[Engine-devel] Question about hasMemoryToRunVM() in RunVmCommandBase

[Doron Fediuck]

----- Original Message -----
> From: "ly pan" <plysab at gmail.com>
> To: engine-devel at ovirt.org
> Sent: Thursday, October 18, 2012 2:43:35 PM
> Subject: [Engine-devel] Question about hasMemoryToRunVM() in RunVmCommandBase
> 
> Hi:
> 
> This is my test environment:
> hardware: Dell PowerEdge 2710 ,Memory 48G
> Software: OVirt engine 3.0 ,VDSM 4.9.4 ,kernel
> 2.6.32-279.2.1.el6.x86_64
> 
> 
> I create 100 vms from pool(each vm has 512M memory and has
> 1M memory garanteed, with guest_overhead = 65 and reserved_mem =
> 256),
> but only 83 vms
> turn out to run. When I run the 84th vm, engine says there is not
> enough memory. However at this time KSM is enabled and 15G free
> memory
> is still left on the host.
> 
> I checked the code and found out that before running a vm, the
> function hasMemoryToRunVM() in RunVmCommandBase would decide whether
> there is enough memory in the selected host to run it.
> The Algorithm in the function is like this:
> mem_avaliable >= mem_required = curVds.getmem_commited() +
> curVds.getpending_vmem_size()
>      + curVds.getguest_overhead() + curVds.getreserved_mem() +
> vm.getMinAllocatedMem();
> 
> And here is my question: curVds.getmem_commited() is caculated from
> the memory of vm
> assigned by user. But when the host is running with KSM enabled, the
> actual memory of each
> running vm would be smaller than the memory the user assigned to. In
> this case, the following
> may occur: engine says there be no more memroy to run any vm while
> the
> host has much free
> space to run more vms(in my case, 15G).
> 
> 
> Therefore I think the actual memory size reported by vdsm(The KSM
> shared memory) should be taken into account when calculating
> curVds.getmem_commited(). Any ideas are welcome.

Hi Ly,
Sorry for the delay. I'd like to look into this issue.
In the meanwhile a few relevant points;

1. You're saying that reserved_mem=256. This is not accurate, since it's
reported by vds as host_mem_reserve(256) + extra_mem_reserve(65) = 321

2. I'm assuming you retried running the 84th vm, just to make sure your
pending_vmem_size is 0 once all 83 VMs are running.

3. So assuming (2) is correct, I'd like to ask what's the max_vds_memory_over_commit
you have for the cluster? By default it's set to 120, and in such a case the 84th
VM should still be running.

Here's a calculation sample based on your data, and the above 3 issues:

vdsCurrentMem =
83 * (512M+65) = 47,891
0
65
321
1
=======
48,278

vdsMemLimit = 
120/100
*
47,321.1328125  // reported by vdsm: 'cat /proc/meminfo | grep MemTotal' / 1024
=========
56,785.359375

So vdsCurrentMem <= vdsMemLimit should be true unless you changed something
such as the MemTotal or the memory_over_commit. You can try and change memory_over_commit
let us know how it works.

Doron