** Changed in: nova
Status: New => Invalid
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https://bugs.launchpad.net/bugs/1244045
Title:
Memory calculation is not accurate for KVM hypervisor
Status in OpenStack Compute (Nova):
Invalid
Bug description:
Currently, the resource tracker get the free memory from virt driver.
In libvirt environment, if the below hypervisor is not Xen, it will
use memory summary of 'MemFree:', 'Buffers:' and Cached:'" for
calculation. However, this is not correct.
I'd give an example below, the environment is a devstack environment,
with host has 8G RAM. Ubuntu 12.04 host with swap disabled (enable
swap should not impact this).
1) Create two flavor, test with 4G memory, and mtest with 2G memory.
2) Create 3 instance, 2 for test flavor, 1 for mtest flavor. The 3
instance can be created successfully.
3) In each guest, execute followed program, which is mainly allocate memory
and populate it.
#include <stdlib.h>
int main() {
int *p;
while(1) {
int inc=1024*1024*sizeof(char);
p=(int*) calloc(1,inc);
if(!p) break;
}
}
4) After sometime, in my test environment, two guest has been killed
because of host OOM.
This is sure to not be good. I don't know why this problem is not
reported before.
The reason for this issue is, the KVM allocate guest memory as a progressive
method:
a) when guest started, qemu will malloc() for the virtual address ranges for
the guest.
b) When guest access the memory, the memory will be populated as needed.
Thus when we create instance, there is not much memory access thus
from host point of view, still a lot of memory existed. However, once
guest begin access the memory as in step 3), host will out of memory.
Thanks
--jyh
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