Tuning PostgreSQL on Linux

I’ve found an interisting documentation page which applies to our storage production environment: it’s about kernel resources.

In brief, on a Linux box we can face three problems:

  • System V IPC Parameters

    The default maximum segment size is 32 MB, which is only adequate for small PostgreSQL installations. However, the remaining defaults are quite generously sized, and usually do not require changes. The maximum shared memory segment size can be changed via the sysctl interface. For example, to allow 128 MB, and explicitly set the maximum total shared memory size to 2097152 pages (the default):

    <samp class="PROMPT">$ sysctl -w kernel.shmmax=134217728
    $ sysctl -w kernel.shmall=2097152

    In addition these settings can be saved between reboots in /etc/sysctl.conf.

    Older distributions might not have the sysctl program, but equivalent changes can be made by manipulating the /proc file system:

    $ echo 134217728 >/proc/sys/kernel/shmmax
    $ echo 2097152 >/proc/sys/kernel/shmall
  • Memory Overcommit

    In Linux 2.4 and later, the default virtual memory behavior is not optimal for PostgreSQL. Because of the way that the kernel implements memory overcommit, the kernel might terminate the PostgreSQL server (the master server process) if the memory demands of another process cause the system to run out of virtual memory.

    If this happens, you will see a kernel message that looks like this (consult your system documentation and configuration on where to look for such a message):

    Out of Memory: Killed process 12345 (postgres). 

    This indicates that the postgres process has been terminated due to memory pressure. Although existing database connections will continue to function normally, no new connections will be accepted. To recover, PostgreSQL will need to be restarted.

    One way to avoid this problem is to run PostgreSQL on a machine where you can be sure that other processes will not run the machine out of memory. If memory is tight, increasing the swap space of the operating system can help avoiding the problem, because the out-of-memory (OOM) killer is invoked whenever physical memory and swap space are exhausted.

    On Linux 2.6 and later, an additional measure is to modify the kernel’s behavior so that it will not “overcommit” memory. Although this setting will not prevent the OOM killer from being invoked altogether, it will lower the chances significantly and will therefore lead to more robust system behavior. This is done by selecting strict overcommit mode via sysctl:

    sysctl -w vm.overcommit_memory=2

    or placing an equivalent entry in /etc/sysctl.conf. You might also wish to modify the related setting vm.overcommit_ratio. For details see the kernel documentation file Documentation/vm/overcommit-accounting.

    Some vendors’ Linux 2.4 kernels are reported to have early versions of the 2.6 overcommit sysctl parameter. However, setting vm.overcommit_memory to 2 on a kernel that does not have the relevant code will make things worse not better. It is recommended that you inspect the actual kernel source code (see the function vm_enough_memory in the file mm/mmap.c) to verify what is supported in your copy before you try this in a 2.4 installation. The presence of the overcommit-accounting documentation file should not be taken as evidence that the feature is there. If in any doubt, consult a kernel expert or your kernel vendor.

  • Resource Limits

    Unix-like operating systems enforce various kinds of resource limits that might interfere with the operation of your PostgreSQL server. Of particular importance are limits on the number of processes per user, the number of open files per process, and the amount of memory available to each process. Each of these have a “hard” and a “soft” limit. The soft limit is what actually counts but it can be changed by the user up to the hard limit. The hard limit can only be changed by the root user. The system call setrlimit is responsible for setting these parameters. The shell’s built-in command ulimit (Bourne shells) or limit (csh) is used to control the resource limits from the command line. On BSD-derived systems the file /etc/login.conf controls the various resource limits set during login. See the operating system documentation for details. The relevant parameters are maxproc, openfiles, and datasize. For example:

    default:\
    ...
    :datasize-cur=256M:\
    :maxproc-cur=256:\
    :openfiles-cur=256:\
    ...

    (-cur is the soft limit. Append -max to set the hard limit.)

    Kernels can also have system-wide limits on some resources.

    On Linux /proc/sys/fs/file-max determines the maximum number of open files that the kernel will support. It can be changed by writing a different number into the file or by adding an assignment in /etc/sysctl.conf. The maximum limit of files per process is fixed at the time the kernel is compiled; see /usr/src/linux/Documentation/proc.txt for more information.
    The PostgreSQL server uses one process per connection so you should provide for at least as many processes as allowed connections, in addition to what you need for the rest of your system. This is usually not a problem but if you run several servers on one machine things might get tight.

    The factory default limit on open files is often set to “socially friendly” values that allow many users to coexist on a machine without using an inappropriate fraction of the system resources. If you run many servers on a machine this is perhaps what you want, but on dedicated servers you might want to raise this limit.
    On the other side of the coin, some systems allow individual processes to open large numbers of files; if more than a few processes do so then the system-wide limit can easily be exceeded. If you find this happening, and you do not want to alter the system-wide limit, you can set PostgreSQL‘s max_files_per_process configuration parameter to limit the consumption of open files.

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