Slashdot Mirror
Rik van Riel on Kernels, VMs, and Linux
Andrea Scrimieri writes "
An Interesting
interview with Rik van Riel, the kernel developer, in which he talks
about the Linux's VM, particurarly about his own implementation (which was
recently adopted in Alan Cox's tree). With some controversy towards Linus
Torvalds.
"
>> (which was recently adopted in Alan Cox's tree).
As I understand it, the Rik VM is what we started the 2.4 series with.
The Andrea VM was adopted in 2.4.10 amidst much controvery, and Alan has kept
the Rik VM as a part in the -ac kernels.
25% Funny, 25% Insightful, 25% Informative, 25% Troll
I saw a post on the linux kernel news groups (you can serch for it) about 2-3 weeks ago where Linus says something like "that's why I don't consider you a kernel developer" he always seems to be wining about something. But hey what do I know I'm still trying to get xscreensver to do a mozilla -remote openurl (some url) for a kiosk :)
Anyways, an enlightening, no-holes-barred interview. Enjoyable.
To be honest though, optimising for machines of different sizes really is a no-brainer compared to having to make the VM work with really diverse workloads ;)
Open Source's biggest PR dilema is this sort of argument.
Make no mistake, every company has developers that do this. There's two differences in the Open Source world: 1) you can't just fire an Open Source developer who won't "play ball" with management's edict 2) it's usually public.
These are actually both really good things. The fact that you can't silence someone leads to repeated analysis of a problem. OSS' biggest benefit is that it brings massive peer review to bare not just on the code, but on the process.
The fact that it's public feeds into that, and is equally good.
The problem is PR. The Linux kernel is starting to look like anarchy to non-developers. I suggest that the process works, so we should all take a deep breath and leave it be. However, we all need to take the front lines on PR. Spin is all-important. This is not a "spat" or a "fight", this is "parallel development" and "peer review". The joy of this kind of spin is that, unlike most spin, it's TRUE! This guy is pissed at Linus. Linus has dumped his code. Yet, the two of them keep working hard to meet their customers' demands and producing what they feel is the best possible product.
Please, don't foster the idea that we're a bunch of anarchists producing code that's any less functional than the rest of industry, because quite the opposite is true.
Rik is an extremely bright (and likeable) guy, but his adherence to the OOM killer concept is disappointing. I've seen a lot of dumb ideas gain currency in the computing community or some part of it; OOM killer is the dumbest. If your process was allowed to exist in the first place, it should not be killed by the VM system. The worst that should happen is that it gets suspended with all of its pages taken away. If that doesn't free up any memory then neither would killing it (modulo some metadata - read on). If there are other processes waiting for the one that's suspended, then eventually they'll go to sleep, their pages will be released, and the suspended process will wake up - which won't happen if you killed it. There are only two differences between the two approaches:
The usual whine from OOM-killer advocates is that you can still get into a situation where all of that retained metadata clogs up the system and essential system functions can't allocate pages. However, that's preventable too. All you need to do is preallocate a special pool of memory that's only available for use by those essential system processes - either individually or collectively. The size of that pool and the exact details of how it gets allocated (e.g. which processes are considered essential) could be treated as site-specific tuning parameters. The same idea can then be further generalized to allow definition of multiple private pools, creating a semi-hard barrier between different sets of tasks running on the system (if you want one; the default pool is still there otherwise). This actually fits in very nicely with other things like processor affinity and NUMA-friendly VM, which I know because I once worked on a kernel that had all of these features.
In short, there's no need for the OOM killer. Plenty of systems, many of which handle extreme VM load much better than Linux, have been implemented without such a crock. Rik contends that a lot of people make suggestions without actually understanding the problem, and he's right, but I also submit that sometimes he also rejects suggestions from people who do know what they're talking about. This row has been hoed before, and Rik's smart enough that he should know to avoid the NIH syndrome that afflicts so many of the other Linux kernel heavyweights.
Slashdot - News for Herds. Stuff that Splatters.
Rik's a really smart guy, but he isn't (or, rather, wasn't) so good at keeping the mainline kernel moving forward. Despite his comment about Linus dropping patches (which is true). What he didn't mention is that he never resubmitted the patches. He tried once and then dropped them.
I thinks Rik's VM will become really really good as he maintains a branch for himself. When it's 95% of the way done, he can then work on merging it into maintstream (ie, the Linus kernel). Then we'll have a really kick-ass VM.
But Rik wasn't working well with the established method for dealing with the Linus kernel. Linus then made the choice to go with a VM from someone who *did* know how to work with the Linus Kernel.
It's not a technical issue, it's a maintence issue.
Read up on the kernel cousin stuff with Rik and Linus talking about this.
Ciao!
The Doctor What (KF6VNC)
That's a little strong. True, I haven't used anything but CVS for the last couple years, but last time I tried common alternatives (namely MS VSS and PVCS), they were major PITAes - slow, unreliable, and not helpful when more than one developer was working on the same file. Not to say that CVS doesn't have its problems, of course, but for a number of years it has been the logical choice for anyone who doesn't want to plunk down hundreds of dollars per seat for a closed-source tool.
> there is indeed a /proc entry >(/proc/sys/vm/overcommit_memory)
That setting doesn't work properly. Linux will just overcommit slightly less.
Pushin' 'n dealin', shovin' 'n stealin'