To be fair, the MM primitives that we have today are the same we had in 1979, while the capabilities of the hardware evolved, and the actual needs evolved, too.

For example, caching. It's essentially impossible to do caching at the application layer right unless your cache is also persistent with no durability or consistency needs whatsoever and can be mmaped (and you don't mind total I/O trashage). The persistence part also implies that you must be able to store the entire cache somewhere.

What an application would want here is a mechanism to allocate cache memory that the kernel can throw away, if need be, and notify the application of that, so that the app can adjust. That's not impossible to do, it's just tricky - and beyond POSIX.

I'm pretty sure "real" engineers would be very much in favor of using a cheap resource lavishly, if the tradeoff is worth it. Adding a million dollars to the design cost in order to save $100,000 in resource costs isn't a good idea, and a good engineer will design accordingly.

The difference is that engineers have codified this and call it "safety factor." Your house is built to carry perhaps ten times the load it truly needs to, because it's cheaper to throw in extra materials than it is to build it to hold precisely what it needs. An airplane is built with much thinner margins, because saving materials is far more valuable, and it's well worth the extra time and attention to.

I suppose that works with RAM and CPU waste, but I notice the people building gigabyte consuming irc clients aren't paying for all their users RAM, so the cost tradeoff is rather different.

Look up Hamming's course on "Learning to learn". An optimal design is not robust. You add a safety factor and this addition is wasteful in some sense (thus not optimal) and necessary in another. Do you design a building so that the toilets are optimally used? Likewise for planes.

But just because you have enough of a cheap resource (RAM) it is not a given that using it in excess is either optimal or robust.

Also, most users aren't really aware what's using up the resources - only that their computer gets slower with time. I actually wish Windows would start naming & shaming applications - e.g. by showing a balloon when the system slows down, that would say that application X and Y are consuming most of the resources.

As it is now, CPU, RAM and storage usage are perfect externalities for developers. The result of their abuse is user frustration, and reduction of the number of things people can do on their computers. That, and wasting user time. Unfortunately, there's no way that would make the developers suffer costs for wasting user's resources, so the only thing that lets us have lean software is engineering competence and human decency (both of which seem to be in short supply).

It's also a lot of fun when someone gets the bright idea of "oh, we'll allocate until we have 75% of RAM and then stop; that will keep us from swapping". Which isn't so terrible until somebody installs a second program using that strategy.

My favourite these days is Linux OOM killer. For some people it's just right to randomly kill a process instead of rejecting the current request. My mind gets a blue-screen every time I'm contemplating this issu.,....ds.fsd... :)

edit: https://discuss.96boards.org/t/yocto/576

For Linux systems in (heavy) interactive use early-oom is a must have, pretty much regardless of how much memory you have.

Wait, what?

Memory allocation "doesn't fail" so they kill a random program?

Can someone link me to an explanation of this?

"Memory allocation "doesn't fail""

That's not quite it. But it does deserve a double-take.

When you request memory, it generally works out fine. But it doesn't actually give you a lock on the pages you need until you request those pages. This leads to weird situations where memory is oversubscribed. When this happens the OS casts one of your processes into the rift. Google for "oom killer".

To me this sounds insane - a basic breach in the contract of malloc. However, it's possible that this is one of those situations that involves a complex tradeoff that you won't grok until you've been the guy building it. It would be interesting to know what other OSs do.

Something that is annoying: as far as I know there is no signal that fires when this happens. Instead, you scrape logs to learn about it. Whereupon you will reboot the box, because you no longer trust its state. You can get a callback when this happens if you use systemtap.

In practice it's less of a problem than you'd think. If you care about the thing you're building, you will have made it host-independent. From the perspective of production software, there's no difference between a rifted process and a motherboard death. If you're using sockets as your IPC mechanism, it doesn't matter why the process died, just that it did.

To elaborate a bit, there are basically three ways to do this stuff.

One is to commit memory on request. If all of your RAM and swap is committed, then malloc will fail. This is probably how we expect it to work, but it can be really wasteful. Some programs will malloc a huge area and only use a small portion of it, and in this scheme, that memory is "in use." It's also worth noting that most programs don't tolerate malloc failure well at all, and will just crash anyway.

Another way is to overcommit, where malloc doesn't actually reserve memory initially but only on first use, but only pause processes when memory fills up. If a process needs more memory and there isn't any more available, that process blocks until more memory becomes available. This means nobody gets killed, but it's not uncommon for the system to just deadlock like this forever once memory fills up.

And the third way is as you describe, to overcommit and kill processes when you run out of memory.

Note that there's no right answer, just different tradeoffs. Linux lets you configure this to an extent as well, so if you don't like the default, you can change it!

> It's also worth noting that most programs don't tolerate malloc failure well at all, and will just crash anyway.

But at least then it's the program that doesn't handle failure that dies, not one that does!

> Some programs will malloc a huge area and only use a small portion of it, and in this scheme, that memory is "in use." It's also worth noting that most programs don't tolerate malloc failure well at all, and will just crash anyway.

...and it would be absurd for bad programs to eat memory and crash? We must have other programs pay for its sins?

Tha program that hits a malloc failure will not necessarily be the program that ate up all the RAM.

Malloc failure is at least synchronous with the out of memory condition. And if your program has reached a fixed state, it may not even be calling malloc.

Ot: your flak blog could benefit from a page showing links to all articles. Currently, hard to read beyond 'recent'.

Don't waste your time with the old crap. :)

Sure, but (a) it can handle the malloc failure and (b) it's pretty easy for the user to find the bad guy.

It might handle the malloc failure by crashing. Some languages (e.g., Rust) and runtimes (e.g., glib) presume that small allocations always succeed; a failure in such an allocation immediately crashes the program (as the underlying API provides no means to report the failure).

Rust the language doesn't know anything about allocation; it's entirely a standard library decision.

(a) it probably won't therefore (b) the "bad guy" is going to be most/all processes.

The OOM killer doesn't pick the process by an entirely random method, it seeks to kill the problem

Android handles this by having a total ordering of the importance of every application process, and building its APIs such that process death of something the user isn't interacting with doesn't lose state.

iOS is like that too. It keeps all programs running by default, and then when memory gets low it'll start killing them. It starts with backgrounded apps that aren't doing anything, and kills the current foreground app as a last resort.

Apps are supposed to be built to tolerate being killed while in the background and restoring their state when relaunched, so in theory this will mostly be invisible to the user other than apps taking longer to reload. In practice, lots of apps don't get this right.

After a fork, does the amount of committed memory increase by the size of the child's allocated virtual memory space?

I'm curious how committed memory interacts with COW fork semantics. After all, a child can just overwrite the COW/copy of the memory it inherited from its parent without allocating anything else.

No, all of the child's memory mappings are shared with its parent. Upon write, though, that becomes a new committed mapping; I believe the entire page is copied, so you could end up committing another 4k with a one-byte write from the child.

Hm. Then suppose I turn off the OOM killer. What happens to a process that writes to its parent's page if the system has no memory left to commit?

COW is usually a kind of loophole. Most systems only count the pages once (otherwise you'd have to reject the fork, but that becomes inconvenient fast).

There's a switch you can toggle that will turn off the OOM (vm.overcommit_memory).

The downside is memory allocations that would oversubscribe memory will immediately fail. That means processes won't be able to allocate tens of gigabytes of virtual memory anymore.

If I understand the limitation correctly, this also means that all process's allocated virtual memory can no longer exceed the amount of physical+swap memory in the machine. I think this probably interacts badly with COW memory semantics for child processes: even though children and parent reuse address space, the kernel can't know whether the child will write to the memory it inherited from its parent's memory map. I think if the kernel is being too careful, all "shared" memory would have to be accounted separately, which would severely limit the number of processes you could have open.

(Is that really what happens? That seems awfully limiting. I hope I'm wrong.)

You can still allocate more virtual memory; it just has to be backed by a file, e.g., through mmap(). So, for example, what mongod does — mmap() in its entire DB, which can be terabytes of RAM — would still work.

You can almost think of file-backed mmaps as swap, in fact.

I appreciate your reply (and replies to your reply), but I must be missing something about why this is a good idea.

Is it just that software is (badly) coded to request lots of memory it doesn't intend to use? The only sensible usecase I can think of for that is oversubscribing VMs, but even there... Doesn't this just crash a VM when there's contention for the memory that's oversubscribed?

I guess that would be okay for the VM case, but why would I want this in a normal usecase? It seems like it's just encouraging/supporting bad memory management in software.

Have I just been lucky that this has never killed a long-running job of mine when I use 90%+ of RAM to do a big job? Or shit, maybe it has and the specific error just was obscured. Ugh, I hate when the system lies by default.

How does that help me as a user?

It does help you as a user, since the OOM killer tries to be somewhat sensible in what it kills. Imagine you have 10 well-behaved programs and 1 program that just leaks 1GB/h. After a few hours your entire RAM has been committed and the next process to request memory will fail. In this scenario, the process that crashes is random - it might be the leaky program, or it might be your desktop shell. The OOM killer on the other hand will definitely kill the leaky program once there is literally no more memory available. It's a combination of programs not using up all pages they mmap and there being no good ordering of which memory requests are more important that leads to overcommitting and the OOM killer being generally good ideas. In my experience that's exactly how it works out - the moment you start something that gobbles up memory due to a bug, it gets killed promptly, rather than some other random program crashing due to being unable to allocate memory.

> Something that is annoying: as far as I know there is no signal that fires when this happens. Instead, you scrape logs to learn about it. Whereupon you will reboot the box, because you no longer trust its state. You can get a callback when this happens if you use systemtap.

It's a SIGKILL, which isn't catchable/handleable by the process (how could it be?).

The machine should still be stable after an OOM, just minus a few process. (In my production setups, usually a supervising daemon can restart the now-dead child, and begin the memory leaking cycle anew.)

The real drawback of disabling memory overcommitting is not that processes can no longer allocate huge amounts of unused memory, but that processes that use more memory that the remaining availabe RAM + swap can no longer call fork (even if the forked process like most forked processes would immediately discard all the shared memory by execing some small program).