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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.
have you tried ionice?
Do you even lift?
These aren't the 'roids you're looking for.
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/
There are some interactive-response fixes queued up for 2.6.37 that may help (a lot!) with this stuff.
Start reading here: http://www.phoronix.com/scan.php?page=news_item&px=ODU0OQ
The kernel will preempt the process calling "readln", in other words putting it to sleep.
The kernel will make sure the I/O happens, allowing other processes to work at the same time.
You only need non-blocking code if your own process needs to other things at the same time.
The process will go to sleep inside the read() system call (inside readln() somewhere presumably). Other processes will be able to run in the meantime. It works by interrupting into kernel code, and the kernel changes the stack pointer (and program counter, and lots of other registers) to that of another process. When the data comes back from the disk, the kernel will consult its tables and see that your process is runnable again, and when the scheduler decides it's its turn, in a timer interrupt, the stack pointer will be switched back to your stack. (So yes, dirty stack manipulation voodoo.) Every modern OS works this way.
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.
I just did it and didn't notice any slowdown.
will force everything back into memory
Yes. Here there is another problem at play: cp reads in the whole (big) file and then writes it out. This brings the whole file into the Linux pagecache (file cache).
That, if the VM is not fully detecting that linear copy correctly, can blow a lot of useful app data (all cached) out of the pagecache. That in turn has to be read back once you click within Firefox, etc. - which generates IO and is a few orders of magnitude slower than reading the cached copy. That such data tends to be fragmented (all around on the disk in various small files) and that there is a large copy going on does not help either.
Catastrophic slowdowns on the desktop are typically such combined 'perfect storms' between multiple kernel subsystems. (for that reason they also tend to be the hardest ones to fix.)
It would be useful if /bin/cp explicitly dropped use-once data that it reads into the pagecache - there are syscalls for that.
And yes, we'd very much like to fix such slowdowns via heuristics as well (detecting large sequential IO and not letting it poison the existing cache), so good bugreports and reproducing testcases sent to linux-kernel@vger.kernel.org and people willing to try out experimental kernel patches would definitely be welcome.
Thanks,
Ingo
I think the Phoronix article you linked to is confusing the IO scheduler and the VM (both of which can cause many seconds of unwanted delays during GUI operations) with the CPU scheduler.
The CPU scheduler patch referenced in the Phoronix article deals with delays experienced during high CPU loads - a dozen or more tasks running at once and all burning CPU time actively. Delays of up to 45 milliseconds were reported and they were fixed to be as low as 29 milliseconds.
Also, that scheduler fix is not a v2.6.37 item: i have merged a slightly different version and sent it to Linus, so it's included in v2.6.36 already: you can see the commit here.
If you are seeing human-perceptible delays - especially in the 'several seconds' time scale, then they are quite likely not related to the CPU scheduler (unless you are running some extreme workload) but more likely to the CFQ IO scheduler or to the VM cache management policies.
In the CPU scheduler we usually deal with milliseconds-level delays and unfairnesses - which rarely raise up to the level of human perception.
Sometimes, if you are really sensitive to smooth scheduling, can see those kinds of effects visually via 'game smoothness' or perhaps 'Firefox scrolling smoothness' - but anything on the 'several seconds' timescale on a typical Linux desktop has to have some connection with IO.
Thanks,
Ingo
It's more than that. Since most Linux systems use ext{2,3,4}, CFQ is designed to behave very well with them. However, XFS and JFS do better with deadline or no-op. In fact, on my Athlon 64 X2 w/ 4G RAM, using XFS and CFQ at 2.5GHz did worse than XFS and deadline at 1GHz. Yes, CFQ and XFS clash that badly.
(Site pimp: I did some of my own testing, and reported on it here. I also provide basic shell scripts, so others can do their own tests.)
For years I've wondered if it was just me; everyone I'd asked naturally denied any problems, when all I had to do was delete a 1GB file and I could kiss goodbye to my system for 20 seconds or so.
That's a very well known ext2/ext3 problem - they're really slow at deleting huge files, and the amount of disk access involved in doing so slows down any other application accessing the disk. ext4 should fix the issue. (There's also another subtle bug, finally fixed in 2.6.36, where heavy disk IO can cause processes that aren't doing any IO to become unresponsive.)
The -tip tree contains development patches for the next kernel version for a number of kernel subsystems (scheduler, irqs, x86, tracing, perf, timers, etc.) - and i'm glad that you like it :-)
We typically send all patches from -tip into upstream in the merge window - except for a few select fixlets and utility patches that help our automated testing. We merge back Linus's tree on a daily basi and stabilize it on our x86 test-bed - so if you want some truly bleeding edge kernel but want proof that someone has at least built and booted it on a few boxes without crashing then you can certainly try -tip ;-)
Otherwise we try to avoid -tip specials. I.e. there are no significant out-of-tree patches that stay in -tip forever - there are only in-progress patches which we try to push to Linus ASAP. If we cannot get something upstream we drop it. This happens every now and then - not every new idea is a good idea. If we cannot convince upstream to pick up a particular change then we drop it or rework it - but we do not perpetuate out-of-tree patches.
So the number of extra commits/changes in -tip fluctuates, it typically ranges from up to a thousand down to a few dozen - depending on where we are in the development cycle.
Right now we are in the first few days of the v2.6.37 merge window and Linus pulled most of our pending trees already in the past two days, so -tip contains small fixes only. While v2.6.37 is being releasified in the next ~2.5 months, -tip will fill up again with development commits geared towards v2.6.38 - and we will also keep merging back Linus's latest tree - and so the cycle continues.
Thanks,
Ingo
Depends on the paranoia of the user. FTFY.
Any "sane" filesystem will simply unlink that entry in the directory or table.
The only reason to be physically overwriting the entire space occupied by the 1GB file is some "super secret secure" filesystem used by people scared of having their porn browsing habits discovered by the FBI.
The problem isn't overwriting the data, it's adding the space previously used by the file to the free space bitmap/list. For a 1GB file on an FS with 1k blocks (not uncommon), you're going to be deallocating about a million blocks. Now, unless your system is fragmented horrendously, a lot of those are going to be hits to the same bitmap block (or similar), but you're still looking at writing about 5,000 or so blocks, probably scattered over several cylinders of your disk (=> more than one seek), so on a typical hard disk the process is going to take tens or hundreds of milliseconds at best. If badly fragmented, this could easily take over a second.
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
Sorry dude, it looks like it's a hardware specific problem. I did that on nearly 700G of large files and then fired up the flight sim while it was still going. The only slow down was on file related activity, which is totally what you'd expect. I had it running full screen across two monitors without any drop in frame rate. AND I'm using economy hardware.
It may also be kernel version dependent - with older kernels still showing this bug.
A lot of work has gone into the Linux kernel in the past 2 years to improve this area - and yes, i think much of the criticism from those who have met this bug and were annoyed by it was fundamentally justified - this bug was real and it should have been fixed sooner.
Kernels post v2.6.33 ought to be much better - with v2.6.36 bringing another set of improvements in this area. The fixes were all over the place: IO scheduler, VM and filesystem code and few of them were simple.
This Slashdot article from 1.5 years ago shows when more attention was raised to this category of Linux interactivity bugs.
Thanks,
Ingo
Such drastic change! I have seen this happen on numerous systems and I just change the elevator to "deadline" and poof! The problem is gone. See this discussion for some details. The CFQ scheduler is great for a Linux server running a database, but it completely sucks for desktop or any server used to write large files to.
I see that the bug entry you referred to contains measurements from early 2010, at which point Ubuntu was using v2.6.31-ish kernels IIRC. (and that's the kernel version that is being referred to in the bug entry as well.)
A lot of work has gone into this area in the past 1-2 years, and v2.6.33 is the first kernel version where you should see the improvements. Slashdot reported on that controversy as well.
If you can still reproduce interactivity problems during large file copies with CFQ on v2.6.36 (and it goes away when you switch the IO scheduler to deadline), please report it to linux-kernel@vger.kernel.org so that it can be fixed ASAP. (You can also mail me directly, i'll forward it to the right list and people.)
Thanks,
Ingo