Am Mittwoch, den 07.11.2007, 12:28 -0500 schrieb Adam Chlipala:
> I've encountered a problem where certain OCaml programs use orders of 
> magnitude more RAM when compiled/run in 64-bit Linux instead of 32-bit 
> Linux.  Some investigation led to the conclusion that the difference has 
> to do with the size of OCaml page tables.  (Here I mean the page tables 
> maintained by the OCaml runtime system, not any OS stuff.)
> 
> A program that should be using just a few megabytes of RAM ends up using 
> 200+ MB to store a page table.  It seems that a C macro is defined by 
> default on 64-bit Linux to use mmap() instead of malloc().  Ironically, 
> a comment says that this was done to avoid being given blocks of memory 
> that are very far apart from each other, forcing the creation of overly 
> large page tables.  It's ironic because that is exactly the problem that 
> is showing up now with mmap().  It ends up called twice for the program 
> I'm looking at, and the two addresses it returns are far enough apart to 
> lead to creation of a 200 MB page table.
> 
> Has anyone else experienced this problem?  Would the runtime system need 
> to be changed to avoid it?

We are using O'Caml on 64 bit Linux, and aren't aware of such problems.

Did you observe a debug GC message that proves it? 200 MB means that an
address space of 200M * 4K = 8E is covered.

Also think of Linux modifications that do address randomization, i.e.
prevent that contiguous addresses are allocated.

Gerd
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