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The Apache HTTP Server Reference Manual
by Apache Software Foundation
Paperback (6"x9"), 862 pages
ISBN 9781906966034
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8.5  In-Memory Caching

Related Modules

Related Directives



Serving directly from system memory is universally the fastest method of serving content. Reading files from a disk controller or, even worse, from a remote network is orders of magnitude slower. Disk controllers usually involve physical processes, and network access is limited by your available bandwidth. Memory access on the other hand can take mere nano-seconds.

System memory isn’t cheap though, byte for byte it’s by far the most expensive type of storage and it’s important to ensure that it is used efficiently. By caching files in memory you decrease the amount of memory available on the system. As we’ll see, in the case of operating system caching, this is not so much of an issue, but when using Apache’s own in-memory caching it is important to make sure that you do not allocate too much memory to a cache. Otherwise the system will be forced to swap out memory, which will likely degrade performance.

8.5.1  Operating System Caching

Almost all modern operating systems cache file-data in memory managed directly by the kernel. This is a powerful feature, and for the most part operating systems get it right. For example, on Linux, let’s look at the difference in the time it takes to read a file for the first time and the second time;

colm@coroebus:~$ time cat testfile > /dev/null  
real    0m0.065s  
user    0m0.000s  
sys     0m0.001s  
colm@coroebus:~$ time cat testfile > /dev/null  
real    0m0.003s  
user    0m0.003s  
sys     0m0.000s

Even for this small file, there is a huge difference in the amount of time it takes to read the file. This is because the kernel has cached the file contents in memory.

By ensuring there is "spare" memory on your system, you can ensure that more and more file-contents will be stored in this cache. This can be a very efficient means of in-memory caching, and involves no extra configuration of Apache at all.

Additionally, because the operating system knows when files are deleted or modified, it can automatically remove file contents from the cache when neccessary. This is a big advantage over Apache’s in-memory caching which has no way of knowing when a file has changed.

Despite the performance and advantages of automatic operating system caching there are some circumstances in which in-memory caching may be better performed by Apache.

Firstly, an operating system can only cache files it knows about. If you are running Apache as a proxy server, the files you are caching are not locally stored but remotely served. If you still want the unbeatable speed of in-memory caching, Apache’s own memory caching is needed.

8.5.2  MMapFile Caching

mod_file_cache provides the MMapFile directive, which allows you to have Apache map a static file’s contents into memory at start time (using the mmap system call). Apache will use the in-memory contents for all subsequent accesses to this file.

MMapFile /usr/local/apache2/htdocs/index.html

As with the CacheFile directive, any changes in these files will not be picked up by Apache after it has started.

The MMapFile directive does not keep track of how much memory it allocates, so you must ensure not to over-use the directive. Each Apache child process will replicate this memory, so it is critically important to ensure that the files mapped are not so large as to cause the system to swap memory.

8.5.3  mod_mem_cache Caching

mod_mem_cache provides a HTTP-aware intelligent in-memory cache. It also uses heap memory directly, which means that even if MMap is not supported on your system, mod_mem_cache may still be able to perform caching.

Caching of this type is enabled via;

# Enable memory caching  
CacheEnable mem /  
# Limit the size of the cache to 1 Megabyte  
MCacheSize 1024

ISBN 9781906966034The Apache HTTP Server Reference ManualSee the print edition