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[Xen-devel] Re: Xen balloon driver discuss

FYI, the balloon driver in 2.6.18 was meant to be working at some point. The xen tree has some drivers which will compile for 2.6.18 externally and will run in HVM mode. More modern kernels need Stefano's pv-on-hvm patch series to be able to access xenstore (which is a requisite for a working balloon driver).


On 28/11/10 02:36, Dan Magenheimer wrote:
Am I understanding correctly that you are running each linux-2.6.18 as
HVM (not PV)? I didn’t think that the linux-2.6.18 balloon driver worked
at all in an HVM guest.

You also didn’t say what version of Xen you are using. If you are
running xen-unstable, you should also provide the changeset number.

In any case, any load of HVM guests should never crash Xen itself, but
if you are running HVM guests, I probably can’t help much as I almost
never run HVM guests.

*From:* cloudroot [mailto:cloudroot@xxxxxxxx]
*Sent:* Friday, November 26, 2010 11:55 PM
*To:* tinnycloud; Dan Magenheimer; xen devel
*Cc:* george.dunlap@xxxxxxxxxxxxx
*Subject:* re: Xen balloon driver discuss

Hi Dan:

I have set the benchmark to test balloon driver, but unfortunately the
Xen crashed on memory Panic.

Before I attach the details output from serial port(which takes time on
next run), I am afraid of I might miss something on test environment.

My dom0 kernel is 2.6.31, pvops.

Well currently there is no driver/xen/balloon.c on this kernel source
tree, so I build the xen-balloon.ko, Xen-platform-pci.ko form

linux-2.6.18.x86_64, and installed in Dom U, which is redhat 5.4.

What I did is put a C program in the each Dom U(total 24 HVM), the
program will allocate the memory and fill it with random string repeatly.

And in dom0, a phthon monitor will collect the meminfo from xenstore and
calculate the target to balloon from Committed_AS.

The panic happens when the program is running in just one Dom.

I am writing to ask whether my balloon driver is out of date, or where
can I get the latest source code,

I’ve googled a lot, but still have a lot of confusion on those source tree.

Many thanks.

*From:* tinnycloud [mailto:tinnycloud@xxxxxxxxxxx]
*Date:* 2010.11.23 22:58
*TO:* 'Dan Magenheimer'; 'xen devel'
*CC:* 'george.dunlap@xxxxxxxxxxxxx'
*Subject:* re: Xen balloon driver discuss

HI Dan:

Appreciate for your presentation in summarizing the memory overcommit,
really vivid and in great help.

Well, I guess recently days the strategy in my mind will fall into the
solution Set C in pdf.

The tmem solution your worked out for memory overcommit is both
efficient and effective.

I guess I will have a try on Linux Guest.

The real situation I have is most of the running VMs on host are
windows. So I had to come up those policies to balance the memory.

Although policies are all workload dependent. Good news is host workload
is configurable, and not very heavy

So I will try to figure out some favorable policy. The policies referred
in pdf are good start for me.

Today, instead of trying to implement “/proc/meminfo” with shared pages,
I hacked the balloon driver to have another

workqueue periodically write meminfo into xenstore through xenbus, which
solve the problem of xenstrore high CPU

utilization problem.

Later I will try to google more on how Citrix does.

Thanks for your help, or do you have any better idea for windows guest?

*Sent:* Dan Magenheimer [mailto:dan.magenheimer@xxxxxxxxxx]
*Date:* 2010.11.23 1:47
*To:* MaoXiaoyun; xen devel
*CC:* george.dunlap@xxxxxxxxxxxxx
*Subject:* RE: Xen balloon driver discuss

Xenstore IS slow and you could improve xenballoond performance by only
sending the single CommittedAS value from xenballoond in domU to dom0
instead of all of /proc/meminfo. But you are making an assumption that
getting memory utilization information from domU to dom0 FASTER (e.g.
with a shared page) will provide better ballooning results. I have not
found this to be the case, which is what led to my investigation into
self-ballooning, which led to Transcendent Memory. See the 2010 Xen
Summit for more information.

In your last paragraph below “Regards balloon strategy”, the problem is
it is not easy to define “enough memory” and “shortage of memory” within
any guest and almost impossible to define it and effectively load
balance across many guests. See my Linux Plumber’s Conference
presentation (with complete speaker notes) here:



*From:* MaoXiaoyun [mailto:tinnycloud@xxxxxxxxxxx]
*Sent:* Sunday, November 21, 2010 9:33 PM
*To:* xen devel
*Cc:* Dan Magenheimer; george.dunlap@xxxxxxxxxxxxx
*Subject:* RE: Xen balloon driver discuss

Since currently /cpu/meminfo is sent to domain 0 via xenstore, which in
my opinoin is slow.
What I want to do is: there is a shared page between domU and dom0, and
domU periodically
update the meminfo into the page, while on the other side dom0 retrive
the updated data for
caculating the target, which is used by guest for balloning.

The problem I met is, currently I don't know how to implement a shared
page between
dom0 and domU.
Would it like dom 0 alloc a unbound event and wait guest to connect, and
transfer date through
grant table?
Or someone has more efficient way?
many thanks.

 From: tinnycloud@xxxxxxxxxxx
 To: xen-devel@xxxxxxxxxxxxxxxxxxx
 CC: dan.magenheimer@xxxxxxxxxx; George.Dunlap@xxxxxxxxxxxxx
 Subject: Xen balloon driver discuss
 Date: Sun, 21 Nov 2010 14:26:01 +0800

 Greeting first.

 I was trying to run about 24 HVMS (currently only Linux, later will
 involve Windows) on one physical server with 24GB memory, 16CPUs.
 Each VM is configured with 2GB memory, and I reserved 8GB memory for
 For safety reason, only domain U's memory is allowed to balloon.

 Inside domain U, I used xenballooned provide by xensource,
 periodically write /proc/meminfo into xenstore in dom
 And in domain 0, I wrote a python script to read the meminfo, like
 xen provided strategy, use Committed_AS to calculate the domain U balloon
 The time interval is ! 1 seconds.

 Inside each VM, I setup a apache server for test. Well, I'd
 like to say the result is not so good.
 It appears that too much read/write on xenstore, when I give some of
 the stress(by using ab) to guest domains,
 the CPU usage of xenstore is up to 100%. Thus the monitor running in
 dom0 also response quite slowly.
 Also, in ab test, the Committed_AS grows very fast, reach to maxmem
 in short time, but in fact the only a small amount
 of memory guest really need, so I guess there should be some more to
 be taken into consideration for ballooning.

 For xenstore issue, I first plan to wrote a C program inside domain
 U to replace xenballoond to see whether the situation
 will be refined. If not, how about set up event channel directly for
 domU and dom0, would it be faster?

 Regards balloon strategy, I would do like this, when there ! are
 enough memory , just fulfill the guest balloon request, and when shortage
 of memory, distribute memory evenly on the guests those request

 Does anyone have better suggestion, thanks in advance.

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