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Date:	Thu, 06 Mar 2014 01:24:09 -0500
From:	Brian Wong <>
Subject: Re: Linux does not use more than the startup RAM under Hyper-V with
 dynamic memory enabled

On 3/6/2014 1:20 AM, Brian Wong wrote:
> I'm new to LKML, so please don't be too hard on me :)
> I'm running Gentoo Linux under Microsoft Client Hyper-V on Windows 8.1
> Pro, and I've noticed some odd behavior with respect to dynamic memory
> (aka memory ballooning). The system will never use more than the startup
> memory defined in the vitual machine's settings.
> For example, if I set the startup memory to 512 MB, and enable dynamic
> memory with a minimum of 512 MB and a maximum of 8192 MB, the system
> will never allocate than 512 MB of physical memory, despite Hyper-V
> assigning more memory to the VM and the added memory being visible in
> the output of "free" and "htop". Attempting to use more memory causes
> the system to start paging to swap, rather than actually allocating the
> memory above the startup memory assigned to the VM.
> The kernel is built with the full set of Hyper-V drivers, including the
> key "Microsoft Hyper-V Balloon Driver" as well as memory hot-add and
> hot-remove functionality. This is happening with both the Gentoo-patched
> 3.10.32 kernel and the vanilla 3.12.5 kernel. The host machine has a
> total of 24 GB of memory.
> For now, I am working around the issue by starting the VM with the
> startup memory set to the maximum and letting Hyper-V take the usused
> memory back when it is not in use. The VM will then get the extra memory
> when it needs it.
> Have I encountered a bug in the Hyper-V balloon driver?

Just a correction: the vanilla kernel version is 3.13.5, not 3.12.5. 
Sorry for any confusion.

Brian Wong
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