Re: [Xen-users] Memory overcommitment in Xen.

["Austin S. Hemmelgarn" <ahferroin7@xxxxxxxxx>]

On 2016-12-04 14:21, Carl Schneider wrote:
> > How Memory overcommitment work in Xen? It is automatically or must be enabled?
> Overcommitment is not a feature, but a state of the system. Basically,
> things like Ballooning allow you to take away Memory from a domain,
> while it "thinks" that it still has it available (From within the
> domain, it looks like the balloon driver allocates a big chunk of
> memory, as far as I know).
> So, if you have e.g. a system with 8GB of memory, and start up 4 domains
> (including dom0) that have an maxmem of 2GB, you are not overcommiting,
> but use up all your available memory.
Actually, that would be over-committed, and you wouldn't be able to 
start all the VM's in that situation unless at least one was 
pre-ballooned (maxmem higher than memory), because you lose part of that 
8G to Xen and the hardware (so 8G of physical RAM is actually about 
7.95G usable under Xen).
> Let us assume that the domains now just needs 1GB, so the ballooning
> driver gives some memory "back" to the hypervisor. Now it is possible to
> start another domain, but the sum of all maxmems is now bigger than 8GB.
> This state is called overcommitment.
>
> Note that this is not only possible with Memory, but also with disk
> storage (LVM thin provisioning, e.g.).
It's also possible (and very commonly used) with processors.
> My opinion about this: If you do not know the workload of your system
> and do not understand the problems of this exactly, do not over-commit
> too much and keep an eye on your system. I do not have any experience
> with the problems of over-committing, so I can not estimate the risks of it.
In general, you can safely get away with over-committing CPU's.  Most 
people who aren't hosting providers do this at least some of the time 
when using Xen (or Hyper-V, or VMWare).  Over-committing disk space is a 
bit riskier, but generally safe (most things will gracefully handle 
unexpectedly running out of disk space) as long as you have Domain-0 and 
the guest domains set up such that unused space gets properly reclaimed 
(this isn't too hard with PV Linux guest domains, but it is seriously 
annoying to set up with Windows guests).  Over-committing memory, 
however, is seriously risky unless you have a well-defined static 
workload and you have a _very_ good understanding of how it behaves and 
expects memory to behave.  If you don't get things correct with memory 
over-commit, you will crash at least a guest domain, and possibly your 
whole system.
> Note that I do not have any noteworthy experience with ballooning or
> overcommitting, so if I made a mistake, please correct me.
> It may also be possible that I am just plain wrong, too. (Take this as a
> disclamer)
>
> Cheers
> CRT
>
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