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SubjectRe: Memory overcommitting (was Re:
On Fri, 21 Feb 1997, Kevin Littlejohn wrote:
> fork, vfork - create a child process
> Under bugs, it mentions that vfork is an alias for fork, but given fork on
> Linux does what vfork on other systems do, I believe you have your vfork :)

False. Linux fork() has a standard fork(), *NOT* a vfork.

All modern *NIX'es uses Copy-on-write fork()'s, but that a very different
animal from a vfork()... Not that many *NIX'es have real vfork()'s...

Cut-n-pasted from a machine with a real vfork and the manual pages to back
it up:

vfork() can be used to create new processes without fully
copying the address space of the old process. It is useful
when the purpose of fork() would have been to create a new
system context for an execve(). vfork() differs from fork()
in that the child borrows the parent's memory and thread of
control until a call to execve() or an exit (either by a
call to _exit() (see exit(2)) or abnormally). The parent
process is suspended while the child is using its resources.

There are at least two possible reasons for using vfork():
1. It doesn't use extra swap-space.
2. The OS doesn't have copy-on-write semantic available.

Neither is relevant for Linux, since it belives in memory over-commitment
and have copy-on-write fork()'s.

> > (2) The claims that statically that this is okay 99.9% is hogwash. The
> > airline industry may sell tickets that way, but they don't want their
> > computers doing it. I've been in line when it has happened to them and
> > its not a pretty site.
> Fact remains - the process is going to segfault somewhere if it needs more
> memory than it can get. Given, from the info in this thread, most *nix's
> (especially modern versions) use this sort of 'overcommitting', I can't

FALSE, for non-overcommited systems. Ever heard of *checking* the
return-value of malloc/calloc/realloc/new?

Most modern *NIX's *allows* you to do memory overcommitment (lazy
swapallocation etc), some have it enabled by default, on some it can be

One thing that is missing in Linux is the possibility to *disable* the
overcommitment, which is very usefull for some applications...

SGI/Irix has probably the cleanest implementation of this of them all
(including Linux):

You can tell it how much overcommited it is allowed to be by setting up
"virtual swap". Without any "virtual swap" you have your old basic
non-overcommited VM subsystem, and with a really large "virtual swap" you
have a overcommited VM subsystem.

It's cute since you can limit the amount of over-allocation it allows, if
you so wish... Many sites run with essentially unlimited overcommitment

> see it's such a bad practice - especially when it allows you to run
> programs (netscape springs to mind) that you would not normally be able to
> run.

Which could also be fixed by having sufficent amount of swap...

> I'd be interested in what, if anything, POSIX has to say about memory
> allocation - I'd be willing to bet nothing or very little, in which case
> I'll contend relying on memory being _there_ just because you asked it be
> available is SUICIDE (hey, I like these caps :) if you intend to write
> portable programs.

You don't even have to turn to the POSIX standard. Take the C standard
instead... If you allocate memory it should be available!

> > (3) This is sure a time waster for porting and developing programs to run
> > on Linux. "Hey, you know that program that worked fine on System A could
> > blow up on Linux." This is espcially true for those people who want
> > to move binaries from some other vendor's OS.
> hrm. Except, under a scheme that definately allocates that memory, the
> program is going to blow up anyway. Think about it - the program blows up
> because the memory is not there. So, if the memory is not there, is any
> allocation routine going to let the program work? Your scheme: "Hey, you
> know that program that worked fine on System A could not run on Linux."

No, it wouldn't "blow up" on System A, the memory allocations would fall,
but these are *usually* checked, and the program would probably protest.

Note that there are a big difference between:
1. Memory allocations can fail
2. Memory you have successfully allocated might not be there, leading to
seg-fault on access..

It's *MUCH* easier to handle case 1, it happens in a few easily located
spots, and the behavior is well-defined. This isn't the case for case 2.

> Instead, we give the program a chance to allocate all the memory it might
> want (a common, still-taught practice), then not touch and not use any of
> that memory, without impacting on the execution of anything else on the
> system. Seems to me to be a very friendly way of doing things.

Yes, but if it use the memory something may blow up... Really bad practice
for some programs.

> Still, if you'd rather dedicate memory to a program based on the
> programmers original decision of what they might need, you're free to
> build/commission a new memory allocation system. Don't expect me to use
> it, though - none of our three commercial servers would survive that sort
> of scheme, and I doubt I could justify hunting down 128Mb sticks of memory
> for them all. And I suppose that is the acid test - in this production
> system, and in many, many other production systems, this scheme works
> without a hitch :)

Hmm, *extra memory*, sounds like your on the wrong track. A
non-overcommited VM usually doesn't need more MEMORY, it need more *SWAP*.

Frankly, at the current HD prices a non-overcommited VM does make sense,
at least as an option...

Torbjörn Lindgren
Karenlyst Allè 5, N-0277 Oslo, Norway Phone: +47 22420102
If Santa ever DID deliver presents on Christmas Eve, he's dead now.

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