macOS Big Sur – update_dyld_sim_shared_cache Taking Up a Lot of Memory

From what I can tell, wired memory belongs to the kernel, the innermost core of Mac OS X. It's many layers removed from the icons in the menubar, which are just ordinary apps showing themselves in an odd way.

Wired memory is used for some of the core functions of the operating system—things like keeping track of all the applications on your system, or open files and network connections, or chunks of memory used by various drivers. The "page tables" that form a map of your system's memory are also stored in wired memory, and a system with more memory needs larger page tables. I suspect that the memory used by the integrated video chips in most Macs is wired as well, but I can't find anything that says that outright. In any case, much of this information is needed to manage and access memory, and so it can't itself be swapped out to disk!

To understand why, imagine an enormous library. Think of, for example, the Bodleian Library at the University of Oxford, which has eleven million printed items. There's no way you could possibly fit all that stuff into a single building—certainly not one in the middle of a university campus.

So instead, imagine that the librarians construct a vast warehouse. (The real Bodleian Library has about forty sub-libraries of various descriptions, plus storage for really rare books, but this is a thought experiment.) Most of the books are kept in the warehouse, but anything that's been used recently is kept in the library. If you show up at the library looking for a book, and it's in the stacks there, you can read it immediately. If not, ask a librarian and the book you want will be transported from the warehouse and given to you the next day.

The information in wired memory, then, would be things like the card catalog, keys to the book delivery trucks, and maps of the route to the warehouse. If you stored these things in the warehouse, you could never retrieve books to bring them back to the library—so they must be kept in the library at all times, lest the whole system break down.

Anyway, getting back to practical considerations: wired memory is basically used by your computer for internal bookkeeping of various sorts. You shouldn't worry about it.

And don't worry if you have lots of "inactive" memory and little "free" memory, either. Inactive memory is basically memory that Mac OS is keeping something in on the off chance it's needed again; if your system needs that memory for something else, it'll be converted to free memory without any performance hit.

To extend the library metaphor, imagine that the library keeps books that have been recently used in the stacks. The space is there in the stacks; there's no use leaving it empty, so you might as well keep the books you already have there. There's no harm in it, and sometimes someone will be able to pick up a book immediately that he would otherwise have had to wait for.

Similarly, inactive memory can only speed things up; it can't hurt your computer, so don't worry about it being too high.

Really, you shouldn't worry about your memory use unless wired + active starts getting close to the 8 GB in your computer. That's when you could get into trouble, because that's when Mac OS will start consuming disk space to add "extra" memory, damaging performance considerably.

Otherwise, you might as well have something in that memory. Ultimately, unused memory is just a waste of power—if you have it, you might as well put it to use.

In my experience, this occurs when my main system hard drive is running low on free space. Operating Systems use the hard drive for extra memory storage, called "virtual memory". (I've definitely always wished that the OS could just reserve enough space for itself, but it just can't predict how many applications we'll be running).

On top of that, it's worth noting that regular web use now requires far more memory than it did in the past. In activity monitor, you'll notice that every single tab & window (every open web page) is it's own process, taking up a significant chunk of memory. On top of that, account for all the ads, movies, flash, scripts, plugins and 360 videos etc. that we expect to run smoothly. New OS's and new web pages just use a lot of memory to provide us with the services we expect to "just work" (eg. syncing across devices, notifications, automatic updating etc. etc.).

In short, in my experience there usually isn't a single process that's suddenly taking up a huge amount of memory (although a leaky program could indeed be a culprit - Sketchup 2016 does this to me, for example). More commonly, it's the additional functionality we expect of many programs/web plugins.

I believe restarting the computer always alleviates this problem for a short while - primarily by unloading all the webpages and apps we had launched over time. But if our expectation of the computer and hardware constraints stay the same (and we run the same number of processes without changing anything else), eventually we'll run into the problem again.

Two solutions that work for me:

1) Open fewer tabs/pages and fewer programs at one time. Close some web-pages/programs before opening the hefty apps, such as MS Office, Parallels, 3D CAD, Adobe programs etc.

2) Free up more space on the system hard drive (eg. move all your music and photos to another drive), to allow the system to handle your typical virtual memory needs. For me, this means my 1TB OS drive needs >20% free space (200GB)! Your requirements may be different. If you're on an older Apple laptop or iMac or Mini, the OWC Data-Doubler is a really fantastic way to accomplish this.

Method (1) is my temporary fix, so that when I eventually enact method (2) I will have restored the snappy performance I expect while running many heavy-duty programs simultaneously.