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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.
This issue got so bad for me I switched to FreeBSD.
Isn't this also relevant when using Linux on a server? I mean, if one process or thread is copying a large file, you don't want your server to come to a crawl.
It doesn't sound like just a "desktop" issue to me.
If Pandora's box is destined to be opened, *I* want to be the one to open it.
have you tried ionice?
Do you even lift?
These aren't the 'roids you're looking for.
..download and compile the 2.6.36 kernel. A feature of the changes can be found at http://www.h-online.com/open/features/What-s-new-in-Linux-2-6-36-1103009.html
A very very easy to follow guide can be found at http://kernel.net/articles/how-to-compile-linux-kernel.html
Sidenote - What is up with not being able to paste links? That's annoying.
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. http://ck.kolivas.org/patches/bfs/ He's also got a patchset out that I use on all my desktops which includes a bunch of tweaks for desktop use. http://www.kernel.org/pub/linux/kernel/people/ck/patches/2.6/
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.)
I've wondered on occasion if this problem is really only due to scheduling. After all, most of us still write our file access code more or less as follows: x=fopen('somefilename'); while ( !eof(x)) { print readln(x,1024); /* ---- */
}
fclose(x);
Point being, there's nothing that tells the marked line that the process should gracefully go to sleep while the drive is doing its thing, and there's no callback vector defined either- nothing that indicates we're dealing with non-blocking I/O. I'd like to think that our compilers have silently been improved to hide those implementation details from us, but I have no proof that this is the case. Unless the system functions use some dirty stack manipulation voodoo to extract the return address of the function and use that as callback vector?
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That was a joke, right? You don't really think that all the millions of desktop Linux users just up and vanished because some idiot at PCWorld wanted a catchy headline?
This is almost certainly not the IO scheduler's problem. IO scheduling priorities are orthogonal to CPU scheduling priorities.
What you are likely running into is the dirty_ratio limits. In Linux, there is a memory threshold for "dirty memory" (memory that is destined to be written out to disk), that once crossed, will cause symptoms like you've described. The dirty_ratio values can be tuned via /proc, but beware that the kernel will internally add its own heuristics to the values you've plugged in.
When the threshold is crossed, in an attempt to "slow down the dirtiers", the Linux kernel will penalized (in rate-limited fashion) any and every task on the system that tries to allocate a page. This allocation may be in response to userland needing a new page, but it can also occur if the kernel is allocating memory for internal data structures in response to a system call the process did. When this happens, the kernel will force that allocating thread (again, rate-limited) to take part in the flushing process, under the (misguided) assumption that whoever is allocating a lot of memory is the same thread that is dirtying a lot of memory.
There are a couple ways to work around this problem (which is very typical when copying large amounts of data). For one, the copying process can be fixed to rate limit itself, and to synchronously flush data at some reasonable interval. Another way that a system administrator can manage this sort of task (if automated of course) is to use Linux's support for memory controllers which essentially isolates the memory subsystem performance between tasks. Unfortunately, it's support is still incomplete and I don't know of any popular distributions that automate this cgroup subsystem's use.
Either way, it is very unlikely to be the IO scheduler.
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
He tried that before. I think he's given up on getting his scheduler (though perhaps not a suspiciously similar one written by Inigo) in the kernel after what happened with CFQ.
I am trolling
I would definitely ditch an OS that fucked up a file copy because I used the computer for something else while I was waiting.
I've encountered situations where I'm trying to do something online and a task starts up due to a cron job that builds some kind of index. The index building should be in the background but somehow takes priority over what I'm doing on the desktop. Those kinds of cron jobs should be default scheduled in the background, not take priority over what is happening on the desktop.
That's great that you post your experiences with server scheduling in a topic about desktop scheduling. It's so relevant. No wait, it's not.
The boundary between the desktop space and the server space is rather fluid, and many of the problems visible on servers are also visible on desktops - and vice versa.
For example 'copying a large amount of data' on a server is similar to 'copying a big ISO on the desktop'. If the kernel sucks doing one then it will likely suck when doing the other as well.
So both cases should be handled by the kernel in an excellent fashion - with an optimization/tuning focus on desktop workloads, because they are almost always the more diverse ones, and hence are generally the technically more challenging cases as well.
Thanks,
Ingo
Generally Windows runs badly without a swap. Don't listen to people who tell you to disable it. You should have a swap file on Windows no matter how much memory you have.
Tweakers who don't really understand anything about Windows paging often conclude turning off the swap is a good idea, because they only run trivial applications and don't experience certain memory backed I/O operation failing with it off. They do see an initial speed boost though. The reason is NT is very pessimistic about memory. Windows assumes you will need to page out to disk. It therefore flush the set of static pages to disk almost right away. This is why there is so much more disk thrashing on Windows than say Linux when you start an application and plenty of memory is free. It will do its best to keep the working set out of the page file of course. This does give Windows a performance advantage under memory pressure however. When there is not enough memory to start a new application Windows can just drop the pages from memory of the application being paged out without the need to flush them to disk because they are already there; Linux will need to write those pages.
Given that Windows boxes (desktops anyway) tend to have large numbers proccess running in the background so they usually are under that memory pressure.
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I would definitely not let a monkey like you get near my computers if some intense file copy was going on and they wanted to start doing other things while that was going on, sure you can do it but that does not make it a prudent thing to do, and the file may copy over just fine, and it may lose a few bits without even reporting any errors and that can happen on any OS, BSD, Linux, Winders & etc...etc...etc...
You sir, are a perfect specimen of a BOFH. You only have a dim notion of what actually goes on inside those mysterious boxes that are unfortunately left under your care. And yet, by some curious accident of nature, you've been entrusted with root passwords for said boxes. You use phrases like "intense file copy" like they mean anything. You place every idiotic restriction that you can think of on the users of said boxes (who, incidentally, are almost always smarter and more qualified than you in whatever field of work they're in) by using words like "prudent" and "safety"... or god forbid... "security". You actually think that because I run a second program along with your "intense" copy, it can result in loss of "a few bits without even reporting any errors" due to what ? The magical fairies that dance inside those little chips getting angry ? Tired ? Can you do everybody a favor and reduce the amount of utter nonsense emanating out of that tiny, befuddled brain ?
He tried that before. I think he's given up on getting his scheduler (though perhaps not a suspiciously similar one written by Inigo) in the kernel after what happened with CFQ.
One reason for why the principle of CFS may seem to you so suspiciously similar to Con's SD scheduler is that i used Con's fair scheduling principle when writing the initial version of CFS. This is credited at the very top of today's kernel/sched.c [the scheduler code]:
* 2007-04-15 Work begun on replacing all interactivity tuning with a
* fair scheduling design by Con Kolivas.
It was added in this commit.
The scheduler implementations (and even the user visible behavior) of the schedulers was and is very different - and there is where much of the disagreement and later flaming came from.
Note that this particular Slashdot article is about IO scheduling though - which is unrelated to CPU schedulers. Neither Con nor i wrote IO schedulers.
There are two main IO schedulers in Linux right now: CFQ and AS, written by Jens Axboe, Nick Piggin, et al.
What adds fuel to the confusion is that it is relatively easy to mix up 'CFQ' with 'CFS'.
Thanks,
Ingo
AFAIK there are only two I/O schedulers remaining in recent Linux (and if you squint you might say that RHEL 5's kernel could have been related to 2.6.34 at one point right? :) - CFQ and deadline (three if you count noop I guess). The anticipatory scheduler was removed in 2.6.33...