Out of memory error on Apache

Here are some possible solutions I've found on my VPS's support page.

Diagnosing and Fixing Memory Issues

When your VPS is running low on physical memory, it may start to "swap thrash." This means it's attempting to use your swap partition heavily instead of real RAM. We recommend you limit your swap partition size to 256 MB; heavy use of swap in a virtualized environment will cause major performance problems.

We strongly recommend running the 32-bit version of your distribution of choice, unless you have a specific reason to run the 64-bit version. The 32-bit version of your distro will use significantly less memory.
Determining Free Memory and Swap Activity

You can use the following command to display memory use on your Linode:

free -m

You can use the following snippet to see a list of your running processes sorted by memory use:

ps -eo pmem,pcpu,rss,vsize,args | sort -k 1 -r | less

To see IO activity on your VPS, you may use the following command (you may need to install the sysstat package under Debian or Ubuntu first):

iostat -d -x 2 5

This will give an extended device utilization report five times at two second intervals. If your VPS is OOMing (running out of memory), Apache, MySQL, and SpamAssassin are the usual suspects.
MySQL Low-Memory Settings

In your MySQL configuration file (typically found in /etc/mysql/my.cnf), change your entries for the various settings shown below to match the recommended values:

key_buffer = 16K
max_allowed_packet = 1M
thread_stack = 64K
table_cache = 4
sort_buffer = 64K
net_buffer_length = 2K

If you don't use InnoDB tables, you should disable InnoDB support by adding the following line:

skip-innodb

Apache 2 Low-Memory Settings

Determine the type of MPM in use by your Apache install by issuing the following command. This will tell you which section to edit in your Apache configuration file.

Debian-based systems:

apache2 -V | grep 'MPM'

Fedora/CentOS systems:

httpd -V | grep 'MPM'

In your Apache 2 configuration file (typically found at /etc/apache2/apache2.conf in Debian and Ubuntu systems, and /etc/httpd/httpd.conf in CentOS and other similar systems), change your entries for the various settings shown below to match the recommended values.

StartServers 1
MinSpareServers 3
MaxSpareServers 6
ServerLimit 24
MaxClients 24
MaxRequestsPerChild 3000

Reducing SpamAssassin Memory Consumption

If you're filtering mail through SpamAssassin in standalone mode and running into load issues, you'll need to investigate switching to something to keep the program persistent in memory as a daemon. We suggest looking at amavisd-newExternal Link.
Troubleshooting Network Issues

If you've added multiple IP addresses to your Linode, you must set up static networking as described in the Linux Static Networking Guide. Please be sure to specify only one gateway. Using multiple gateways frequently causes problems.

If you just added an IP address to your Linode, please be sure to reboot before attempting to use it. This is required to properly route the IP address on our network.

If you've added a private IP address, please be sure to use the network settings shown in the "Remote Access" of the Linode Manager, paying special attention to the subnet mask. Note that private IP addresses do not require a gateway (nor should one be specified).

http://library.linode.com/troubleshooting/memory-networking#diagnosing_a...