On Mon, Apr 27, 2009 at 03:47:42PM +0100, Jamie Lokier wrote: > Personally, I'm interested in the following: > > - A process with RT I/O priority and RT CPU priority is reading > a series of files from disk. It should be very reliable at this. > > - Other normal I/O priority and normal CPU priority processes are > reading and writing the disk. > > I would like the first process to have a guaranteed minimum I/O > performance: it should continuously make progress, even when it needs > to read some file metadata which overlaps a page affected by the other > processes. That's pretty easy. The much harder and much more interesting problem is if the process with RT I/O and CPU priority is *writing* a series of files to disk, and not just reading from disk. > I don't mind all the interference from disk head seeks and > so on, but I would like the I/O that the first process depends on to > have RT I/O priority - including when it's waiting on I/O initiated by > another process and the normal I/O priority queue is full. > > So, I'm not exactly sure, but I think what I need for that is: > > - I/O priority boosting (re-queuing in the elevator) to fix the > inversion when waiting on I/O which was previously queued with > normal I/O priority, and > > - Task priority boosting when waiting on a filesystem resource > which is held by a normal priority task. For the latter, I can't think of a filesystem where we would block a read operation for long time just because someone was holding some kind of filesytem-wide lock. A spinlock, maybe, but the only time it makes sense to worry about boosting an I/O priority is if we're going to be blocing a filesystem for milliseconds or more, and not just a few tens of microseconds. All of the latency problems people have been complaining about, such as the infamous firefox fsync() problem, all involved write operations, and specifically fsync(), and maybe a heavy read-workload interfered with a write, but I can't think of a situation where a real-time read operation would be disrupted by normal priority reads and writes. For the former, where a real-time read request gets blocked because the read request for that block had already been submitted --- at a lower priority --- that's something that should be solvable purely in core block layer and in the I/O scheduler layer, I would expect. - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
