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Subject: Re: Large arrays (again), problem case for GNAT
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From: Mark Lorenzen <mark.lorenzen@surfpost.dk>
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Date: 15 Apr 2005 20:30:20 +0200
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Date: 2005-04-15T20:30:20+02:00
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Robert A Duff <bobduff@shell01.TheWorld.com> writes:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
> 
> > On 14 Apr 2005 11:18:22 -0400, Robert A Duff wrote:
> > 
> > > The reason I like allocate-on-write is that it is very useful to
> > > allocate a huge array, and then only use some part of it.  That way, the
> > > program can deal with enormous input data, without paying the cost when
> > > the input data is small.  Physical memory is 1000 times more expensive
> > > than address space.
> > 
> > I think this approach is flawed. The contract of malloc is to return memory
> > not the address space. 
> 
> Well, I'm not an expert on malloc.  Where is it documented that this is
> the contract?  Apparently the Linux folks don't think that's the
> contract.

Exactly. As mentioned in another post, it is a popular implementation
strategy to only allocate memory on write in order to save the system
from memory exhaustion.

> 
> I *am* an expert on Ada, and I know that "new" does not necessarily
> reserve real memory -- it might just reserve address space.
> 
> >...That must be just another system call.
> 
> I agree that there *should* be two separate calls: one reserves address
> space, and the other reserves real memory.  Or one call with a Boolean
> parameter.

A simple "man malloc" specifies that the default overcommitting
behaviour can be turned of by writing the letter "2" to the file
/proc/sys/vm/overcommit_memory . F.x.:

echo 2 > /proc/sys/vm/overcommit_memory

- Mark Lorenzen

<snip>