Re: [Users] Ovirt Memory Page Sharing or to less memory
On 10/17/2012 11:16 PM, Alan Johnson wrote:
On Mon, Oct 15, 2012 at 6:51 PM, Mathok <mathok1(a)googlemail.com
<mailto:mathok1@googlemail.com>> wrote:
It is possible to increase the level of memory page sharing on a
cluster above the 200% with a option and start the vms after that
change ? or it is possible to deactivate the memory threshold with
any option?
First, let me be clear that this is complete speculation. That said,
my interpretation of these features based on how the GUI behaves is that
the Cluster Memory Optimization (CMO) setting changes the
default Physical Memory Guarantee (PMG) on new VMs under the Resource
Allocation tab. However, you could set that number manually to whatever
you want. I have been setting it to 1/4 of the Memory Size (MS) on the
General tab as a general rule, but varying it by VM as needed.
that is correct. the default value for "reserved memory" at vm level is
based on cluster over commit.
cluster over commit can be set to a custom value via the api/sdk/cli.
you should validate ksm is up and running.
you should also note ksm by default only kicks in around 80% host memory
utilization. so if you want to be more aggressive on sharing memory
pages (at the expense of a bit more cpu cycles for ksm), you can
configure it at vdsm level to start at a lower threshold.
also, you should note that during vm startup, until vm goes to up,
engine assumes it is taking all of its defined RAM, then moves to check
actual ram consumption of the VM.
so between ksm configuration, and the time VMs go to up state, you may
need to throttle the launch of multiple VMs on same host.
So, to restate, when CMO is set to None, PMG=MS when creating a new VM.
When CMO is set to Server, PMG=MS/150%. When CMO is Desktop,
PMG=MS/200%. But you can still set the PMG to any custom value any time
the VM is not running. That much I am sure of. What I have not tested
is my expectation that the engine will pile the VMs on the hosts based
purely on their the greater of PMG and actual VM use. It would be
reasonably plausible that the engine limits VMs on the hosts based on
CMO as well as PMG and actual use, but I just don't know for sure either
way.
CMO - total potential RAM of the VMs on the host should not exceed host
ram * CMO.
PMG - total reserved RAM of the VMs on the host, should not exceed host ram
(that's "close enough to the truth", calculation is taking some
overheads into account, etc.)
I would greatly appreciated it if some guru could school me on this point.
_______________
Alan Johnson
alan(a)datdec.com <mailto:alan@datdec.com>
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