on 26/07/2012 01:08 Alexander Motin said the following: > Different controllers have different command queueing limitations. If you are > testing with ahci(4) driver and modern disks, then their 32 command slots per > port can be enough for many workloads to enqueue all commands to the hardware > and leave queue empty as you've described. But if you take harder workload, or > controller/ device without command queueing support, extra requests will be > accumulated on that bioq and sorted there.
Alexander, thank you for the reply. Indeed, using 64 parallel dd processes with bs=512 I was able to 'kick in' the disksort logic. But I am not sure if the disksort algorithm makes much difference in this case given the number of commands that a disk firmware can internally re-order. (Not mentioning that potentially disksort could starve some I/O bound processes in favor of others -- but that's a totally different topic). But then, of course, for the less capable hardware the disksort could still be a significant factor. -- Andriy Gapon _______________________________________________ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "freebsd-hackers-unsubscr...@freebsd.org"
