Re: [Xen-API] [XCP] Frequently-Asked Questions (FAQ) for Dynamic Memory Control (DMC)

[Jonathan Knowles <jonathan.knowles@xxxxxxxxxxxxx>]

Jonathan Knowles ÐÐÑÐÑ:
> > However one thing not obvious from the FAQ is that the memory
> > ballooning daemon is a replaceable component, built separately
> > from Xapi, and running as a separate service on each XCP host.
George Shuklin replied:
> I'm sorry, I think this point is wrong.
> Balloon is not a server/dom0 component, it is a kernel module
> for domU kernel. It reads from XenStore variables:
>
> static-max = "4194304"
> target = "1048576"
> dynamic-min = "1048576"
> dynamic-max = "1048576"
When I talk about the "memory ballooning daemon", I'm referring
to the following service that runs within domain 0:

    /etc/init.d/squeezed

The ballooning daemon is reponsible for choosing, for every VM
running on a host, a target that lies between dynamic-min and
dynamic-max. The ballooning daemon writes targets into XenStore.

The ballooning daemon's default behaviour is to assign targets
proportionally, as described in the previous mail. You can find
more information about it its design here:

    http://wiki.xensource.com/xenwiki/squeezed
    (courtesy of Dave Scott)

Of course, DMC also relies on each guest having a balloon driver
that responds appropriately to changes in its XenStore target.

> So when someone set xe vm-memory-target-set (or dynamic) it
> simply order to xapi master to order xapi slaves change those
> values and balloon driver in domU will do all work.
The default behaviour of:
    xe vm-memory-target-set target=<x>

is equivalent to:
    xe vm-memory-dynamic-range-set min=<x> max=<x>

They both directly update the VM.memory-dynamic-{min,max} fields
with the same value.

See the following FAQ section for more details:

    http://wiki.xensource.com/xenwiki/Dynamic_Memory_Control

    Is there an equivalence between target mode and
    dynamic range mode?

George Shuklin replied:
> I'd like to announce our own system named MOD (Memory on Demand)
> wich one use xenballoon for graceful memory allocating to guest
> machines.
> Right now it under research and testing, but it looks really
> good: it never steal memory to cause oom killer doing it dirty
> job.
> Main idea: we looking to memory data, calculating free memory
> and setting target for xenballoon to preserve some amount of
> free memory (We use static-min static-max values as borders
> for allowed changes).
I'm intrigued.

Have you considered using dynamic-{min, max} for limits, rather
than static-{min, max}?  Using dynamic limits instead of static
limits may give administrators a little more flexibility when
constraining their VMs' memory allocations. (The current design
of XCP allows you to change the dynamic limits while a VM is
running, but changing the static limits requires you to reboot
the VM.)

> Right now we using strightforward algorithm 'preserve X MiB of
> free memory', in future I think we can switch to some more
> intellectual algorithm with prediction system.
Out of interest, how do you measure "free" memory? Do you intend
to count buffer memory as "free"?

I'd be very interested to learn more about your design.

Cheers,
Jonathan

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