Messages in this thread | | | Date | Thu, 8 Jul 2010 16:12:01 -0700 (PDT) | From | Dan Magenheimer <> | Subject | RE: [Xen-devel] Re: GSoC 2010 - Migration from memory ballooning to memory hotplug in Xen |
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> From: Andi Kleen [mailto:andi@firstfloor.org] > > Daniel Kiper <dkiper@net-space.pl> writes: > > > > OK, let's go to details. When I was playing with Xen I saw that > > ballooning does not give possibility to extend memory over boundary > > declared at the start of system. Yes, I know that is by desing > however > > I thought that it is a limitation which could by very annoing in some > > enviroments (I think especially about servers). That is why I decided > to > > develop some code which remove that one. At the beggining I thought > > that it should be replaced by memory hotplyg however after some test > > and discussion with Jeremy we decided to link balooning (for memory > > removal) with memory hotplug (for extending memory above boundary > > declared at the startup of system). Additionaly, we decided to > implement > > this solution for Linux Xen gustes in all forms (PV/i386,x86_64 and > > HVM/i386,x86_64). > > While you can do that the value is not very large because you > could just start the guests with more memory, but ballooned in > the first place (so that they don't actually use it) > > The only advantage of using memory hotadd is that the mem_map doesn't > need to be pre-allocated, but that's only a few percent of the memory. > > So it would only help if you want to add gigantic amounts of memory > to a VM (like >20-30x of what it already has).
One can envision a scenario where a cloud customer launches a business-critical VM with some reasonably large "maxmem" set, balloons up to the max, then finds out it isn't enough after all and would like to avoid rebooting. Or a cloud provider might charge for a specific maxmem, but allow the customer to increase maxmem if they pay more money.
Dan
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