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Using swap file or not for VM

Source: microsoft.public.virtualpc
Sent: 10/14/2008
From: Magnusb <(email address - cut out)>
Message:

Before I have always setup the o/s in the guest environment with it's
own swap file, but is it really necessary?

Would it perhaps be better to setup the guest VM to use no swap and with
a sufficient mempory setting like 2 GB (or maybe even 4 GB) and instead
let the host manage the virtual memory.

Otherwise you will have virtual memory management in both host and
guest.

Or does Virtual PC somehow grab physical memory without going through
the host o/s (don't think that is possible though).

I am currently running a VPC session with XP with no swap file with the
VM setup to use 1 gb RAM. Witin the VM 300 MB is allocated however the
Virtual PC session in the host is only using 50 MB ram or so.

I am using Vista as Host o/s on a laptop with 4 GB memory and using XP
in my VMs.

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Related Messages

Re: Using swap file or not for VM

Sent: 10/14/2008
From: (email address - cut out)
Message:

On Oct 14, 4:50=A0am, Magnusb <(email address - cut out)> wrote:

The paging file is needed on the guest just like it's needed on the
host. Leave it alone

Show quoted text

> In article <48f4535a$0$6586$(email address - cut out)>,
> (email address - cut out) says...
>
> > Yes, because some apps still rely on a swap file present, even if you
> > have enough RAM.
>
> Really? I didn't think that applications cared at all about the presence
> of swap file, just the memory virtual or not. =A0
>
> Can you give any example so I can see if it is something I use?

Re: Using swap file or not for VM

Sent: 10/14/2008
From: "Melelina" <(email address - cut out)>
Message:

"Magnusb" <(email address - cut out)> wrote in message
news:(email address - cut out)...

My host machine (XP Pro) is crawling right now. I am very low on memory. I
can't imagine not having a paging file on the Vista Ultimate guest. I had to
drastically increase the paging file on the host (which I had custom set)
when I got Virtual PC and installed Vista Ultimate (that was after I got an
out of memory notice on the host machine and XP resized my paging file). I
have a VMWare guest also running (XP Pro) and Dell limited this fancy 2.5
years old gaming machine to 2GB RAM...why I don't know. So, it is very easy
if I have this Vista guest, and another guest open, to have very little RAM
left. I can have THREE guests, running two versions of XP, and one Ubuntu
open, and no RAM problems but Vista needs a lot more RAM than Linux or XP.

Show quoted text

> In article <7ae0c67e-62a2-4d3e-9ae0-68e88c293450
> @y21g2000hsf.googlegroups.com>, (email address - cut out) says...
>> The paging file is needed on the guest just like it's needed on the
>> host. Leave it alone
>
> You are maybe right about best thing is to leave it alone.
>
> However, it is still interesting to know which would be most efficient,
> handling virtual memory in host or both guest/host.
>
> I don't see why swap file is needed. Enough memory is needed and if you
> don't have enough physical memory and no swap file bad things will
> happen though.
>
> But if memory is set high enough for the VM then it would be like a
> physical machine without a swap file, but with the hist machine
> providing memory which can be virtual.
>
> That is if VPC doesn't do something special with memory by it self.
>
> Might still be better to have a swap fil for the VM though. If you are
> getting low on physcial memory then your entire machine will be less
> responsive instead of just the VM.
>

Re: Using swap file or not for VM

Sent: 10/14/2008
From: DevilsPGD <(email address - cut out)>
Message:

In message <(email address - cut out)> Magnusb
<(email address - cut out)> was claimed to have wrote:

I would very highly recommend enabling a pagefile within your guest OS,
and I would only allocate the amount of memory needed, I wouldn't
recommend over-allocating RAM to a guest under the assumption that the
host can move those pages to disk.

Windows will attempt to use all of the memory it has available. Free
memory is wasted memory. Vista does a better job at this then XP, but
in either OS, if the OS knows you have a ton of available memory,
Windows will leave more pages in memory devoted to nothing more then
disk caching.

When a memory shortage does occur in a guest, the guest OS will
intelligently dump it's disk cache first, then other pages starting from
the least recently accessed. The guest OS also knows which pages are
dirty (need to be written to disk) and which can be dumped (already
written to disk, were read from disk originally and never modified in
memory, etc) painlessly. If the host happens to have a bunch of
available memory, it's disk cache will actually catch the guest's
pagefile and when the guest goes to bring pages back into memory, it
will move from the host's disk cache to the guest's memory far faster
then hitting the underlying disk.

In other words, a memory shortage in a guest isn't the end of the world.

Conversely, the host OS has no idea what is going on within the guest
OS, and from the host's point of view, all pages assigned to a VM are
dirty and must be written to disk before being dumped. So imagine what
happens when the host OS becomes starved for memory and starts paging
out a VM, it may actually start writing to disk a copy of a VM's disk
cache. Later, when there is activity in the VM, the VM may decide to
dump that disk cache and cache a different file, which results in the
host reading the pages back from disk into memory, then the guest dumps
those pages, reads data from disk into memory, leaving the host OS to
start all over again.

Now note that I spend most of my time in VMWare, which definitely does
allow some VM memory to be paged out. I'm actually not sure if this is
even possible in Virtual PC, I have a vague recollection that VPC memory
cannot be paged out at all, which eliminates the above described case
where a disk cache gets written back to disk. However, VPC's disk
access definitely is cached in memory, so having an excess of host
memory will definitely include improve a guest's performance.

Hopefully I am writing coherently, but if not, it's been a long day and
I do apologize.

Show quoted text

>Before I have always setup the o/s in the guest environment with it's
>own swap file, but is it really necessary?
>
>Would it perhaps be better to setup the guest VM to use no swap and with
>a sufficient mempory setting like 2 GB (or maybe even 4 GB) and instead
>let the host manage the virtual memory.
>
>Otherwise you will have virtual memory management in both host and
>guest.
>
>Or does Virtual PC somehow grab physical memory without going through
>the host o/s (don't think that is possible though).
>
>I am currently running a VPC session with XP with no swap file with the
>VM setup to use 1 gb RAM. Witin the VM 300 MB is allocated however the
>Virtual PC session in the host is only using 50 MB ram or so.
>
>I am using Vista as Host o/s on a laptop with 4 GB memory and using XP
>in my VMs.

Re: Using swap file or not for VM

Sent: 10/14/2008
From: DevilsPGD <(email address - cut out)>
Message:

In message <(email address - cut out)> Magnusb
<(email address - cut out)> was claimed to have wrote:

Well, a couple notes. First, it's a pagefile, not a swap file.

What happens is that when an application requests memory, if there is
enough space available in the pagefile then Windows approves the request
even if there isn't enough available physical RAM, and Windows will
normally start copying dirty pages from memory to disk (if needed, very
often it's not) so that if the application which requested RAM actually
starts using it, Windows will be able to allocate the physical memory
more quickly by dumping pages.

In this way Windows is able to over-commit the physical RAM. Because of
this behaviour, many applications request memory that they're not yet
ready to use, and may never need, since it's harmless.

If you don't have a pagefile, then Windows knows it may not be able to
back the memory allocation requests with physical RAM when the time
comes, so Windows may actually decline requests for memory.

As a result, without a pagefile, applications actually end up "using"
more physical RAM then they would need of there was a pagefile.

Show quoted text

>In article <48f4535a$0$6586$(email address - cut out)>,
>(email address - cut out) says...
>
>> Yes, because some apps still rely on a swap file present, even if you
>> have enough RAM.
>
>Really? I didn't think that applications cared at all about the presence
>of swap file, just the memory virtual or not.
>
>Can you give any example so I can see if it is something I use?

Re: Using swap file or not for VM

Sent: 10/14/2008
From: Robert Comer <(email address - cut out)>
Message:

>I don't see why swap file is needed. Enough memory is needed and if you

If you have a known working set and you know you never will stray from
that, then no page file will be the most efficient, but you run the
risk of crashing the VM being doing something even slightly different.

It doesn't other than to virtualize some physical RAM locations.

That's my take on it -- I always make sure the host has enough RAM to
operate efficiently, then I worry about the VM's. I usually allocate
less RAM in a VM than I would if I put that OS on a real machine...

--
Bob Comer

On Tue, 14 Oct 2008 13:03:43 +0200, Magnusb <(email address - cut out)> wrote:

Show quoted text

>don't have enough physical memory and no swap file bad things will
>happen though.
>That is if VPC doesn't do something special with memory by it self.
>Might still be better to have a swap fil for the VM though. If you are
>getting low on physcial memory then your entire machine will be less
>responsive instead of just the VM.
>In article <7ae0c67e-62a2-4d3e-9ae0-68e88c293450
>@y21g2000hsf.googlegroups.com>, (email address - cut out) says...
>> The paging file is needed on the guest just like it's needed on the
>> host. Leave it alone
>
>You are maybe right about best thing is to leave it alone.
>
>However, it is still interesting to know which would be most efficient,
>handling virtual memory in host or both guest/host.
>
>I don't see why swap file is needed. Enough memory is needed and if you
>don't have enough physical memory and no swap file bad things will
>happen though.
>
>But if memory is set high enough for the VM then it would be like a
>physical machine without a swap file, but with the hist machine
>providing memory which can be virtual.
>
>That is if VPC doesn't do something special with memory by it self.
>
>Might still be better to have a swap fil for the VM though. If you are
>getting low on physcial memory then your entire machine will be less
>responsive instead of just the VM.

Re: Using swap file or not for VM

Sent: 10/14/2008
From: Robert Riebisch <(email address - cut out)>
Message:

Magnusb wrote:

Yes, because some apps still rely on a swap file present, even if you
have enough RAM.

--
Robert Riebisch

Show quoted text

> Before I have always setup the o/s in the guest environment with it's
> own swap file, but is it really necessary?

Re: Using swap file or not for VM

Sent: 10/14/2008
From: Magnusb <(email address - cut out)>
Message:

In article <48f4535a$0$6586$(email address - cut out)>,
(email address - cut out) says...

Really? I didn't think that applications cared at all about the presence
of swap file, just the memory virtual or not.

Can you give any example so I can see if it is something I use?

Show quoted text

> Yes, because some apps still rely on a swap file present, even if you
> have enough RAM.

Re: Using swap file or not for VM

Sent: 10/14/2008
From: Magnusb <(email address - cut out)>
Message:

In article <7ae0c67e-62a2-4d3e-9ae0-68e88c293450
@y21g2000hsf.googlegroups.com>, (email address - cut out) says...

You are maybe right about best thing is to leave it alone.

However, it is still interesting to know which would be most efficient,
handling virtual memory in host or both guest/host.

I don't see why swap file is needed. Enough memory is needed and if you
don't have enough physical memory and no swap file bad things will
happen though.

But if memory is set high enough for the VM then it would be like a
physical machine without a swap file, but with the hist machine
providing memory which can be virtual.

That is if VPC doesn't do something special with memory by it self.

Might still be better to have a swap fil for the VM though. If you are
getting low on physcial memory then your entire machine will be less
responsive instead of just the VM.

Show quoted text

> The paging file is needed on the guest just like it's needed on the
> host. Leave it alone

Re: Using swap file or not for VM

Sent: 10/14/2008
From: Magnusb <(email address - cut out)>
Message:

In article <(email address - cut out)>,
(email address - cut out) says...

Was a very good explanation raising some good points and I think you are
right. It is most certainly better to have paging activated for the
guest.

Not that I actually intended to run without a paging file. But started
thinking of it while I was setting up a serie of differencing disks and
had swap file turned off to cut down size on parent vhd file.

Might still be an option though if you only work within VPC and only
using the host as a shell to start VPC.

Show quoted text

> Hopefully I am writing coherently, but if not, it's been a long day and
> I do apologize.

Re: Using swap file or not for VM

Sent: 10/15/2008
From: FACE <(email address - cut out)>
Message:

On Tue, 14 Oct 2008 09:36:34 +0200, in microsoft.public.virtualpc,
Magnusb <(email address - cut out)>, wrote

I am not saying that VPC does, but why do you think that is not possible?

FACE

Show quoted text

>Or does Virtual PC somehow grab physical memory without going through
>the host o/s (don't think that is possible though).

Post Reply | This is Spam! | Mark as Spam

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