Linux – Page allocation failure – Am I running out of memory

linuxmemorytroubleshooting

Lately, I've noticed entries like this one in the kern.log of one of my servers:

Feb 16 00:24:05 aramis kernel: swapper: page allocation failure. order:0, mode:0x20

I'd like to know:

What exactly does that message mean?
Is my server running out of memory?

The swap usage is quite low (less than 10%), and so far I haven't noticed any processes being killed because of lack of memory.

Additional information:

The server is a Xen instance (DomU) running Debian 6.0
It has 512 MB of RAM and a 512 MB swap partition
CPU load inside the virtual machine shows an average of 0.25

Best Answer

Debian bug 666021 seems to be a report of this same issue. The suggestion there is:

#change value for this boot
sysctl -w vm.min_free_kbytes=65536

#change value for subsequent boots
echo "vm.min_free_kbytes=65536" >> /etc/sysctl.conf

http://russ.garrett.co.uk/2009/01/01/linux-kernel-tuning/ has some discussion of when altering this setting may be useful, reproduced here:

This tells the kernel to try and keep 64MB of RAM free at all times. It’s useful in two main cases:

Swap-less machines, where you don’t want incoming network traffic to overwhelm the kernel and force an OOM before it has time to flush any buffers.

x86 machines, for the same reason: the x86 architecture only allows DMA transfers below approximately 900MB of RAM. So you can end up with the bizarre situation of an OOM error with tons of RAM free.

I applied this setting on my 3.2.12-gentoo x86 machine, but I'm still getting these errors.

Related Solutions

Linux – Running out of memory running fsck on large filesystems

A 64 bit kernel and large quantities of RAM will allow the fsck to finish nice and fast. Alternately, there's now an option in e2fsck that'll tell it to store all of it's intermediate results in a directory instead of in RAM, which helps immensely. Create /etc/e2fsck.conf with the following contents:

[scratch_files]
directory = /var/cache/e2fsck

(And, obviously, make sure that directory exists, and is on a partition with a good few GB of free space). e2fsck will run SLLOOOOWWWWWWW, but at least it'll complete.

Of course, this won't work with the root FS, but if you've got swap then you're past mounting the root FS anyway.

Linux – What does pdflush attempt to do when a CentOS server runs out of memory and does not have a swap partition

pdflush's primary duty is flushing the disk buffer cache (AFAIk this is its only duty, but a Linux guru may correct me on this). The cache it flushes includes files that have been written to but not yet committed to disk.
If I recall correctly kswapd is the virtual process responsible for dealing with swap space & shuffling memory pages from RAM to disk & back again.

For the health of your system, please set up a swap partition for your server (or swapon the one you have)

Unix-like systems expect to be able to swap. Bad Things can happen when there is no available swap (like the disk buffer cache getting squeezed down to a tiny bit of memory, which can really make pdflush have a bad day).
Even if you'll "never" use the swap space, disk is cheap enough that you can throw 2G at it.

Best Answer

Related Solutions

Linux – Running out of memory running fsck on large filesystems

Linux – What does pdflush attempt to do when a CentOS server runs out of memory and does not have a swap partition

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