Re: [Caml-list] Understanding usage by the runtime

["Richard W.M. Jones" <rich@annexia.org>]

On Sun, Jan 08, 2012 at 05:50:40PM -0500, orbitz@ezabel.com wrote:
> Isn't the goal of compaction to keep all of these blocks of memory
> as close as possible?  I should have noted the fragmentation of my
> heap after compaction, but it seems unlikely that my < 1meg of actual
> data could be fragmented across 400megs worth of chunks.

I might not have been clear: memory can only be given back to the OS
at the C / malloc allocator level.  OCaml compaction has nothing to do
with this because C allocations (and data structures used by malloc
itself) cannot ever be moved.

However you can get a clearer picture if you look at /proc/<pid>/maps
or smaps and also if you have a debugging malloc implementation.

> > Here's the news: the OS doesn't need you to give back
> > memory.  Because of virtual memory and swap, the OS will quite happily
> > take back your memory whenever it wants without asking you. 
>
> For most cases this is true, however in my case (which is not the
> usual case), my OS has no swap.  We actually prefer things to fail
> than to be swapped because we are doing computations could take months
> if we get into a swapping situation.  I'm no linux expert so our
> solution is to not have swap and to keep our VMs light when it comes
> to I/O.  Perhaps this is a poor solution but it does change things for
> us.

Sure, this is a perfectly valid case, we have many customers who use
RHEL like this.

Rich.

-- 
Richard Jones
Red Hat