Just What is Wired Memory Anyway?

Jeffrey, I think what you're refering to is memory protection. Mac OS X, Windows NT and later, Unix and Linux (and any other decent modern OS) all have memory protection. The "classic" Mac OS and old versions of Windows did not.

Basically memory protection puts up walls to protect areas of memory from other processes. User programs (like Safari, iTunes, etc) can't mess with each other's memory directly, nor can they mess with the operating system's memory directly.

This prevents both malicious programs from messing with things, and bugs from causing crashes in programs other than itself. Without memory protection, a program with a certain kind of bug could accidently corrupt memory anywhere in the system, which could cause the whole computer to crash. Now the worst it can do is crash itself.

What's more of interest to me is not so much the wired memory, but the inactive memory, and trying to figure out why my machine seems to freeze up every few seconds when it's running with small Free but large Inactive. Especially when using things like Aperture.

The Bits article has a comment that points to a utility iFreeMem

http://www.apple.com/downloads/macosx/system_disk_utilities/ifreemem.html

that gives potential clues: "If you are in the middle of using an application and the system becomes unresponsive for several seconds it could be the memory manager working and claiming back some of the Inactive memory for your application to use". And it sounds like this little util may help with that (freeing up inactive memory all at once). I'll have to try that out tonight.

> This isn't quite right. Wired memory is memory
> that can't be paged out to the disk, it has to
> stay in the RAM. This makes it untouchable in a
> way, but not in the "another application can't
> 'borrow' it" way.

Right. What I meant by this is that wired memory is what the OS needs to manage running applications. Application data can be swapped out, but page tables etc. are needed by the OS to handle address translation, dynamic linking and all of that other good stuff that modern OS's do.

Jeffrey: Wired memory is a concept that exists in OS X because of its UNIX underpinnings (BSD). As to Windows instability tangentially: it's a stretch to equate the two (http://en.wikipedia.org/wiki/General_Protection_Fault).

"[Wired memory is] used by the OS [which] is pretty much untouchable. Another application can't 'borrow' wired memory"

This isn't quite right. Wired memory is memory that can't be paged out to the disk, it has to stay in the RAM. This makes it untouchable in a way, but not in the "another application can't 'borrow' it" way.

http://docs.info.apple.com/article.html?artnum=107918

nice article, i checked up on my activity monitor and killed some of the memory eating apps and processes that i didnt need and it helped a lot!

I am the LEAST tech savy person to ever post her (but I run circles aroun my parents)

I always thought that the fact that Apple's strict adherance to this "wired memory" that no developers could touch is why the mac platform is more stable than windows of the past. I'm not sure if it's true, but someone told me that in th past, every windows developer (not MS, just external) was using this memory, and this is why there were so many conflicts between programs storing and accessing in "protected" ares.


Is this true, or am I WAY off the boat?