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[Xen-users] RE: [Xen-devel] How to request contiguous memory for a domu

To: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
Subject: [Xen-users] RE: [Xen-devel] How to request contiguous memory for a domu
From: "Fan, Huaxiang" <hufan@xxxxxxxxxxxx>
Date: Thu, 21 Jul 2011 08:57:11 +0000
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Thread-topic: [Xen-devel] How to request contiguous memory for a domu
Hi Ian,

Thanks for your quick reply.

We are running cpu/memory intensive application/services on a domu, we are 
worry about non-contiguous memory allocation might affect the performance. 

We are migrating the application/services from a single CPU socket (4 cores, 
i.e. 8 ht cores) hardware platform to a dual CPU socket (8 cores, i.e., 16 ht 
cores) hardware platform. And we saw performance degradation. As you know each 
physical and a half memory constitute a node. We also worry about the domu 
memory allocation across nodes boundary.


-----Original Message-----
From: Ian Campbell [mailto:Ian.Campbell@xxxxxxxxxx] 
Sent: Thursday, July 21, 2011 4:45 PM
To: Fan, Huaxiang
Cc: xen-users@xxxxxxxxxxxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-devel] How to request contiguous memory for a domu

On Thu, 2011-07-21 at 09:28 +0100, Fan, Huaxiang wrote:
> Hi gurus,
> I used xen 3.4.2. I noticed that, every time I reboot my box, the dom0
> occupy all available physical memory (except those used by xen
> hypervisor), then /etc/init.d/xendomains try to boot domu specified
> in /etc/xen/auto, then dom0 balloon out to free a portion of memory
> (of course the resulting memory should be above dom0_min_mem) to meet
> the memory requirement of incoming domu.
> My question is, is there a guarantee that the domu get contiguous
> physical memory?


>  Or how can I guarantee that?

A guest can try to exchange memory for contiguous memory using
XENMEM_exchange but there are limits on how bug a single contiguous
region can be requested that way (I think it's around order 9 or so).

Why do you want domU memory to be contiguous? There is no general
requirement on dom0 or domU for this to be the case and the p2m/m2p
mechanisms exist to allow non-contiguous machine memory. In general
guests only need to arrange for contiguous memory for specific DMA
buffers to specific physical devices which require it.

Please explain what you are actually trying to achieve so we can best
advise you.

> (My memory capacity is big enough for meeting all domus and dom0
> memory requirements.)
> What about xen 4.1.0?
> Thanks in advance.
> Software Engineer II
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