On Tuesday, Mar 25, 2003, at 14:16 US/Pacific, Shawn Erickson wrote: On Tuesday, March 25, 2003, at 12:06 PM, Joey Echeverria wrote: I'm talking about the low level scheduling of writing logical blocks out to disk. For example, FIFO, SSTF, LOOK, C-LOOK, etc. The reason that I'm interested is that we are looking into power efficiency in operating systems. One thing we wanted to investigate is if there is any effect to the energy usage when using different algorithms. One idea that we had was to be able to queue requests for longer in an operating system buffer and only write to disk when the queue becomes a certain size. We were thinking that under such a scheme, you could keep the disk stopped for longer periods of time to save the energy from keeping it spinning. To my knowledge Mac OS X does no buffering of writes to disk, I think that the kernel actually does buffer writes to disk, in the "unified buffer cache", using variations on 'classical' Unix cache-management schemes. Depending on what the data is, and how it's being written, it could be a while before the data ends up on the disk. There are, however, no scheduling mechanisms of the sort referred to above. I believe the reasoning is that, these days, the OS does not have a good idea of the actual disk geometry and other performance characteristics, and so we leave that level of scheduling up to the controllers in the disk drive itself. I think, for example, that recent IBM drives have some variant of OS/2 running in the controller. Since the OS knows nothing about heads, tracks, cylinders for modern commodity disks, it's futile to try to schedule I/O for them. it simply uses a write through caching scheme (file system structure updates may be cached for a short bit I think). I also believe it does no scheduling or coalescing of IO requests, just simply FIFO (or whatever the bus/disk driver is implemented to do). It leaves the scheduling up to the disk / array controller. Coalescing is, in fact, done by the kernel, as part of the UBC operation. As for the rest, the controllers tend to be sufficiently intelligent that the OS can only confuse the issue by trying to apply strategy. Corrections welcome. Regards, Justin -- Justin C. Walker, Curmudgeon-At-Large * Institute for General Semantics | When LuteFisk is outlawed | Only outlaws will have | LuteFisk *--------------------------------------*-------------------------------* _______________________________________________ darwin-kernel mailing list | darwin-kernel@lists.apple.com Help/Unsubscribe/Archives: http://www.lists.apple.com/mailman/listinfo/darwin-kernel Do not post admin requests to the list. They will be ignored.