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Re: [Xen-users] Purpose of mem-max command

On 11/10/15 21:45, Marco Guazzone wrote:
On Sat, Oct 10, 2015 at 6:36 PM, Simon Hobson <linux@xxxxxxxxxxxxxxxx> wrote:
Marco Guazzone <marco.guazzone@xxxxxxxxx> wrote:

I don't understand the behavior of the "xl mem-max" command.

The help says: "Set the maximum amount reservation for a domain."
But if I try to use it I see no effect on the target domain.

It seems that only the "xl mem-set" command has effect on the VM.
That sounds correct. mem-max sets the maximum that mem-set can configure. I 
can't see that it's that useful from the command line, but I have mem-max set 
is several of my config files where I want to start a machine with a configured 
amount of ram, but have the option to increase it later (typically because I've 
had to cut some machines down to shoehorn another VM onto the host)

Hi Simon,

What I don't understand is how to use it effectively.
Could you provide a practical example of how to use it?

Let me show you some of tests I've done (sorry for the length of the
message). Before performing a test, I always reboot the VM to cancel
the effects of previous tests

My CentOS 7 VM, called 'memtest', is statically configured (in the
config file) to have 8GB of memory:
   maxmem = 8192
   memory = 8192
Indeed, once booted I get
$ xl list
   Name                                        ID   Mem VCPUs    State    
   Domain-0                                     0 71845    16
r-----   12630.1
   memtest                                     35  7935     4
-b----      10.0

and inside the VM

[root@memtest ~]# free -h
                 total        used        free      shared  buff/cache
   Mem:           7.5G        106M        7.4G        8.3M         89M
   Swap:          1.0G          0B        1.0G

- TEST #1
$ xl mem-max memtest 3g
libxl: error: libxl.c:3916:libxl_domain_setmaxmem: memory_static_max
must be greater than or or equal to memory_dynamic_max
: Success
cannot set domid 35 static max memory to : 3g

Try this test, in the config file:

  maxmem = 20480
  memory = 8192

When you start the VM, it should have 8G

Then, you can allocate some extra RAM (note, you are changing the amount of memory, not the "maximum" amount that could be allocated.

xl mem-set memtest 10g

Now you should see the VM has extra RAM available.

So, from these test it seems that the take-home lesson is to never use
"xl mem-max" since it has no effect.

Where am I wrong?

Thank you very much for your help.



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