Heap memory limit?

[Steven Schveighoffer]

On Tue, 23 Feb 2010 09:34:19 -0500, bearophile <bearophileHUGS at lycos.com>  
wrote:

> Steven Schveighoffer:
>
>> The GC is open source.  Submit a patch if you want.
>
> It can be nice if the the C heap too is counted in such max memory used.  
> I don't know if this is possible.

I don't know if this is possible.  D would have to replace C's malloc in  
order to do this, but I don't think that's a good idea.  Also, however,  
C's malloc isn't as likely to explode memory usage because items should be  
freed when they are no longer used.  GC-based languages tend to use more  
memory because there are at any one time a lot of unused pieces of memory  
waiting to be cleaned.

>> DMD is not in charge of the GC, it is runtime code, I don't think we  
>> need a specific option for it.<
>
> - D2 code has safe modules too, they are meant to turn D code into  
> something a little safer than usual C code.
> - I think using too much memory is a unsafe situation, a small risk. You  
> are free to not agree.

It is a risk, but it is entirely dependent on factors outside the  
program's control.  For example, how much memory is available.  If you  
limit to 200MB, what if you have to run 50 programs that were built from D  
with the same default limit, that's 10GB limit.  I just don't think it's  
realistic to come up with a simple per-program limit that solves the  
problem.

> - If it's seen as a real small risk, then if you want to do something  
> concrete to avoid this risk, you need such memory limit enforced by  
> default (if it's not a default, very few people will use it, and this  
> small risk will be often ignored). And it can be positive if all D  
> implementations share have this safety. That's why I was talking about a  
> compiler argument (plus eventually a small program to change at runtime  
> the limit of an already compiled program. Another silly possible  
> solution is to use  small program to modify the max value contained into  
> the binary itself, a kind of binary patching).

Again, this can be done in the GC code itself, it does not need to be a  
compiler-defined limit.  My preference is for the default to be unlimited,  
since this is the most common case.

>> I also don't think that limits to memory are too important for most
>> applications, only for specific ones.
>
> It's not just my desire to copy Java, I have made my PC unstable few  
> times because a bug in my code has caused a D proggy to eat too much  
> memory, so having a default limit of memory used seems useful to me.

This isn't a common case though.  An artificial limit that has no  
consideration of available memory seems like it would lead to more  
puzzlement than help.

-Steve