15.6.2. Using memcached

15.6.2.1. memcached Deployment
15.6.2.2. Using Namespaces
15.6.2.3. Data Expiry
15.6.2.4. memcached Hashing/Distribution Types
15.6.2.5. Using memcached and DTrace
15.6.2.6. Memory Allocation within memcached
15.6.2.7. memcached Thread Support
15.6.2.8. memcached Logs

To start using memcached, 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 a privileged user (root) to run memcached except 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 nonprivileged (that is, non-root) user:

shell> memcached

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

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 11211 on the IP address 192.168.0.110, running as a background daemon:

shell> memcached -d -m 1024 -p 11211 -l 192.168.0.110

To ensure that memcached is started up on boot, check the init script and configuration parameters.

memcached supports the following options:

-u user

If you start memcached as root, use the -u option to specify the user for executing memcached:
```
shell> memcached -u memcache
```
-m memory

Set the amount of memory allocated to memcached for object storage. Default is 64MB.

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 is.
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 uses swap space, and not physical RAM. This may lead to delays when storing and retrieving values, because data is 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> vmstatkthr      memory            page            disk          faults      cpur b w   swap  free  re  mf pi po fr de sr s1 s2 -- --   in   sy   cs us sy id0 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 cannot 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 -p11211shell> memcached -m 1024 -p11212shell> memcached -m 1024 -p11213
```
Note

On all systems, particularly 32-bit, ensure that you leave enough room for both memcached application in addition to the memory setting. For example, if you have a dedicated memcached host with 4GB of RAM, do not set the memory size above 3500MB. Failure to do this may cause either a crash or severe performance issues.
-l interface

Specify a network interface/address to listen for connections. The default is to listen on all available address (INADDR_ANY).
```
shell> memcached -l 192.168.0.110
```
Support for IPv6 address support was added in memcached 1.2.5.
-p port

Specify the TCP port to use for connections. Default is 18080.
```
shell> memcached -p 18080
```
-U port

Specify the UDP port to use for connections. Default is 11211, 0 switches UDP off.
```
shell> memcached -U 18080
```
-s socket

Specify a Unix socket to listen on.

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).
-a mask

Specify the access mask to be used for the Unix socket, in octal. Default is 0700.
-c connections

Specify the maximum number of simultaneous connections to the memcached service. The default is 1024.
```
shell> memcached -c 2048
```
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.
-t threads

Specify the number of threads to use when processing incoming requests.

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. To increase or decrease the number of threads, use the -t option:
```
shell> memcached -t 8
```
-d

Run memcached as a daemon (background) process:
```
shell> memcached -d
```
-r

Maximize the size of the core file limit. In the event of a failure, this attempts to dump the entire memory space to disk as a core file, up to any limits imposed by setrlimit.
-M

Return an error to the client when the memory has been exhausted. This replaces the normal behavior of removing older items from the cache to make way for new items.
-k

Lock down all paged memory. This reserves the memory before use, instead of allocating new slabs of memory as new items are stored in the cache.

Note

There is a user-level limit on how much memory you can lock. Trying to allocate more than the available memory fails. You can set the limit for the user you started the daemon with (not for the -u user user) within the shell by using ulimit -S -l NUM_KB
-v

Verbose mode. Prints errors and warnings while executing the main event loop.
-vv

Very verbose mode. In addition to information printed by -v, also prints each client command and the response.
-vvv

Extremely verbose mode. In addition to information printed by -vv, also show the internal state transitions.
-h

Print the help message and exit.
-i

Print the memcached and libevent license.
-I mem

Specify the maximum size permitted for storing an object within the memcached instance. The size supports a unit postfix (k for kilobytes, m for megabytes). For example, to increase the maximum supported object size to 32MB:
```
shell> memcached -I 32m
```
The maximum object size you can specify is 128MB, the default remains at 1MB.

This option was added in 1.4.2.
-b

Set the backlog queue limit. The backlog queue configures how many network connections can be waiting to be processed by memcached. Increasing this limit may reduce errors received by the client that it is not able to connect to the memcached instance, but does not improve the performance of the server. The default is 1024.
-P pidfile

Save the process ID of the memcached instance into file.
-f

Set the chunk size growth factor. When allocating new memory chunks, the allocated size of new chunks is determined by multiplying the default slab size by this factor.

To see the effects of this option without extensive testing, use the -vv command-line option to show the calculated slab sizes. For more information, see Section 15.6.2.8, "memcached Logs".
-n bytes

The minimum space allocated for the key+value+flags information. The default is 48 bytes.
-L

On systems that support large memory pages, enables large memory page use. Using large memory pages enables memcached to allocate the item cache in one large chunk, which can improve the performance by reducing the number misses when accessing memory.
-C

Disable the use of compare and swap (CAS) operations.

This option was added in memcached 1.3.x.
-D char

Set the default character to be used as a delimiter between the key prefixes and IDs. This is used for the per-prefix statistics reporting (see Section 15.6.4, "Getting memcached Statistics"). The default is the colon (:). If this option is used, statistics collection is turned on automatically. If not used, you can enable stats collection by sending the stats detail on command to the server.

This option was added in memcached 1.3.x.
-R num

Sets the maximum number of requests per event process. The default is 20.
-B protocol

Set the binding protocol, that is, the default memcached protocol support for client connections. Options are ascii, binary or auto. Automatic (auto) is the default.

This option was added in memcached 1.4.0.