Messages in this thread |  | | Date | Wed, 26 Nov 2003 00:17:08 +0100 | From | "Ihar 'Philips' Filipau" <> | Subject | Re: 2.2/2.4/2.6 VMs: do malloc() ever return NULL? |
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Richard B. Johnson wrote: > > As documented, malloc() will never fail as long as there > is still address space (not memory) available. This is > the required nature of the over-commit strategy. This is > necessary because many programs never even touch all the > memory they allocate. >
We are reading different mans? My man malloc(3) clearly states that malloc() can return NULL. (*)
May I ask you one question? Did you were ever doing once graceful failure of application under memory pressure? Looks like not.
I can guess why sendmail allocates memory it never touches - memory pools. There are situations where you really cannot fail - and memory allocation failures are really nasty. Do you wanna to lose your e-mails? No? So then think twice, while implementing lazy allocators.
So from my tests I see that by default Linux is not safe. You allocate memory - malloc() != NULL. Then later you try to write to this memory and you get killed by oom_killer. What is the point of this? Your reasoning doesn't sound to me.
Memory pools used by applications exactly to make grace error handling under memory pressure - but it looks like this stuff under Linux gets no testing at all. And default settings could make from simple bug complete disaster.
> You can turn OFF over-commit by doing: > > echo "2" >proc/sys/vm/overcommit_memory > > However, you will probably find that many programs fail > or seg-fault when normally they wouldn't. So, if you don't > mind restarting sendmail occasionally, then turn off over-commit. >
I shall try overcommit_memory == 2 tomorrow and say what I see.
P.S. For example application I have ported right now to kernel space has a limitiation - it must never ever allocate memory: memory consumption is known, protocol just have no situation like ENOMEM - it _must_ fail to initialize on start-up. No - not to being killed by oom_killer in middle of processing. think carrier grade and/or just good programming technics.
(*) Great optimization opportunities: remove from all programmes checks of the return value if malloc(). As by your words - why not?
-- Ihar 'Philips' Filipau / with best regards from Saarbruecken. -- _ _ _ Because the kernel depends on it existing. "init" |_|*|_| literally _is_ special from a kernel standpoint, |_|_|*| because its' the "reaper of zombies" (and, may I add, |*|*|*| that would be a great name for a rock band). -- Linus Torvalds
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