[CentOS] top and allocation issues

Brunner, Brian T. BBrunner at gai-tronics.com
Thu Mar 3 20:11:08 UTC 2011

centos-bounces at centos.org wrote:
> Yes, I do expect to do a bit of arithmetic.  I will need
> several blocks of about 0.5G, and I am checking the limits.
> Is it true, then, that I won't really know if I succeeded with
> the allocation until I try to write the memory? What will
> happen then? Is there a way to check without actually writing?

That's where the malloc() bug can hit you.  The bits I spoke of are only
in the table of pages of your process' virtual memory.  That's limited
to 4GB without some mechanism getting past the 32-bit-addressing-snag.
If you use calloc(), you discover at page-allocation-time that either
your virtual space, or the system virtual space, is exhausted; this is
preferable to discovery in the nth iteration of some deeply-buried loop
that wrote to the 'one page too many'.

Stephen: Good clue on the overcommit_momory setting.  I'd forgotten it.

Can you work on only one file at a time, each file under GOB, or do you
must-have more than GOB of data in front of you at a time?

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