Comments on: Hello Dynamic Memory?

By: az első Hyper-v patch kapcsán

az első Hyper-v patch kapcsán — Sun, 12 Jun 2011 21:23:39 +0000

[...] “A Memory overcommitment potenciálisan veszélyes, éles környezetben nagyon kockázatos” A minap olvastam, hogy mégiscsak lesz Hyper-V 2-ben hasonó feature… Most már nem [...]

By: Tweets that mention Post-R2 release of Hyper-V may include memory overcommit - Dynamic Memory | VCritical -- Topsy.com

Sat, 06 Feb 2010 16:30:27 +0000

[...] This post was mentioned on Twitter by Eric Gray, Eric Gray, Eric Gray, William Lam, Andrew Storrs and others. Andrew Storrs said: RT @eric_gray: VCritical: Hello Dynamic Memory? http://bit.ly/91tAAs [...]

By: Phil

Phil — Fri, 05 Feb 2010 16:56:44 +0000

Mark: “That’s where VMware DRS comes in…”

or PROTips in a Hyper-V environment.

By: Fernando

Fernando — Thu, 04 Feb 2010 10:31:14 +0000

Brilliant explanation

By: Mark Chuang

Mark Chuang — Wed, 03 Feb 2010 20:08:26 +0000

@James

It feels like saying that “VMware’s approach can lead to paging” while “MS’s approach won’t” misses the bigger picture.

Let’s look at the scenario that would lead to paging for VMware, i.e. the VMs on a host cumulatively need more memory than exists on the host. In the MS scheme (if I’m interpreting your explanation correctly), the MS balloon driver will try to free up memory, but it won’t be able to because all of the memory is actively being used. So you end up with Hyper-V VMs that need more memory but can’t get it, so the performance for those VMs are essentially capped as well.

So in both approaches, the bigger picture/solution is that you’ll need to move 1 or more VMs to another host (since AFAIK, neither company has figured out how to defy the laws of physics and create more physical memory out of thin air). That’s where VMware DRS comes in…

(Disclosure: Yes, I work for VMware.)

By: Anton Zhbankov

Anton Zhbankov — Wed, 03 Feb 2010 18:10:29 +0000

James, I know how can you give VM more memory – Windows 2003 Enterprise and above support memory hot-add. But how can you hot remove memory?
I see only one way to do this – ballooning = swapping.

By: Anton Zhbankov

Anton Zhbankov — Wed, 03 Feb 2010 18:06:56 +0000

It’s dynamic memory!

By: James O'Neill

James O'Neill — Wed, 03 Feb 2010 17:21:47 +0000

Anton, you’re right we will probably see lots of arguments about the correct usage of “commit” and “Overcommit”. Overcommitment (at least when I was introduced to the term in the context of non-virtualized OSes) is the point where paging begins.

Far be it from me to advise VMware on how to market their products but it would seem to me that the whole upside on page sharing is that it allows (sum of granted) > (memory physical) without actually being overcommitted and needing to page (bad).

Microsoft’s dynamic memory (as seen in 2008) didn’t page – so it reduced what was granted to one VM to increase what was granted to another. Different beasts.

By: Fernando

Fernando — Tue, 02 Feb 2010 22:39:26 +0000

So, what is it then ?

By: Anton Zhbankov

Anton Zhbankov — Tue, 02 Feb 2010 16:47:59 +0000

James,
AFAIR VMware defines Memory Overcommitment as (Sum of memory granted) > (Memory physical). So if we have 32GB of memory and 40GB granted to running VMs – memory is overcommited, even if working set is 15GB.

TSP is included to Memory Overcommitment techniques with ballooning. So actually it’s just a question of terms.

Marketing always will find something to say, no matter what engineers do.