updated 05:00 am EDT, Wed April 18, 2012
Metro app 'suspension' allows RAM to be reclaimed
A new paper and accompanying video on Microsoft's developer network has laid out the new system of memory management for Metro-style apps on Windows 8 which closely mimics Apple's iOS "app suspension" model, allowing inactive apps to surrender most of their RAM in favor of running apps to avoid "memory pressure." While Windows 8 will attempt to regulate physical memory allocation for all apps, Metro-style apps will use the iOS-like suspension technique to free up RAM.
Once in the suspended state, Metro apps hold on to very little physical RAM -- but can quickly reclaim its working memory once it is reactivated. The Metro-style apps can have their working state written to disk when needed (similar to OS X's writing of RAM states when going to sleep) and use that information to quickly reclaim sufficient RAM when re-activated, meaning the app can return to nearly full speed in just moments.
As explained by Microsoft's Bill Kargounis in a companion video (seen below), the suspension is done automatically when Metro apps are not in the foreground. Only when the system detects "memory pressure" does Windows check Metro apps and write their working state to disk, allowing even that bit of RAM that was being used to preserve the working state to be released. Due to fast I/O in hard drives and SSDs, the delay before a suspended app has reclaimed its working RAM is expected to be about a second.
Not explained is how the system would react to users re-opening Metro apps (which will need their working RAM back) while the system is still experiencing "memory pressure" (insufficient RAM to handle the number of programs opened, or a non-Metro app that is using up most RAM resources). Microsoft is continuing to refine the technique, and Windows 8 is not expected until the fall at the earliest.
Users can test the Metro-style app RAM allocation process for themselves in the latest Windows 8 Consumer Preview.
Metro-style app with working RAM suspended
Metro-style app reclaiming working RAM