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

[Doron Fediuck]

Hi Ly,
So basically just to be clear, indeed memory_over_commit is the solution
for memory over commitment. Changing it enabled you to run the amount of
VMs you need.

----- Original Message -----
> From: "ly pan" <plysab at gmail.com>
> To: "Doron Fediuck" <dfediuck at redhat.com>
> Cc: engine-devel at ovirt.org 
> Sent: Wednesday, October 31, 2012 12:32:48 PM
> Subject: Re: [Engine-devel] Question about hasMemoryToRunVM() in	RunVmCommandBase
> 
> Hi Doron,
> 
> Sorry for the so late reply.
> These days I looked into the code again and found it's my setting not
> rightly configured.
> 1. You opinion about reserved_mem is right.
> 2. Yes, pending_vmem_size is 0 before the 84th runs.
> 3. memory_over_commit is where my problem is.
> 
> In webadmin UI, I set the cluster's memory optimization
> to none, thus memory_over_commit is 100 in DB(the dafault value is
> 100
> in my env).
> when I set the cluster's mem_opt to 'for server load',
> memory_over_commit grows to 150
> and when I set to 'for desktop load', memory_over_commit grows to
> 200.
> 
> Both 'for server load' and 'for desktop load' have let the
> vdsMemLimit
> grows dramaticaly,
> thus the 84th vm managed to run.
> So I realized the memory optimization option in cluster is for KSM
> function.
> 
> Thank you.
> 
> ly pan
> 
> 
> 2012/10/24 Doron Fediuck <dfediuck at redhat.com>:
> > ----- 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
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