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RE: [Xen-devel] Re: [Xen-users] Max. PV and HVM Guests

To: "Pasi Kärkkäinen" <pasik@xxxxxx>, Nick Couchman <Nick.Couchman@xxxxxxxxx>
Subject: RE: [Xen-devel] Re: [Xen-users] Max. PV and HVM Guests
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Mon, 9 Nov 2009 07:39:48 -0800 (PST)
Cc: Robert, xen-devel@xxxxxxxxxxxxxxxxxxx, "Mr. Teo En Ming \(Zhang Enming\)" <space.time.universe@xxxxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx, Dunkley <Robert@xxxxxxxxx>
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Hmmm... I'll bet the problem with dom0 crashing is that
dom0 is pv_ops (2.6.31-based) and this patch, which
has been in 2.6.18.8-based dom0 for some time, has never
been put into pv_ops:

http://lists.xensource.com/archives/html/xen-devel/2008-04/msg00143.html

That would explain why some people are seeing this problem
and others are not, and why setting dom0_mem seems to
solve the problem (as dom0_mem effectively shuts off
ballooning in dom0).

Nick and others, if you are interested in detail on
ballooning and improving memory utilization within
and between guests, see:

http://wiki.xensource.com/xenwiki/Open_Topics_For_Discussion?action=AttachFile&do=get&target=Memory+Overcommit.pdf

and

http://oss.oracle.com/projects/tmem

(Tmem is in xen-unstable and in Oracle VM 2.2... it takes
a few steps to set it up.)

> -----Original Message-----
> From: Pasi Kärkkäinen [mailto:pasik@xxxxxx]
> Sent: Monday, November 09, 2009 8:18 AM
> To: Nick Couchman
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx; Mr. Teo En Ming (Zhang Enming);
> xen-users@xxxxxxxxxxxxxxxxxxx; Robert Dunkley
> Subject: Re: [Xen-devel] Re: [Xen-users] Max. PV and HVM Guests
> 
> 
> On Mon, Nov 09, 2009 at 08:06:54AM -0700, Nick Couchman wrote:
> > 
> > 
> > >>> On 2009/11/09 at 05:05, Pasi Kärkkäinen<pasik@xxxxxx> wrote: 
> > > On Mon, Nov 09, 2009 at 08:01:00PM +0800, Mr. Teo En Ming (Zhang
> > Enming) 
> > > wrote:
> > >>    No, I didn't limit dom0 memory in grub.conf.
> > >> 
> > > 
> > > You should. 
> > 
> > Really?  I thought current conventional wisdom was to allow Xen to
> > self-manage memory in both dom0 and domUs, and not to 
> manually adjust
> > this?  I run several Xen systems with anywhere from 8 to 24 
> GB of RAM
> > and 20 to 30 domUs on some of these systems and have 
> *never* specified
> > the dom0 memory at boot time - the Xen ballooning has 
> always functioned
> > perfectly fine, and never crashed my dom0.  
> >
> 
> Yes, Xen is totally OK with this, but dom0 Linux has more problems..
> 
> > Furthermore, while I'm not
> > Linux developer and so not familiar with how Linux 
> calculates buffering
> > and caching, I do know that my Linux systems dynamically 
> manage buffers
> > and caches, and when memory is reduced or some application 
> requires a
> > larger amount of physical memory, Linux reduces the amount 
> of data in
> > buffers and caches.
> > 
> 
> Yeah, it has to do with sizing the network buffers, caches etc.. 
> 
> It shouldn't _crash_, so Teo is seeing some bug I believe. But it has
> always been "best practice" to limit dom0 memory - and prevent weird
> things happening later (like "memory squeeze in netback driver").
> 
> > Of course, a lot of this depends on what you're doing in 
> dom0 - on my
> > Xen servers, my dom0 is strictly for Xen management - I'm 
> not running
> > anything else in dom0 that would require large amounts of 
> memory, memory
> > buffers and caches, etc.
> > 
> 
> Teo is running graphical stuff, X etc so it's a bit different..
> 
> -- Pasi
> 
> > 
> > > 
> > > If dom0 has all the memory at boot time, you need to balloon down
> > dom0 
> > > memory every time you create a new guest - this can (and 
> will) cause
> > 
> > > problems with the dom0 linux kernel.
> > > 
> > > Linux calculates some internal parameters/buffers/values based on
> > the
> > > _boot time_ amount of memory. And when the amount of 
> memory goes down
> > to
> > > only a small fraction of that while creating new guests bad things
> > can
> > > happen..
> > > 
> > > It still shouldn't crash though.. I bet your problem will 
> get fixed
> > when
> > > you limit the dom0 memory to say dom0_mem=512M and reboot.
> > > 
> > > -- Pasi
> > > 
> > >>    Here's my xm info output after I have shutdown all the virtual
> > machines.
> > >> 
> > >>    [root@fedora11-x86-64-host ~]# xm list
> > >>    Name                                        ID   Mem 
> VCPUs     
> > State
> > >>    Time(s)
> > >>    Domain-0                                     0  2812     2    
> > r-----
> > >>    3242.5
> > >>    [root@fedora11-x86-64-host ~]# xm info
> > >>    host                   : fedora11-x86-64-host
> > >>    release                : 2.6.30-rc3-enming.teo-tip
> > >>    version                : #1 SMP Wed Aug 19 23:14:15 SGT 2009
> > >>    machine                : x86_64
> > >>    nr_cpus                : 2
> > >>    nr_nodes               : 1
> > >>    cores_per_socket       : 2
> > >>    threads_per_core       : 1
> > >>    cpu_mhz                : 2800
> > >>    hw_caps                :
> > >>   
> > 
> bfebfbff:20100800:00000000:00000140:0400e3bd:00000000:00000001
> :00000000
> > >>    virt_caps              : hvm hvm_directio
> > >>    total_memory           : 6039
> > >>    free_memory            : 3124
> > >>    node_to_cpu            : node0:0-1
> > >>    node_to_memory         : node0:3124
> > >>    xen_major              : 3
> > >>    xen_minor              : 5
> > >>    xen_extra              : -unstable
> > >>    xen_caps               : xen-3.0-x86_64 xen-3.0-x86_32p
> > hvm-3.0-x86_32
> > >>    hvm-3.0-x86_32p hvm-3.0-x86_64
> > >>    xen_scheduler          : credit
> > >>    xen_pagesize           : 4096
> > >>    platform_params        : virt_start=0xffff800000000000
> > >>    xen_changeset          : Tue Sep 01 11:34:31 2009 +0100 
> > > 20143:a7de5bd776ca
> > >>    xen_commandline        : iommu=1
> > >>    cc_compiler            : gcc version 4.4.1 20090725 (Red Hat
> > 4.4.1-2)
> > >>    (GCC)
> > >>    cc_compile_by          : root
> > >>    cc_compile_domain      : (none)
> > >>    cc_compile_date        : Thu Sep 10 07:01:13 SGT 2009
> > >>    xend_config_format     : 4
> > >> 
> > >>    --
> > >>    Mr. Teo En Ming (Zhang Enming) Dip(Mechatronics)
> > BEng(Hons)(Mechanical
> > >>    Engineering)
> > >>    Alma Maters:
> > >>    (1) Singapore Polytechnic
> > >>    (2) National University of Singapore
> > >>    My Primary Blog:
> > [1]http://teo-en-ming-aka-zhang-enming.blogspot.com
> > >>    My Secondary Blog: [2]http://enmingteo.wordpress.com
> > >>    My Youtube videos: [3]http://www.youtube.com/user/enmingteo
> > >>    Email: [4]space.time.universe@xxxxxxxxx
> > >>    Mobile Phone (Starhub Prepaid): +65-8369-2618
> > >>    Street: Bedok Reservoir Road
> > >>    Country: Singapore
> > >> 
> > >>    On Mon, Nov 9, 2009 at 7:54 PM, Pasi Kärkkäinen 
> <[5]pasik@xxxxxx>
> > wrote:
> > >> 
> > >>      On Mon, Nov 09, 2009 at 06:52:37PM +0800, Mr. Teo En Ming
> > (Zhang 
> > > Enming)
> > >>      wrote:
> > >>      >    Hi,
> > >>      >
> > >>      >    Please watch this 4-minute video at
> > >>      >    [1][6]http://www.youtube.com/watch?v=LbLaPpwNAx4
> > >>      >
> > >>      >    I have only started 3 HVM Linux guests with 1 
> GB ram each.
> > I can't
> > >>      start
> > >>      >    the 4th HVM guest. If I attempt to start the 
> 4th instance,
> > it will
> > >>      crash
> > >>      >    dom0.
> > >>      >
> > >>      >    Are there anything in the xm dmesg output that could
> > explain the
> > >>      low limit
> > >>      >    to the number of VMs that I could start before dom0
> > becomes
> > >>      unresponsive?
> > >>      >
> > >> 
> > >>      Have you limited dom0 memory (by specifying 
> dom0_mem=XMB option
> > in
> > >>      grub.conf for xen.gz) ?
> > >> 
> > >>      What does "xm info" say about free memory before 
> starting any
> > guests?
> > >>      -- Pasi
> > >> 
> > >> References
> > >> 
> > >>    Visible links
> > >>    1. http://teo-en-ming-aka-zhang-enming.blogspot.com/
> > >>    2. http://enmingteo.wordpress.com/
> > >>    3. http://www.youtube.com/user/enmingteo
> > >>    4. mailto:space.time.universe@xxxxxxxxx
> > >>    5. mailto:pasik@xxxxxx
> > >>    6. http://www.youtube.com/watch?v=LbLaPpwNAx4
> > 
> > 
> > 
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