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16.2.1 memcached Deployment »
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16.2. Using memcached

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16.2.1. memcached Deployment
16.2.2. Memory allocation within memcached
16.2.3. Using namespaces
16.2.4. Data Expiry
16.2.5. memcached Hash Types

To start using memcached, you must start the memcached service on one or more servers. Running memcached sets up the server, allocates the memory and starts listening for connections from clients.

Note

You do not need to be privileged user (root) to run memcached unless you want to listen on one of the privileged TCP/IP ports (below 1024). You must, however, use a user that has not had their memory limits restricted using setrlimit or similar.

To start the server, run memcached as a non-privileged (i.e. non-root) user: 

shell> memcached

If you start memcached as root, use the -u option to specify the user for executing memcached: 

shell> memcached -u memcache

By default, memcached uses the following settings:

  • Memory allocation of 64MB

  • Listens for connections on all network interfaces, using port 11211.

  • Supports a maximum of 1024 simultaneous connections.

To increase the amount of memory allocated for the cache, use the -m option to specify the amount of RAM to be allocated (in megabytes). The more RAM you allocate, the more data you can store and therefore the more effective your cache will be.

Warning

Do not specify a memory allocation larger than your available RAM. If you specify too large a value, then some RAM allocated for memcached will be using swap space, and not physical RAM. This may lead to delays when storing and retrieving values, because data will be swapped to disk, instead of storing the data directly in RAM.

You can use the output of the vmstat command to get the free memory, as shown in free column: 

shell> vmstat
kthr      memory            page            disk          faults      cpu
r b w   swap  free  re  mf pi po fr de sr s1 s2 -- --   in   sy   cs us sy id
0 0 0 5170504 3450392 2  7  2  0  0  0  4  0  0  0  0  296   54  199  0  0 100

For example, to allocate 3GB of RAM: 

shell> memcached -m 3072

On 32-bit x86 systems where you are using PAE to access memory above the 4GB limit, you will be unable to allocate RAM beyond the maximum process size. You can get around this by running multiple instances of memcached, each listening on a different port: 

shell> memcached -m 1024 -p11211
shell> memcached -m 1024 -p11212
shell> memcached -m 1024 -p11213

To specify a specific network interface, use the -l option to specify the IP address of the desired interface: 

shell> memcached -l 192.168.0.110

To specify an alternate port to listen on, use the -p option: 

shell> memcached -p 18080

If you are running memcached on the same server as the clients, you can disable the network interface and use a local UNIX socket using the -s option: 

shell> memcached -s /tmp/memcached

Using a UNIX socket automatically disables network support, and saves network ports (allowing more ports to be used by your web server or other process).

To specify the maximum number of simultaneous connections to the memcached service, use the -c option: 

shell> memcached -c 2048

You should use this option, either to reduce the number of connections (to prevent overloading memcached service) or to increase the number to make more effective use of the server running memcached server.

By default, memcached is configured to use 4 concurrent threads. The threading improves the performance of storing and retrieving data in the cache, using a locking system to prevent different threads overwriting or updating the same values. You may want to increase or decrease the number of threads, use the -t option: 

shell> memcached -t 8

To run memcached as a daemon (background) process, use the -d option: 

shell> memcached -d

Typically, you would specify the full combination of options that you want when starting memcached, and normally provide a startup script to handle the initialization of memcached. For example, the following line starts memcached with a maximum of 1024MB RAM for the cache, listening on port 11121 on the IP address 192.168.0.110, running has a background daemon: 

shell> memcached -d -m 1024 -p 11121 -l 192.168.0.110

To ensure that memcached is started up on boot you should check the init script and configuration parameters. On OpenSolaris, memcached is controlled by SMF. You can enable it by using: 

root-shell> svcadm enable memcached