Linux 2.2 and 2.4 VM Systems Compared

Derek Glidden writes "I got sick of trying to figure out from other people's reports whether or not the 2.4 kernel VM system was broken or not, so I decided to run my own tests, write them up and post them online. The short conclusion is that the 2.4 VM rocks when compared with 2.2, but there's more to it than just that."

Better but bad

[Chocky2]

2.4 VM is, IMO, a significant improvement over the 2.2 VM, but completely rewriting something as important as VM management is intrinsicaly risky and it's difficult to predict with even the slightest confidence many of the consequences of such a change. This sort of thing should be left for major revisions.

BSoD

[parc]

From the article:

These were pretty uninteresting - just sitting there watching the kernel compile. Except that at one point, while running the 2.4.13 kernel, the hard drive started grinding away with the drive light pegged on continuously, the display became extremely sluggish and quickly froze up entirely, and about ten minutes later, the hard drive light went off but the machine remained unresponsive, requiring a hard reboot. I don't think this was related to anything I was doing as I wasn't actually doing a compile run at the time - probably just a random occurance, but worth mentioning.

So the machine essentially BSoD'd, but it's not interesting?

why not more than one?

[Karmageddon]

when I learned computer science--which I admit was a long time ago, but that means the "gurus" have all had plenty of time to catch up--they taught us that if you obeyed the principles of modularity that you could have more than one implementation of something and use what was appropriate for the particulars of a given situation....

...so why does linux have 1 VM? it seems that 2 of them exist, and the BSD's have more... guys, "gimme a hunk" and "page fault" aren't exactly rocket science anymore, particularly with hardware support... the fact that there is room to make a big deal out of this is the problem, not the VMs.

VM Trouble and Linux 2.5

[PineHall]

I am glad to see the 2.4 VMs doing so well. I assume that Linus is not at all satisfied with the VM code and that is the reason the 2.5 branch is not started. Hopefully it will start soon when the VM trouble is solved!

The one major difference

[matty]

...is that Linux's warts are fully out in the open for all to see. Microsoft would never admit to such failings openly, even though anyone who has used Windows extensively is painfully aware of them.

And it's been my experience that you don't hear, "Linux never crashes" that much anymore. At least I don't say it anymore, whereas I used to. I would still say that a properly configured Linux box is more stable than any Windows box, but I've had my share of lockups. (on the desktop anyway. You'll notice my server has been up for 140+ days. The last reboot was when the power supply died [it's a patched together P166] which interrupted 243 days uptime)

All the mailing lists are public, and all of Linux's problems are there for anyone to see. This allows people to make truly informed decisions about which version of Linux to use, or whether to even use it at all. (Yes, of course these things are also true of *BSD) The current issues are why I still run 2.2.19 on my servers, since none of them get anywhere near enough load to need the newer VM's. "Stable" is definitely a relative term.

Re:why not more than one?

[Anonymous Coward]

"Too ugly for words." -Alan Cox [zork.net]

Re:Would also be interesting...

[TheAwfulTruth]

Heh, If I ever said anything like this at work I'd get a "Are you afraid to code" out of my boss...

Systems are getting more complex and demands on them get more complex. People have to plan harder and think harder to keep up. It's time to step up to the plate or go home. Years ago Linux didn't need a VM that worked with 4 way SMP and 2 Gig of ram. Today it does.

Claiming that modularity is (I'm paraphrasing) "too hard" comes off more as a cop-out than a reason. If it's hard, do a better job at it. I don't think anyone is claiming that the VM should only take a weekend to do... They just want it done and done right. The argument FOR modularity put forth above is a solid one. Whatever planning/archeteting/coding/testing and debugging that takes. Just do it[tm].

Re:This is all well and good

[Anonymous Coward]

Unfortunately, it wasn't resolved. It's only getting close now because Linus made the "outrageous" move of introducing a new one in the middle of a stable cycle. Whether you agreed with that move or not, it's working out brilliantly. Hopefully after this stabilizes a bit, LVM and devfs, both rat's nests of bugs, can be the subjects of the next holy war.

Re:Would also be interesting...

[haruharaharu]

Claiming that modularity is (I'm paraphrasing) "too hard" comes off more as a cop-out than a reason

Yes, let's make a fundamental, pervasive, part of the kernel hot pluggable, introduce tons of potential bugs and incompatibilities and create lots of work, all for questionable benefit. Engineering involves a series of tradeoffs and, in this case, most people see the pain as being too great for the potential payoff.

But, if you still want to, go right ahead. That's one of the cool thing about Linux: if nobody else wants to do it, you still can.

What about the VM work done in other *nixes?

[hargettp]

I do not follow *BSD nearly enough to make this kind of observation, but I thought I recalled that when Universal Virtual Memory [nec.com] was rolled into NetBSD, it was widely regarded as a good design. Anybody with much more VM design knowledge able to comment on how suitable a design like that one (or other well-regarded VM design from other Unixes) would be for the Linux kernel?

Re:Linux needs professionalism in release manageme

[leku]

And that professionalism comes in form of distributors like Redhat/Mandrake/whoever. When they release updated kernels those kernels work as you'd expect.

I don't think you should expect every "business" to go compile every single kernel update that comes along except of course when there are some serious security/whatever issues involved - in which case the distributor releases an updated kernel.

People who like to experiment or live on the edge compile and use all the hottest kernels and they are also the ones who report of problems, which then get fixed. I don't know how professional this is but I'm happy to see new kernels being released often rather than waiting and wondering for weeks/months if there's going to be a new release some day or not.

Hmm, my first post, hope this works.. :-)

Re:Better but bad

[Anonymous Coward]

Linux 2.4 is a major revision. If we waited until Linux 3.0 until a new VM subsystem came out, we might as well call Linux 2.4 "Linux ME" and Linux 3.0 "Linux ... 2007"? Remember that the Linux development cycle is vastly different to other operating systems (FreeBSD included), and Linus is rather pragmatic about changes. (2.4.10, with the new Andrea VM, should be proof of that.) The Linux VM needed rewriting, so it was rewritten. It's that simple.

Re:Why is the VM important?

[Ziviyr]

Working set or rarely used bloat?

It wouldn't be horribly useful if the CPU was fixated on disk bound data. But for lazy programs with little concept of memory efficiency. Swap out, and if its needed again swap in.

All the more space for disk buffers.