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    SubjectRe: tuning/tweaking VM settings for low memory (preventing OOM)
    Hi Toon,

    On Sat, Jan 13, 2007 at 02:16:01PM +0100, Toon van der Pas wrote:
    > On Sat, Jan 13, 2007 at 08:22:18AM +0100, Willy Tarreau wrote:
    > > >
    > > > Which makes me think that we aren't writing back fast enough. If I
    > > > mount the drive "sync" the issue clearly goes away.
    > > >
    > > > It appears from an strace we are doing ftruncate64(5, 178257920) when
    > > > we OOM.
    > > >
    > > > Any ideas on VM parameters to tweak so we throttle this from occurring?
    > >
    > > Take a look at /proc/sys/vm/bdflush. There are several useful parameters
    > > there (doc is in linux-xxx/Documentation). For instance, the first column
    > > is the percentage of memory used by writes before starting to write on
    > > disk. When using tcpdump intensively, I lower this one to about 1%.
    > >
    > > Willy
    >
    > Hi Willy,
    >
    > I know you're doing a great job on keeping the 2.4 kernel in shape,
    > but do you also have a good advice for people with more recent
    > kernels? (hint: the file /proc/sys/vm/bdflush is missing)

    OK OK OK... Next time I will have coffee *before* replying :-)

    Check /proc/sys/vm/dirty_ratio and dirty_background_ratio. Both are
    percentage of total memory. The first one is for "foreground" writes
    (ie the writing process may block) while the second one is for
    "background" writes :

    $ uname -a
    Linux hp 2.6.16-rc2-pa1 #1 Fri Feb 3 23:34:56 MST 2006 parisc unknown
    $ cat /proc/sys/vm/dirty_ratio
    40
    $ cat /proc/sys/vm/dirty_background_ratio
    10

    Again, lowering those values should help writing data to disk sooner.
    Also you should take a look at min_free_kbytes (although I've not played
    with it yet) :

    [from Documentation/sysctl/vm.txt] :
    min_free_kbytes:

    This is used to force the Linux VM to keep a minimum number
    of kilobytes free. The VM uses this number to compute a pages_min
    value for each lowmem zone in the system. Each lowmem zone gets
    a number of reserved free pages based proportionally on its size.

    Docuemntation/filesystems/proc.txt is your friend here too.

    Regards,
    Willy

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