Warning

IMPORTANT: DOING THIS TOO OFTEN WILL SIGNIFICANTLY REDUCE PERFORMANCE. Furthermore, the equivalent happens automatically. THIS IS A TOY ONLY.

Fun With Debugging Tools

Do you like it when Activity Monitor reports large amounts of free memory, regardless of actual performance? Try this: open Terminal and just type “purge” (without quotes).

OMG WTF BBQ

Pretty neat, eh?

Details

The purge(8) utility is a debugging tool which interacts with the Unified Disk Cache. Basically, Mac OS X (and all modern OSes) keeps recently (or frequently) used files in RAM, since it is fairly likely that they’ll be used again very shortly. This does, in fact, happen more often than not.

Most files are tiny and read once and “never” again. (We’re talking about seconds here, so re-opening the same file tomorrow or after lunch is like talking about how your house will look in the year 2256. Its just as good to say “never.”) These files drop out of the disk cache quickly enough, since they’re closed (programatically) as soon as they’re opened. Other files are enormous (many megabytes) and are rarely merely loaded and closed. Instead, these files are frequently read partially, again and again, and often modified in-place. Often, these files aren’t ever read start-to-finish, and certainly aren’t closed once read. Examples include databases, any sort of for-editing media (audio, video, images, animation, &c.), and even not-for-editing consumer-targetted media files (like a 1.4GB movie downloaded from iTunes).

Most applications interact with files in ways that lets Mac OS X know which category they fall into: open and “discard”, or frequent access. Even when they don’t, Mac OS X is fairly good at guessing.

When an application behaves as though it is clearly within the frequent access category (i.e., working with enormous files that simply couldn’t fit into RAM at all), an app may nevertheless explicitly tell the system that it is really in the other category. Creating a new ZIP or TAR archive, or exporting from source material to ProRes, are some examples of this sort of exception.

The problem comes in when an application is working with those aforementioned enormous files, but neglects to tell the system that it really should be in the other category. The purge(8) utility exists so that a developer or system administrator can debug this situation.

Purge(8)

The purge(8) utility merely empties the aforementioned disk cache. That’s all.

Just like all caches, the purpose of the cache is to increase performance. And, just like all caches, emptying the cache is usually pointless and generally unnecessary. Just like all caches, it will clean itself over time. Unlike the cache in a web browser, though, “over time” is minutes not days or weeks. However, just like the cache in a web browser, it is extremely unlikely that you will get any performance improvement by manually emptying it.

In rare situations, however, a developer or system administrator may know specifically of a situation where the built-in heuristics will guess wrong, or at least not guess quickly enough.