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[Xen-users] Shoehorning a domU and "missing" memory (Xen 4.0.1)

  • To: xen-users@xxxxxxxxxxxxx
  • From: Andrew Wade <andrew@xxxxxxxxxx>
  • Date: Fri, 03 Aug 2012 11:33:23 +0100
  • Delivery-date: Fri, 03 Aug 2012 10:35:01 +0000
  • List-id: Xen user discussion <xen-users.lists.xen.org>


I'm seeing an issue with 4.0.1 on Debian 6 regarding "missing" memory.

My set up:

 * Xen 4.0.1 on Debian 6.0.5
 * dom0-min-mem 512, enable-dom0-ballooning no, dom0_mem=512M (GRUB
config). (I also tried with dom0-min-mem 0)
 * Host server has 32GB RAM

# xm list
Name                                        ID   Mem VCPUs      State
Domain-0                                     0   501     8     r-----

# xm info|grep memory
total_memory           : 32758
free_memory            : 31832

32758 (total) - 31832 ('free_memory') - 512 (dom0) = 414MB unaccounted.
(No domus are running)

I created an HVM domU with 31488 MB RAM (N.B. this is less than 31832
reported by xm info free_memory plus I calculated about ~250MB memory
overhead) and 4 VCPUs but it wouldn't start due to insufficient memory
available. I expected it to fit.

Is there an official calculation for the memory overhead (for
tables/caches etc)?

Can anyone explain why a domu with 31,488MB won't start when 31,832MB is
free? I'm trying to calculate the most amount of RAM a domU can have
(i.e. to occupy an entire server)


Andrew Wade

Memset Ltd., registration number 4504980. 25 Frederick Sanger Road,
Guildford, Surrey, GU2 7YD, UK.

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