[Xen-devel] problem to balloon memory beyond initial allocation

[Xuehai Zhang <hai@xxxxxxxxxxxxxxx>]

Hi all,

I did the following experiment to test booting a domU with less memory
than its maximum then growing its memory via balloon driver later. I
followed by the suggestions from:
 http://lists.xensource.com/archives/html/xen-users/2005-04/msg00294.html
 http://lists.xensource.com/archives/html/xen-devel/2005-04/msg00634.html

The xen version I use is 2.0 testing tree wit "Latest ChangeSet:
2005/06/08". Both dom0 and domU kernel version is 2.6.11.11.

The memory allocation with only dom0 running looks as following:

# xm list
Name              Id  Mem(MB)  CPU  State  Time(s)  Console
Domain-0           0      379    0  r----    194.0

# xm info | grep mem
memory                 : 1023
free_memory            : 622

The configuration file for the domU (named vm1) looks like:

[snip]
kernel = "/boot/vmlinuz-2.6.11-xenU"
memory = 128
[snip]
root = "/dev/sda1 ro"
extra = "mem=268435456" #256MB
[snip]

I pass "mem=268435456" to vm1's kernel command line, which will set the
size of mem_map or maxmem to 256MB. However, "memory=128" will only
allocate 128MB memory to vm1 at its creation time.

After vm1 is created, I checked its memory allocation status.

- from vm1 console:

# dmesg | grep mem
Kernel command line:
ip=128.135.164.83:1.2.3.4:128.135.164.65:255.255.255.192::eth0:off
root=/dev/sda1 ro mem=268435456
vmalloc area: d0800000-fbff9000, maxmem 34000000
Memory: 125376k/262144k available (1630k kernel code, 5324k reserved, 440k
data, 112k init, 0k highmem)
xen_mem: Initialising balloon driver.
Freeing unused kernel memory: 112k freed

# cat /proc/xen/balloon
Current allocation:   131072 kB
Requested target:     131072 kB
Low-mem balloon:      131072 kB
High-mem balloon:          0 kB
Xen hard limit:          ??? kB

The "dmesg" output line "Memory: 125376k/262144k available" confirms 128MB
out of 256MB is currently allocated to vm1.

The "/proc/xen/balloon" output line "Low-mem balloon: 131072 kB" shows
(if I understand correctly) there is 128MB unused memory left which can
be added to the existing 128MB allocation. BTW, can anyone explain the
meanings of last three entries, Low-mem balloon, High-mem balloon, and Xen
hard limit? One question I have here is, what is the real maxmem value
(the value we can set with "xm maxmem vm1" command too) for vm1 at this
stage? Is it 128MB (according to Xen users' manual, "Xen associates a
memory size limit with each domain. By default, this is the amount of
memory the domain is originally started with, preventing the domain from
ever growing beyond this size") or 256MB (because we pass "mem=256MB" as
kernel command line)?

- from dom0

# xm list
Name              Id  Mem(MB)  CPU  State  Time(s)  Console
Domain-0           0      379    0  r----    196.5
vm1               12      127    0  -b---      1.0    9612

# xm info | grep mem
memory                 : 1023
free_memory            : 495

Output from dom0 indicates vm1 is allocated 128MB memory.


Next I tried to increase vm1's memory. Because I am not sure if vm1's
maxmem is set to 128MB or 256MB (as discussed above), I ran the "xm
maxmem" command first from dom0.

# xm maxmem vm1 256

When going back to vm1's console to check "/proc/xen/balloon", it keeps
the same:

# cat /proc/xen/balloon
Current allocation:   131072 kB
Requested target:     131072 kB
Low-mem balloon:      131072 kB
High-mem balloon:          0 kB
Xen hard limit:          ??? kB

Then in dom0 I tried to increase vm1's memory to 200MB by running "xm
balloon" command:

# xm balloon vm1 200

# xm list
Name              Id  Mem(MB)  CPU  State  Time(s)  Console
Domain-0           0      379    0  r----    197.4
vm1               12      127    0  -b---      1.0    9612

# xm info | grep mem
memory                 : 1023
free_memory            : 494

Surprisingly, vm1's memory is still 128MB (not 200MB we want to increase
to) and the free memory is still similar as before.

When going back to vm1's console to check "/proc/xen/balloon" again, I
found "Xen hard limit" is set with "128MB" value instead of "???" before.
It seems to me "Xen hard limit" will forbid vm1's memory go beyond 128MB.
The requested target is also changed, which reflects the new memory value
we want to increased to, 200MB.

# cat /proc/xen/balloon
Current allocation:   131044 kB
Requested target:     204800 kB
Low-mem balloon:      131100 kB
High-mem balloon:          0 kB
Xen hard limit:       131044 kB (inc.       28 kB driver headroom)

My above experiment is not very successful. I will appreciate if you can
point out what I did wrong. Thanks in advance!

Xuehai


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