free memory - use it temporarily

[David Timms]

Peter Arremann wrote:
> 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
There is a program to do this already:
yum install numactl
free
memhog 4g
free

This way you can convince the buffers and file cache to be cleared, but 
sort of pushes stuff into swap.
more fun: ask it for more than ram+swap {ok, that might be bad}.

DaveT.