Hi Alistair On Wed, Aug 8, 2012 at 1:44 PM, Alistair Lynn <arplynn@gmail.com> wrote:

I've been digging about a little, but to no avail. Whereabouts in the kernel might I find the I/O scheduler?

I assume you mean disk I/O scheduling? It's a bit dated, but Amit Singh's Mac OS X Internals book mentions that there is no explicit disk scheduler in OSX. (p. 1366) Quote: Note that Mac OS X does not use explicit disk scheduling. In particular, I/O requests are not explicitly reordered, although the non-I/O Kit parts of the kernel may defer a request in order to combine several requests into a single large request. To the best of my knowledge, nothing has changed about this since 10.4, which was the latest version at the time the book was published. Maybe someone more knowledgeable can point out where in the VFS/UBC system the request combining happens - all I can say is I've certainly seen it in action from the requests it generates. There is also speculative read-ahead that causes I/O request sizes to grow exponentially when you read large files sequentially, which is presumably done in the same place. (you can easily observe this in action using dd and a filter scheme driver that logs requests) The IOBlockStorageDriver class (much further down the stack) does handle request deblocking, that is, ensuring that requests are aligned to sector boundaries, and splitting them so they don't exceed hardware limits for maximum request size etc. but that's not really scheduling as such. I suspect the reason why Apple never bothered to implement a scheduler might be that storage devices these days have hardware support for scheduling requests (SATA NCQ/SCSI TCQ) anyway, and requests to solid-state storage don't need scheduling anyway. The storage stack also doesn't support insertion of barriers, which would make reordering dangerous. (IOStorage::synchronizeCache() essentially acts as a crude barrier, though its semantics are a little fuzzy - see darwin-drivers/darwin-kernel list archives from many years ago for details) As a comparison, Linux actually disables its I/O scheduler on devices which support NCQ/TCQ. I think it does the same for non-rotational devices. Hope that helps (and I hope I got all of the above right) phil _______________________________________________ Do not post admin requests to the list. They will be ignored. Darwin-kernel mailing list (Darwin-kernel@lists.apple.com) Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/darwin-kernel/site_archiver%40lists.... This email sent to site_archiver@lists.apple.com