Kirk McKusick e03486d198 This checkin reimplements the io-request priority hack in a way ... that works in the new threaded kernel. It was commented out of the disksort routine earlier this year for the reasons given in kern/subr_disklabel.c (which is where this code used to reside before it moved to kern/subr_disk.c): ---------------------------- revision 1.65 date: 2002/04/22 06:53:20; author: phk; state: Exp; lines: +5 -0 Comment out Kirks io-request priority hack until we can do this in a civilized way which doesn't cause grief. The problem is that it is not generally safe to cast a "struct bio *" to a "struct buf *". Things like ccd, vinum, ata-raid and GEOM constructs bio's which are not entrails of a struct buf. Also, curthread may or may not have anything to do with the I/O request at hand. The correct solution can either be to tag struct bio's with a priority derived from the requesting threads nice and have disksort act on this field, this wouldn't address the "silly-seek syndrome" where two equal processes bang the diskheads from one edge to the other of the disk repeatedly. Alternatively, and probably better: a sleep should be introduced either at the time the I/O is requested or at the time it is completed where we can be sure to sleep in the right thread. The sleep also needs to be in constant timeunits, 1/hz can be practicaly any sub-second size, at high HZ the current code practically doesn't do anything. ---------------------------- As suggested in this comment, it is no longer located in the disk sort routine, but rather now resides in spec_strategy where the disk operations are being queued by the thread that is associated with the process that is really requesting the I/O. At that point, the disk queues are not visible, so the I/O for positively niced processes is always slowed down whether or not there is other activity on the disk. On the issue of scaling HZ, I believe that the current scheme is better than using a fixed quantum of time. As machines and I/O subsystems get faster, the resolution on the clock also rises. So, ten years from now we will be slowing things down for shorter periods of time, but the proportional effect on the system will be about the same as it is today. So, I view this as a feature rather than a drawback. Hence this patch sticks with using HZ. Sponsored by: DARPA & NAI Labs. Reviewed by: Poul-Henning Kamp <phk@critter.freebsd.dk>		2002-10-22 00:59:49 +00:00
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spec_vnops.c	This checkin reimplements the io-request priority hack in a way	2002-10-22 00:59:49 +00:00