Slashdot Mirror
Jens Axboe On Kernel Development
BlockHead writes "Kerneltrap.org is running an interview with Jens Axboe, 15 year Linux veteran and the maintainer of the linux kernel block layer, 'the piece of software that sits between the block device drivers (managing your hard drives, cdroms, etc) and the file systems.' The interview examines what's involved in maintaining this complex portion of the Linux kernel, and offers an accessible explanation of how IO schedulers work. Jens details his own CFQ, or Complete Fair Queue scheduler which is the default Linux IO scheduler. Finally, the article examines the current state of Linux kernel development, how it's changed over the years, and what's in store for the future."
FreeBSD dispensed with them altogether years ago...
Character devices only, thank you very much.
*Duck*
In Soviet Washington the swamp drains you.
"the piece of software that sits between the block device drivers (managing your hard drives, cdroms, etc) and the file systems.'"
That sounds REALLY hard. I'd be more interested if there's a development strategy he could recommend re: complex development projects.
stuff |
I thought the title was: Ewe Boll On Kernel Development...
If core changes of such magnitude are no longer sufficient to merit a dev branch or even a major point release, why bother with the "2.6" designation at all? Just pull a Solaris and call the next release "Linux 20" or "Linux XX."
-Isaac
I am not a lawyer, and this is not legal advice. For Entertainment Purposes Only.
I did a double take when I saw this, as Jens was an exchange student at my high-school way back when. Small internet.
An excellent read!
Something exciting about delving in the low level logic that gives you the feeling that there's always something more to learn !
I guess always being two steps behind is the motivation that makes it all worth while.
15 year Linux veteran and the maintainer of the linux kernel block layer,...
In the interview he says he is now 30 years old. Wow that means he started working in Linux at the age of 15 - a real prodigy. A very interesting interview.
Btw, it is nice that kerneltrap.org has finally had a make over. The earlier website design looked rather drab.
Linux Help
for all things on Linux
I wonder, if the originating process' priority is taken into account at all... It has always annoyed me, that the "nice" (and especially the idle-only) processes are still treated equally, when it comes to I/O...
In Soviet Washington the swamp drains you.
Anticipatory is, according to my menuconfig:
.config
The anticipatory I/O scheduler is the default disk scheduler. It is
generally a good choice for most environments, but is quite large and
complex when compared to the deadline I/O scheduler, it can also be
slower in some cases especially some database loads.*
Anticipatory is also preselected with a fresh
Am I the only one that misread that as "an interview with Jens Axboe, 15 year old Linux veteran" ?
Is this the part of the kernel that's responsible for making systems really slow during extended disk writes, while the CPU utilization is minimal?
At risk of starting a holy war, is there any reason why one approach would be superior? And do they lend themselves to different methods of scheduling? In TFA, Axboe talks about [1] the scheduling mechanism used in later versions of the 2.6 kernel series, which alleviates a problem that I (and most other people, probably) have run into before.
I'm curious, because although I don't use any of the 'real' BSDs very often -- I spend most of my time (at home, anyway) using either Mac OS X, which uses the Mach/XNU kernel (which is derived from 4.3BSD, although I don't know if the I/O scheduler has been rewritten since then), or Linux with the 2.6 kernel, and it seems to me that OS X's disk I/O leaves something to be desired compared to Linux's.
Does BSD handle I/O differently in some fundamental fashion than Linux? It sounds like, by eliminating block devices, that they basically remove the kernel from doing any re-ordering or caching of data, which makes things "safer" (in the event of a crash) but seems like it would have big performance penalties when using drives that aren't very smart, and don't do a lot of caching and optimization on their own. It seems like getting rid of I/O scheduling altogether is a stiff price to pay for "safety."
[1] (quoting because there doesn't seem to be anchors in TFA)
"Ladies and gentlemen, my killbot features Lotus Notes and a machine gun. It is the finest available."
I think it would be more correct to say:
[His] is the the part of the kernel that's responsible for making systems slightly less slow during extended disk writes, while the CPU utilization is minimal.
And even that's not quite true, where the scheduler really comes into play is when you have two or more processes trying to access the disk at the same time. During an extended, sustained read or write, the scheduler probably just needs to stay the hell out of the way and pass data as fast as it can.
You could also say, that as a secondary priority, he's responsible for keeping the CPU utilization minimal, during those disk writes...
"Ladies and gentlemen, my killbot features Lotus Notes and a machine gun. It is the finest available."
As a native English speaker, comfortable with Spanish and aware of the basics of French (so I'm not entirely uneducated), I am entirely unequipped to reason the pronunciation of "Jens Axboe." Can someone help me out?
My Freakin Blog
Back in school we pronounced it with a "y" sound for the "j": "Yens" rhymed with "mens." Now, as to weather that was actually the correct pronunciation or merely something close enough that he didn't bother correcting us; I couldn't say.
Thank you very much. Much of this article is informative, technical and really, really nerdy. I for one sit through dupes and rubbish like today's meaningless benchmarking of differing minor kernel versions in the hope of reading articles like this.
BTW, does anyone have a good set of benchmarks of the performance of different IO schedulers when running one or two or three IO intensive tasks, when running one intensive and many small tasks, etc.? That would actually help me decide whether to rebuild my kernel with CFQ.
Also, ionice would have made my old machine much more usable when doing backups... Oh well.
# cat
Damn, my RAM is full of llamas.
Now, as to weather that was actually the correct pronunciation or merely something close enough ...
;-)
Close enough.
Are there any hard metrics on what the performance advantages are of various schedulers, under typical load conditions?
Reading TFA piqued my interest into I/O scheduling and I've been doing some reading on it, and it seems like there are several competing schools of thought, of which Axboe (and potentially the Linux kernel developers generally) are only one.
An alternative view, such as this from Justin Walker (a Darwin developer) on the darwin-kernel mailing list, holds that it's not worthwhile for the OS kernel to do much disk scheduling, since "the OS does not have a good idea of the actual disk geometry and other performance characteristics, and so we [kernel developers] leave that level of scheduling up to the controllers in the disk drive itself. I think, for example, that recent IBM drives have some variant of OS/2 running in the controller. Since the OS knows nothing about heads, tracks, cylinders for modern commodity disks, it's futile to try to schedule I/O for them." (written Mar 2003)
Axboe seems to acknowledge that this may sometimes be the case, because they do have the 'non-scheduling scheduler,' which he recommends only for use with very intelligent hardware. However, it seems like some people think that commodity drives are already 'smart enough' to do their own scheduling.
It seems like determining which approach was superior would be relatively straightforward, and yet I've never seen it done (although maybe I'm just not looking in the right places). Anecdotally, I'm tempted to agree with Axboe, since it seems like, when doing things where several processes are all thrashing the disk simultaneously, my Linux machine feels faster than my OS X one, but this is by no means scientific (they don't have the same drives in them, not working with the same datasets, etc.).
On what drives, and under what conditions, is it advantageous to have the OS kernel perform scheduling, and on which ones is it best just to pass stuff to the drive and let the controller do all the thinking?
"Ladies and gentlemen, my killbot features Lotus Notes and a machine gun. It is the finest available."
I am not proficient in sound-writing. (Or whatever it is called..)
But since Jens is a Dane, like myself I'd give it a shot.
Yens Aksbo
Where Yens is pronounced with the pressure on the e.
Yêns
And boe is pronounced without the e, and with the pressure on the a.
âksbo
Hope this helps.
Jens is NOT pronounced "Djens". "J" is pronounced as a Palatal approximant in Danish - just like "y" in English. Yens is somewhat more correct, but the "e" has to be pronounced like the IPA [æ]. Danish is not logic at all. If it was, "Jens" would be spelled with a "æ". Take a look at Jens.
IPA: [jæns]
Axboe is more complicated:
IPA transcription of Axboe would be something like: Open back unrounded vowel + [ksbø]
(I can't get the IPA sign for "Open back unrounded vowel" to display in Slash)
"It sounds like, by eliminating block devices, that they basically remove the kernel from doing any re-ordering or caching of data, which makes things "safer""
No; FreeBSD's shifted the buffer cache away from individual devices and into the filesystem/VM, where it caches vnodes rather than raw data blocks. The IO queue (below all this block/character/GEOM stuff) is scheduled using a standard elevator algorithm called C-LOOK. It's showing it's age in places, and there's been some effort towards replacing/improving it, making it pluggable etc (e.g. Hybrid); sadly it's a tricky problem to solve properly. See this recent thread.
That's awesome! Thanks.
My Freakin Blog
I made a mistake. In this case (Axboe) "oe" is just pronounced "o". This is the Close-mid back rounded vowel IPA: [o].
Last time I checked, there are approximately 5-10 major distros. The vendors have to do their due diligence on any kernel. It's not like RedHat picks a kernel and sits on it for several years. Cherry picking fixes isn't tons of fun, but it's not impossible and if the bugs don't affect anyone why exactly do they need to be fixed again? It's not like those people don't communicate or you can't work with them if you choose to. SuSE and RedHat guys talks. SuSE and Gentoo talk. Having everyone attempt to stabilize their trees independently is insane... that's why I'm saying everyone attempting to do their own is crazy. But attempting to make one group of folks stabilize one tree for the whole world is at least as insane.
Pretty much everybody else bases their kernel sources off of one of those 10-20 trees (counting major kernel developers that maintain reasonable stable trees). There are no free lunches RedHat and SUSE get free engineering and major movement forward. To aquire that they take on QA and stabilization work. More importantly they get to pick when they suffer their pain. Attempting to state that the developers have to absorb all that and never allow anything that isn't stable and tested out the door is insane, we just left that madness. It wasn't stable then anyways, and pretending that you can merely engineer your way towards perfect software that is moving as fast as Linux currently does is ignoring reality. They get to pick what they feel has the most stable base to move to. It's an engineering decision. The forward advancements that Linux is making make moving forward worth the effort and expense of stabilizing a version every 18 months or so.
In terms of Solaris or OpenBSD, vote with your feet. Move towards them and let the people who have figured out a way to deal with the pains you describe do it. Linux works, because it works... That sounds like a tautology, but it's true. What they are doing works for them. By nearly all accounts, everyone is much happier with the current model then the disaster that was the 2.2 to 2.4 transition. It took forever, and when you finally got it wasn't stable until the vendors did their vendor thing to it. I ran the gambit of kernels between 1.3 and 2.4... 2.6 is a much nicer program to be on...
The 2.6 kernel is moving forward and getting major developments out into the hands as broad a set of people as possible. It's a thing of beauty.
Kirby
I was a little disappointed when he said filesystems like Reiser4 and ZFS don't affect the block layer. I'm not sure about ZFS, either, but I do know that Reiser4 can do stuff above and beyond what the block device layer can do, these days.
How do I know? Why, it's on the Namesys webpage!
Hi John!
That is correct, like a "y", rhymes with "mens". I saw another question on the lastname, I typically tell foreigners that it is pronounced ax-bow. Europeans often think the 'oe' is like the Danish "ø", however that is not the case.
He there, good to see you still exist! So, is kernel.dk always STFU or is that just up for /.ing?