEGV). This is essentially the same as -9, with the exception that it creates a coredump. Saves a lot of hassle with manually attaching GDB.

Yup, there are risks too though with that approach. You should be sure that ulimit for core size is large enough (not default almost always). Also, a core dump can take a very long time if your programs address space is large. It might involve writing tens of gigabytes to disk. So not only do you need the file system space, but you also need to be prepared to wait tens of minutes while your program is dumping core.

Re: When should I not kill -9 a process?

#19

Post by phireal »

justizin wrote: post #7

Over 15 years ago, as a teenager, I taught Linux / UNIX Admin courses, and worked as a consultant advising folks, and in the late 90s I was very adamant that you should never -9 anything unless you know exactly what you are doing. As infrastructures have grown, and I have managed large applications involving tens to hundreds, often over a thousand servers, and I have grown to accept that a power supply can fail and a…

I know that grammar comments are probably not welcome here on HN, but I think that since you seem to have an interest in it, I'd point this out:

"Its" only has an apostrophe when it's a contraction of "it is" (or "it has"), the possessive is always "its" (without an apostrophe).

Re: When should I not kill -9 a process?

#20

Post by btmorex »

TillE wrote: post #10
btmorex wrote: post #4

As if often the case with stackoverflow answers, all of them are wrong in different ways. You should only kill -9 when every other signal the program is likely to respond to has not worked. kill -9 is likely to leave program in a state that requires manual intervention, especially if that program is a database. If you're a developer, before you kill -9 a program send SIGTERM (ie kill without args or kill -15). If the…

You can't really corrupt a database that easily, can you? That's half the point of using a database, so you have transactions, etc.

not corrupt != everything is peachy

At the very least, you need to be prepared for a possibly very long replay of logs. Also, a huge number of people run databases in a configuration that doesn't make those guarantees. For example, many people will run mysql (esp. less performant slaves) with innodb_flush_log_at_trx_commit = 0 for performance with the understanding that a failure might require manual fixes.

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