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)

Locking: Performance Considerations Small lock granularity decreases lock contention, increases potential parallelism. Also increased scope for stuffing up. Even with careful design hard to avoid bottlenecks (``convoys''). Important to measure lock contention. Instrument lock ops to keep stats. Illustrative example Windows-NT Kernel dispatcher lock[PS96]. DEC people investigated performance problems of Microsoft's SQL server running on Alphas under NT. No access to source code. Used tool to patch executable code (OS and apps). Instrumented code logged change of control flow to memory buffer. Reconstructed instruction trace from log. Visualised results. Result for 4-CPU system Summary of results: Second box shows convoy effect on KiDispatcherLock. Lock held for 200-900 cycles. Partially due to interrupts being enabled during critical section. Disk interrupt serviced while holding lock. Lock held for about 45% of total time. 16% of time spent spinning. This one lock limits OS scalability to about 6 CPUs! Shows the necessity of keeping critical sections short.