Vanguard (microkernel) - Chaining

Chaining

One major addition to the IPC system under Vanguard, as opposed to V, was the concept of message chains, allowing a single message to be sent between several interacting servers in a single round-trip. In theory, chaining could improve the performance of common multi-step operations.

Consider the case where a client application wishes to read a file. Normally this would require one message to the kernel to find the file server, then three additional messages to the file server itself - one to resolve the file name into an object id, another to open that id, then finally another to read the file. Using Vanguard's chaining, a single message could be constructed by the client that contained all of these requests. The message would be sent to the kernel, and then passed off to the file server who would handle all three requests before finally returning data.

Much of the performance problem normally associated with microkernel systems are due to the context switches as messages are passed back and forth between applications. In the example above running on a V system, there would have to be a total of eight context switches; two for each request as the client switched to and from the kernel. In Vanguard using a chain would reduce this to only three switches; one out of the client into the kernel, another from the kernel to the file server, and finally from the server back to the client. In some cases the overhead of a context switch is greater than the time it takes to actually run the request, so Vanguard's chaining mechanism could result in real-world performance improvements.