Date: Fri, 5 Jan 2007 10:34:17 -0800 From: Luigi Rizzo <rizzo@icir.org> To: Robert Watson <rwatson@freebsd.org> Cc: freebsd-current@freebsd.org, lulf@stud.ntnu.no, freebsd-geom@freebsd.org Subject: Re: Pluggable Disk Schedulers in GEOM Message-ID: <20070105103417.B91349@xorpc.icir.org> In-Reply-To: <20070105140941.B98541@fledge.watson.org>; from rwatson@freebsd.org on Fri, Jan 05, 2007 at 02:14:26PM %2B0000 References: <20070105015800.s3rqdzgm8k8owk4s@webmail.ntnu.no> <20070105140941.B98541@fledge.watson.org>
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On Fri, Jan 05, 2007 at 02:14:26PM +0000, Robert Watson wrote: > > On Fri, 5 Jan 2007, lulf@stud.ntnu.no wrote: > > > Anyway, I'd like to research a bit on this topic to just see how much it > > does matter with different I/O scheduling for different purposes. > > I think working on this is interesting, but the one caution I'd have is that > it's possibly to introduce serious priority inversions through any complex > scheduling scheme for I/O. In our VFS, I/O is frequently performed while > holding locks or things that act like locks -- for example, during a directory > lookup, while pulling an inode off the disk, etc. The I/O will be initiated > by one thread, but then other threads will end up waiting for it also. If > there is a naive mapping of initiating thread priority to I/O request > priority, then you can end up with high priority threads being blocked on a > low priority tasks, leading to nasty starvation effects, especially if the > scheduler allows indefinite waiting for I/O at a low priority. This, at a that's a problem with priority based schedulers. neither the elevator nor the proportional-fair scheduler in Hybrid, nor a plain FIFO scheduler soffer from this 'indefinite waiting' problem. If i remember well the elevator code in freebsd had some support for 'prioritized' requests that could go in front of the queue no matter what, but i think that part was not really used. cheers luigi
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