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[Xen-devel] 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.
 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
 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
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
> Hi:
> 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
> dom0.
> 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
> 0(/local/domain/did/memory/meminfo).
> 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
> target.
> 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
> inflation.
> Does anyone have better suggestion, thanks in advance.

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