Virtual Memory Primitives for User Programs Andrew W. Appel and Kai Li Presented by: Khanh Nguyen

Plan of Attack Virtual Memory Primitives Virtual Memory Applications  Shared Virtual Memory  Concurrent Garbage Collection  Generational Garbage Collection  Heap Overflow Detection Applications Patterns VM Primitive Performance

Virtual Memory Primitives TRAP: handle page-fault traps in user mode PROT1: decrease the accessibility of a page PROTN: decrease the accessibility of N pages UNPROT: increase the accessibility of a page DIRTY: return a list of dirtied pages since the previous call. MAP2: map the physical page at two different virtual addresses, at different levels of protection, in the same address space.

Shared Virtual Memory Useful for splitting a task into multiple computers working together over the network.

Shared Virtual Memory (cont.) Each processor has its own memory Each memory basically represents the big cache for the shared virtual memory. Read-only page can have copies residing in the physical memories of many processors at the same time. When a processor trying to access to the page, but it’s not on the its memory, it will trigger a fault that invoke Mapping manager to go out the network to get the up-to-date page. If it’s a write access, Mapping Manager send out a message to invalidate all other copies. Uses PROT1, TRAP, and UNPROT

Concurrent Garbage Collection Base on Baker’s algorithm Divides the memory heap into from-space and to-space At the beginning of a collection, all objects are in the from-space and to-space is empty. Collectors copy any reachable object from from-space to to-space. Any remaining objects are garbage. Remove garbage and Flip the space around. Any new allocate object will be created in to-space. Any access to the old object that is still referenced to the from-space will be move to-space. This is done by inserting instruction to check every fetch or virtual memory Uses PROTN, TRAP, UNPROT

Generational Garbage Collection Two properties of dynamically allocated object  Younger records are much more likely to die soon than older records.  Younger records tend to point to older records. Allocated objects are kept in area based on their age. This area is called generation Collector usually collect in the youngest generation. If it finds any object in the youngest generation that is pointed by the older generation, it will keep that object. The only way to find if a young generation is pointed by an older generation, we need to check every assignment operation. Or protect the older generations with “no write” flag. Uses PROTN, TRAP, UNPROT, or DIRTY

Heap Overflow Detection Ordinarily, heap overflow is detected by compare and conditional-branch on every memory allocation operation Problem: overhead Similar to stack overflow detection Mark the top page with “no access” Use PROT1 and TRAP Doesn’t use UNPROT because the protection will never be removed.

Application Patterns Algorithm NameProtection Shared Virtual MemoryPROT1 Concurrent GCPROTN Generational GCPROTN Heap Overflow DetectionPROT1

VM Primitive Performance