[CentOS-devel] Malloc behavior

On 11/03/07, Leonardo Vilela Pinheiro <leopinheiro at gmail.com> wrote:
> Hi,
>
> I'm sorry for a so big email, but this might be a bug.
>
> As some of you may know, there is a characteristic of malloc called
> overcommit, which lets a program "allocate" more virtual memory than is
> available on the system. I've set overcommit_memory=2 and turned off swap,
> but I believe the behaviour of malloc is not the expected.
>
> ...
>
> Conditions:
> Kernel: 2.6.9-42.0.10.ELsmp
> RAM memory: 2GB
> Swap: OFF
> overcommit_memory=2 (changed by me)
> overcommit_ratio=50 (default)
>
> ' free '  yields:
>                      total       used       free     shared    buffers
> cached
> Mem:       2065252     602872    1462380          0      12372     283248
> -/+ buffers/cache:     307252    1758000
> Swap:                0          0          0
>
> There is about 1.4GB free RAM memory.
>
> Then I've executed the test code got:
> malloc 178 MB - (nil)
> Returned null
>
> According to the kernel document above:
> "The total address space commit for the system is not permitted to exceed
> swap + a
> configurable percentage (default is 50) of physical RAM. Depending on the
> percentage
> you use, in most situations this means a process will not be killed while
> accessing pages
> but will receive errors on memory allocation as appropriate."
>
> Shouldn't I have got something about 1000 MB instead of 178 MB, because 1000
> MB is 50% of  physical memory? As long as I have 1.4GB free, I should be
> able to allocate those 1000 MB. Are there other (configurable or not) limits
> for memory allocation? (ulimit is not being used)

I think the memory available with overcommit_memory=2 is for system as
a whole, not individual processes ...

You need to increase the overcommit_ratio number to get more memory -
strangely this number can be greater than 100

James Pearson