Advanced Operating Systems   COMP9242 2002/S2

UNSW

Printer-Friendly Version Administration                - Notices - Course Intro - Consultations # On-line Survey (closed) - Survey Results   Work - Lectures - Milestone 0 - Project Admin - Project Spec - Project FAQ - Exam   Documentation - ASysT Lab - L4 source browser - Sulima ISA Simulator - R4x00 ISA Summary  - MIPS R4700 Reference - MIPS R4000 User Manual  - Network Driver - GT64111   Related Info - Aurema OS Prize - OS Hall of Fame   History - 2000 - 1999 - 1998   Staff - Gernot Heiser (LiC)

Virtual Cache Issues Homonyms: Same VA corresponds to several PAs standard situation in multitasking systems (not SASOS!) Problem: tag may not uniquely identify cache data! Homonyms lead to cache accessing the wrong data! Homonym prevention: tag virtual address with address-space ID (ASID) disambiguates virtual addresses (makes them globally valid) use physical tags flush cache on context switch use a SASOS Synonyms (aliases): Several VAs map to the same PA frames shared between processes multiple mappings of a frame within an address space Synonyms in cache may lead to accessing stale data: same data may be cached in several lines on write, one synonym is update a subsequent read on the other synonym returns the old value Physical tagging doesn't help! ASIDs don't help either! Whether synonyms are a problem depends on cache size and page size! Example: MIPS R4x00 synonyms Virtually-indexed, physically tagged on-chip L1 cache 16kB cache with 32B lines 4kB (base) page size Remember, location of data in cache determined by index Tag only confirms whether it's a hit! Synonym problem iff VA VA Synonyms... Avoiding synonym problems: hardware synonym detection flush cache on context switch doesn't help for aliasing within address space detect synonyms and ensure all read-only, OR only one synonym mapped restrict VM mapping so synonyms map to same cache set e.g., R4x00: ensure that VA PA