Re: [Xen-users] memory=x & maxmem=y

[Paul Archer <parcher@xxxxxxx>]

> On Tue, 22 May 2007 13:26:09 +1000
> Brad Plant <bplant@xxxxxxxxxxxx> wrote:
>
> > On Mon, 21 May 2007 21:56:22 -0500 (CDT)
> > Paul Archer <tigger@xxxxxx> wrote:
> > 
> > > After doing some more research, I finally found out what's going on.
> > > In order to allocate more memory to a domU, it has to be freed from
> > > somewhere else first. In other words, if memory=256 and maxmem=512,
> > > then before you run 'xm mem-set mydomU 512', you have to run 'xm
> > > mem-set 0 <current memory for dom0 - 256>'.
> > > I suppose this would work just as well by taking the memory from
> > > another domU (or some combination of domU's and dom0), but I haven't
> > > tried that.
> > 
> > I don't think this is the issue here since the dom0 memory usage will
> > balloon down automatically to free up more RAM for the domU. When I
> > attempted to get this to work yesterday, dom0 had over 2GB of RAM
> > allocated to it, so it had heaps to spare when it ballooned down.
> > 
> > The problem isn't with trying to allocate memory to a domU, it is
> > with trying to get domU to use or recognise the extra memory. I was
> > able to allocate the extra memory ok, but running free -m in the domU
> > would not show the extra memory as being available.
>
> Sorry Paul, you were right. It seems that dom0 didn't automatically 
> balloon down to free up the extra ram. I am now passing the dom0_mem 
> variable to the hypervisor so that I don't have to worry about reducing 
> its usage first.
No problem. I thought this was a bug at first, but I realized that it 
makes some sense, because you might want to pull memory from one domU to 
give it to another, so you have to do the allocation yourself.
What is messed up is the reporting. 'xm list' doesn't necessarily reflect 
reality:
tiva:/etc/xen # xm mem-set 0 999999
tiva:/etc/xen # xm list
Name                                      ID   Mem VCPUs      State 
Time(s)
Domain-0                                   0 999999     1     r----- 
12119.9
c5                                        10   256     1     -b---- 
2.6
centos5                                   11   256     1     -b---- 
2.5
tiva:/etc/xen # cat /proc/meminfo |head
MemTotal:      1545148 kB
MemFree:        276396 kB
Buffers:         35080 kB
Cached:         599996 kB
SwapCached:          0 kB
Active:         549724 kB
Inactive:       530676 kB
HighTotal:     1203788 kB
HighFree:        34820 kB
LowTotal:       341360 kB


Paul

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