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The State of Linux IO Scheduling For the Desktop?
pinkeen writes "I've used Linux as my work & play OS for 5+ years. The one thing that constantly drives me mad is its IO scheduling. When I'm copying a large amount of data in the background, everything else slows down to a crawl while the CPU utilization stays at 1-2%. The process which does the actual copying is highly prioritized in terms of I/O. This is completely unacceptable for a desktop OS. I've heard about the efforts of Con Kolivas and his Brainfuck Scheduler, but it's unsupported now and probably incompatible with latest kernels. Is there any way to fix this? How do you deal with this? I have a feeling that if this issue was to be fixed, the whole desktop would become way more snappier, even if you're not doing any heavy IO in the background."
Update: 10/23 22:06 GMT by T : As reader ehntoo points out in the discussion below, contrary to the submitter's impression, "Con Kolivas is still actively working on BFS, it's not unsupported. He's even got a patch for 2.6.36, which was only released on the 20th. He's also got a patchset out that I use on all my desktops which includes a bunch of tweaks for desktop use." Thanks to ehntoo, and hat tip to Bill Huey.
If the CPU utilization is that low, it's an I/O scheduling problem. See Linux I/O scheduling.
The CFQ scheduler is supposed to be a fair queuing system across processes, so you shouldn't have a starvation problem. Are you thrashing the virtual memory system? How much I/O is going into swapping. (Really, today you shouldn't have any swapping; RAM is too cheap and disk is too slow.)
FYI, the IO scheduler and the CPU scheduler are two completely different beasts.
The IO scheduler lives in block/cfq-iosched.c and is maintained by Jens Axboe, while the CPU scheduler lives in kernel/sched*.c and is maintained by Peter Zijlstra and myself.
The CPU scheduler decides the order of how application code is executed on CPUs (and because a CPU can run only one app at a time the scheduler switches between apps back and forth quickly, giving the grand illusion of all apps running at once) - while the IO scheduler decides how IO requests (issued by apps) reading from (or writing to) disks are ordered.
The two schedulers are very different in nature, but both can indeed cause similar looking bad symptoms on the desktop though - which is one of the reasons why people keep mixing them up.
If you see problems while copying big files then there's a fair chance that it's an IO scheduler problem (ionice might help you there, or block cgroups).
I'd like to note for the sake of completeness that the two kinds of symptoms are not always totally separate: sometimes problems during IO workloads were caused by the CPU scheduler. It's relatively rare though.
Analysing (and fixing ;-) such problems is generally a difficult task. You should mail your bug description to linux-kernel@vger.kernel.org and you will probably be asked there to perform a trace so that we can see where the delays are coming from.
On a related note i think one could make a fairly strong argument that there should be more coupling between the IO scheduler and the CPU scheduler, to help common desktop usecases.
Incidentally there is a fairly recent feature submission by Mike Galbraith that extends the (CPU) scheduler with a new feature which adds the ability to group tasks more intelligently: see Mike's auto-group scheduler patch
This feature uses cgroups for block IO requests as well.
You might want to give it a try, it might improve your large-copy workload latencies significantly. Please mail bug (or success) reports to Mike, Peter or me.
You need to apply the above patch on top of Linus's very latest tree, or on top of the scheduler development tree (which includes Linus's latest), which can be found in the -tip tree
(Continuing this discussion over email is probably more efficient.)
Thanks,
Ingo