On Apr 20, 2010, at 11:48 AM, Jens Alfke wrote: > > On Apr 20, 2010, at 10:36 AM, Bill wrote: > >>> And what make NSCache and NSDiscardableContent not appropriate here ? >> >> They are not inappropriate, but they require a lot more work than a simple >> warning sent via a notification or other means. > > The iPhone has a much simpler memory model than Mac OS (or Windows or Linux). > It doesn’t have virtual memory, and it also has very limited RAM. This makes > low-memory warnings necessary. > > With virtual memory, things are more complicated. The good news is that on > Mac OS your process won’t “run out of memory” until it allocates about 3GB of > heap if it’s 32 bit; if it’s 64-bit it will never run out of memory*. The bad > news is that before that happens, the VM paging is likely to slow your > machine to a crawl, as everything else on the system gets paged out. > > So the danger on Mac OS X is not running out of memory, it’s using too much > RAM and swap space, causing performance to suffer. Avoiding this is tricky > because it depends on the differences between address space, RAM and backing > store. You can allocate a gigabyte of address space with no ill effects as > long as you don’t map any of it in. But conversely, merely accessing a large > already-malloced block might cause your process to block for hundreds of > milliseconds if that address space was paged out. > > NSCache and NSDiscardableContent are high-level constructs that try to > simplify these decisions and make them easier to use. Yes, they’re more > complex than a simple “please free up memory right now” warning. But they > make a lot more sense on Mac OS. (And in practice, they end up automating > what you’d probably already be doing in response to a low-memory warning, > i.e. looking for data that can be safely thrown away and freeing it.) > > —Jens > > * It will, however, eventually fill up your hard disk with VM backing store > files. When this happens the OS starts suspending processes while it puts up > a critical alert asking you to quit apps or free up disk space.
That helps and explains a lot. Thanks! -Bill_______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
