[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] can not use all available memory
> From: Pasi Kärkkäinen [mailto:pasik@xxxxxx] > Subject: Re: [Xen-devel] can not use all available memory > > On Mon, Nov 26, 2012 at 12:20:40PM -0800, Dan Magenheimer wrote: > > > From: Sander Eikelenboom [mailto:linux@xxxxxxxxxxxxxx] > > > Subject: Re: [Xen-devel] can not use all available memory > > > > > > Monday, November 26, 2012, 5:58:28 PM, you wrote: > > > > > > >> From: Ian Campbell [mailto:Ian.Campbell@xxxxxxxxxx] > > > >> Sent: Friday, November 23, 2012 6:34 AM > > > >> To: Pasi Kärkkäinen > > > >> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx; Alexander Bienzeisler > > > >> Subject: Re: [Xen-devel] can not use all available memory > > > >> > > > >> On Fri, 2012-11-23 at 13:29 +0000, Pasi Kärkkäinen wrote: > > > >> > On Fri, Nov 23, 2012 at 01:15:53PM +0000, Ian Campbell wrote: > > > >> > > On Fri, 2012-11-23 at 12:55 +0000, Pasi Kärkkäinen wrote: > > > >> > > > > > >> > > > > This has been discussed at length on the list before, please > > > >> > > > > check the > > > >> > > > > archives. > > > >> > > > > > > > >> > > > > > > >> > > > I tried googling quickly but I didn't find anything relevant.. > > > >> > > > > > >> > > I found > > > >> > > http://web.archiveorange.com/archive/v/zCKz5T3PLvtyZDSPQc9i > > > >> > > in a matter of seconds, then: > > > >> > > http://lists.xen.org/archives/html/xen-devel/2011-11/msg01415.html > > > >> > > http://lists.xen.org/archives/html/xen-users/2012-05/msg00139.html > > > >> > > http://lists.xen.org/archives/html/xen-users/2012-05/msg00146.html > > > >> > > > > > >> > > > To me this behaviour still seems wrong. What's the point of > > > >> > > > autoballoon=1 trying to > > > >> > > > balloon down dom0 if the hypervisor already has enough free > > > >> > > > memory for the VM ? > > > >> > > > > > > >> > > > In this case: > > > >> > > > - dom0_mem=2G > > > >> > > > - new VM to launch with size 16 GB. > > > >> > > > - Xen has 28 GB of free memory. > > > >> > > > > > > >> > > > So clearly there's no need to try to balloon down dom0.. > > > >> > > > > > >> > > Right, so don't set autoballoon then. > > > >> > > > > > >> > > > > >> > It's enabled as a default.. so many people hit this problem. > > > >> > > > > >> > > > > >> > > > not-yet-implemented check/feature in xl, or a bug? > > > >> > > > > > >> > > Neither, it is the intended behaviour of xl autoballoon, this > > > >> > > option > > > >> > > means exactly "take the required memory from dom0". > > > >> > > > > > >> > > > > >> > http://xenbits.xen.org/docs/4.2-testing/man/xl.conf.5.html > > > >> > > > > >> > "autoballoon=BOOLEAN > > > >> > > > > >> > If disabled then xl will not attempt to reduce the amount of > > > >> > memory assigned to domain 0 in order to create free memory when > > > >> > starting a new domain. You should set this if you use the dom0_mem > > > >> > hypervisor command line to reduce the amount of memory given to > > > >> > domain > > > >> > 0 by default. > > > >> > > > > >> > Default: 1" > > > >> > > > > >> > > > > >> > I think we should modify that to say "You should set autoballoon=0 > > > >> > if you use the dom0_mem > > > >> hypervisor command line .." > > > >> > At least I understood that text in the opposite way.. > > > >> > > > >> Yes, there should be a s/set/unset/ in there I think. > > > >> > > > >> > Also: What happens if you have autoballoon=1 and you start some VMs, > > > >> > then stop the VMs, > > > >> > so most of the memory is now free in Xen.. > > > >> > > > >> xl balloons dom0 back up when it destroy domains with autoballoon=1. > > > >> > > > >> > and then you try to start a big VM ? > > > >> > Aren't you going to hit the same problem as in this thread? > > > > > > > Hmmm... this behavior and default may make sense for the Citrix > > > > memory model (single machine, dom0 is "the user" so you want it > > > > to always hold most of physical RAM not used by guests). But > > > > probably not so for a more cloud-like memory model. > > > > > > > Is there any (easy) way to force autoballoon=0 if the hypervisor > > > > dom0_mem boot option is specified? Or is there some reasonably > > > > sane case I am missing where a user would want both dom0_mem > > > > and autoballoon=1? > > > > > > > Oracle VM always boots servers with dom0_mem= set so (if/when > > > > OVM switches to use xl), OVM will always set autoballoon off. > > > > So it's the large number of non-Citrix-non-Oracle Xen-as-a-service > > > > providers I am trying to help here. > > > > > > > > > > > > Hmm i was bitten by this 2 weeks ago, i found it a bit not intuitive that: > > > - While the guest i was trying to start required less memory than was > > > freely available (according > to > > > xentop) outside of dom0 > > > - It would fail, because xl started to try to balloon dom0 down which > > > failed. > > > > Hi Sander -- > > > > I could be wrong (and I am confident someone will correct me if I am) but > > I think this is because the Citrix memory model assumes there is an > > inference-driven policy engine for load-balancing memory across competing > > virtual machines ("squeezed"). I suspect squeezed returns unallocated > > xen "free" memory to dom0. I forgot... it is called Dynamic Memory Control (DMC), not squeezed in the XenServer product. > > IMHO, such policy engines are good for demos and so salespeople can > > say "yes, this product supports memory overcommit" but Transcendent > > Memory goes quite a bit further (albeit not for guests with proprietary > > OS's). > > Afaik XenServer/XCP and XenClient both use dom0_mem= option for Xen. Hmmm... looks like you are right. Dan _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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