Squid tends to be a bit of a memory hog. It uses memory for many different things, some of which are easier to control than others. Memory usage is important because if the Squid process size exceeds your system’s RAM capacity, some chunks of the process must be temporarily swapped to disk. Swapping can also happen if you have other memory-hungry applications running on the same system. Swapping causes Squid’s performance to degrade very quickly.

An easy way to monitor Squid’s memory usage is with standard system tools such as top and ps. You can also ask Squid itself how much memory it is using, through either the cache manager or SNMP interfaces. If the process size becomes too large, you’ll want to take steps to reduce it. A good rule of thumb is to not let Squid’s process size exceed 60% to 80% of your RAM capacity.

One of the most important uses for memory is the main cache index. This is a hash table that contains a small amount of metadata for each object in the cache. Unfortunately, all of these “small” data structures add up to a lot when Squid contains millions of objects. The only way to control the size of the in-memory index is to change Squid’s disk cache size (with the cache_dir directive). Thus, if you have plenty of disk space, but are short on RAM, you may have to leave the disk space underutilized.

Squid’s in-memory cache can also use significant amounts of RAM. This is where Squid stores incoming and recently retrieved objects. Its size is controlled by setting the cache_mem directive. Note that the cache_mem directive only affects the size of the memory cache, not Squid’s entire memory footprint.

Squid also uses some memory for various I/O buffers. For example, each time a client makes an HTTP request to Squid, a number of memory buffers are allocated and then later freed. Squid uses similar buffers when forwarding requests to origin servers, and when reading and writing disk files. Depending on the amount and type of traffic coming to Squid, these I/O buffers may require a lot of memory. There’s not much you can do to control memory usage for these purposes. However, you can try changing the TCP receive buffer size with the tcp_recv_bufsize directive.

If you have a large number of clients accessing Squid, you may find that the “client DB” consumes more memory than you would like. It keeps a small number of counters for each client IP address that sends requests to Squid. You can reduce Squid’s memory usage a little by disabling this feature. Simply put client_db off in squid.conf.

Another thing that can help is to simply restart Squid periodically, say, once per week. Over time, something may happen (such as a network outage) that causes Squid to temporarily allocate a large amount of memory. Even though Squid may not be using that memory, it may still be attached to the Squid process. Restarting Squid allows your operating system to truly free up the memory for other uses.

You can use Squid’s high_memory_warning directive to warn you when its memory size exceeds a certain limit. For example, add a line like this to squid.conf:

high_memory_warning 400 MB

Then, if the process grows beyond that value, Squid writes warnings to cache.log and syslog if configured.