Re: free memory
On Sunday 16 November 2008 08:30:51 am Steve West wrote:
I am running Fedora 9 x86 64 bit. I have 8 gigs of memory in the
system.I
wrote a program to malloc 4 gigs of memory space. Free shows nearly the
same result before and during the execution of the program. Why does free
not show that 4 gigs where allocated and used? How do I show the true
memory usage?
Steve
What you encounter is a documented, intended behavior. Different people refer
to it by different names like delayed allocation or memory overcommit, but it
addresses all the same issue. Essentially, there are so many programs out
there that ask for memory and never use it, the kernel will overcommit
memory. That means that malloc (with few exceptions) will succeed even if you
don't really have that memory available. It isn't actually allocated until
your process uses it.
Change your program to touch every page of the memory you allocated (simple
for loop accessing one word in each page is sufficient) and you'll see the
memory usage of your program increase steadily.
Search google for memory overcommit or delayed allocation.
http://opsmonkey.blogspot.com/2007/01/linux-memory-overcommit.html
Peter.