3 ms·

The thing is: There isn't really any such thing as "the memory of a process". What about a page of a shared library that hasn't been loaded yet, but is mapped i

The thing is: There isn't really any such thing as "the memory of a process". What about a page of a shared library that hasn't been loaded yet, but is mapped in more than one process, for example? Also, if memory is guaranteed to be available at a later point, it cannot really be used for anything, as there is nowhere to put the contents of that memory the moment the process it's reserved for wants to use it. But there are two APIs that can be used to implement the same goal: Linux has a procfs setting "oom_score_adj" that can be used to decrease the risk of a particular process being killed (commonly used for sshd for obvious reasons) and there are the mlock()/mlockall() syscalls that you can use to make sure some address space is backed by actual RAM rather than potentially swap.

btilly 11y ago
Nothing you said disagreed with what I said. It is true that the reserved memory could not be used for anything. This is a feature. The memory has been reserved for emergencies, and will keep part of the system usable when everything else goes belly up. The two alternate methods that you specify are not as useful. I cannot with oom_score_adj have a shell that can be logged in to and be entirely usable during a fork bomb. Yes, you can ssh in. But good luck running arbitrary commands. With mlock()/mlockall() I can guarantee that a process is responsive during heavy memory pressure, but good luck if it needs to allocate more memory. And neither system makes it particularly easy to set things up so that the OOM killer will avoid killing all daemons whose memory usage remained stable during memory pressure. (And that is the biggest problem with the OOM killer, that it killed random things you'd have preferred stayed up.)
- zAy0LfpBZLC8mAC 11y ago
  > I cannot with oom_score_adj have a shell that can be logged in to and be entirely usable during a fork bomb. Yes, you can ssh in. But good luck running arbitrary commands. But that's due to PID exhaustion, not due to memory exhaustion. > With mlock()/mlockall() I can guarantee that a process is responsive during heavy memory pressure, but good luck if it needs to allocate more memory. Well, good luck if the process needs more memory than it had reserved in your scheme. Obviously, you have to mlock() sufficient memory for peak need. > And neither system makes it particularly easy to set things up so that the OOM killer will avoid killing all daemons whose memory usage remained stable during memory pressure. (And that is the biggest problem with the OOM killer, that it killed random things you'd have preferred stayed up.) No, it doesn't kill random things, it kills things that are easy to reconstruct (little CPU use so far) and that have lots of memory allocated, and where the oom_score_adj doesn't tell it to spare the process for other reasons.
  - btilly 11y ago
    I have definitely been on a system with plenty of pids available, that was completely unusable due to memory exhaustion. As for the OOM killer, its logic has changed over time. All that I definitely know is that if a slow leak in application code causes Apache processes to grow too quickly over time, it is usually a good idea to reboot the machine because you never know what random daemon got killed before the actual culprit. Of course you also need to fix application code... The logic that is applied probably works well in the desktop/developer case where what went wrong probably went wrong recently and there is a person who can notice. That isn't the context where I've usually encountered it.

btilly 11y ago

Nothing you said disagreed with what I said. It is true that the reserved memory could not be used for anything. This is a feature. The memory has been reserved for emergencies, and will keep part of the system usable when everything else goes belly up. The two alternate methods that you specify are not as useful. I cannot with oom_score_adj have a shell that can be logged in to and be entirely usable during a fork bomb. Yes, you can ssh in. But good luck running arbitrary commands. With mlock()/mlockall() I can guarantee that a process is responsive during heavy memory pressure, but good luck if it needs to allocate more memory. And neither system makes it particularly easy to set things up so that the OOM killer will avoid killing all daemons whose memory usage remained stable during memory pressure. (And that is the biggest problem with the OOM killer, that it killed random things you'd have preferred stayed up.)

zAy0LfpBZLC8mAC 11y ago
> I cannot with oom_score_adj have a shell that can be logged in to and be entirely usable during a fork bomb. Yes, you can ssh in. But good luck running arbitrary commands. But that's due to PID exhaustion, not due to memory exhaustion. > With mlock()/mlockall() I can guarantee that a process is responsive during heavy memory pressure, but good luck if it needs to allocate more memory. Well, good luck if the process needs more memory than it had reserved in your scheme. Obviously, you have to mlock() sufficient memory for peak need. > And neither system makes it particularly easy to set things up so that the OOM killer will avoid killing all daemons whose memory usage remained stable during memory pressure. (And that is the biggest problem with the OOM killer, that it killed random things you'd have preferred stayed up.) No, it doesn't kill random things, it kills things that are easy to reconstruct (little CPU use so far) and that have lots of memory allocated, and where the oom_score_adj doesn't tell it to spare the process for other reasons.
- btilly 11y ago
  I have definitely been on a system with plenty of pids available, that was completely unusable due to memory exhaustion. As for the OOM killer, its logic has changed over time. All that I definitely know is that if a slow leak in application code causes Apache processes to grow too quickly over time, it is usually a good idea to reboot the machine because you never know what random daemon got killed before the actual culprit. Of course you also need to fix application code... The logic that is applied probably works well in the desktop/developer case where what went wrong probably went wrong recently and there is a person who can notice. That isn't the context where I've usually encountered it.