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Trap, Communication and Page Fault Times

The performance of applications using the Cache Kernel is most dependent on the trap handling, signal delivery and page fault handling times.

The cost of a simple trap from a UNIX program to its emulator is 37 microseconds, effectively the cost of a getpid operation. This time is 12 microseconds longer than the same operation performed in Mach 2.5 running on a NextStation with a comparable speed processor. For most system calls, the extra trap overhead is insignificant compared to the system call processing time, and that processing time is largely independent of the Cache Kernel.

The time to deliver a signal from one thread to another running on a separate processor is 71 microseconds, composed of 44 microseconds for signal delivery and 27 microseconds for the return from signal handler. These measurements are in agreement with (in fact slightly better than) those reported for a similar implementation of memory-based messaging [7]. Because the communication schemes are identical at the higher levels, and no Cache Kernel involvement occurs on data transfer, the communication using the Cache Kernel is as efficient as the communication in the V implementation of memory-based messaging.

The basic cost of page fault handling is 99 microseconds, which includes 32 microseconds for transfer to the application kernel and 67 microseconds for the optimized mapping load operation. This cost is comparable to the page fault cost of Ultrix and other conventional systems and also comparable to the cost of the comparable operation for external page cache management in the V kernel as described by Harty and Cheriton [16]. A page fault that entails page zeroing, page copying or page read from backing store incurs costs that make the Cache Kernel overhead insignificant. Extrapolating from the application-level performance measured by Harty and Cheriton [16] indicates that performance of applications on the Cache Kernel will be comparable to that of conventional operating systems on the same hardware.

Next: Related Work Up: Evaluation Previous: Caching Performance

Tue Oct 4 12:01:58 PDT 1994