Re: [Xen-devel] hypervisor memory usage

[Vladimir Zidar <mr_w@xxxxxxxxxxxxx>]

I have nailed the problem down to RHEL version of 
compute_dom0_nr_pages() function.
Vanilla xen uses something like this to reserve up to 128MB of ram for 
DMA etc. The same alg. is used in rhel <= 5.2 and also in official xen 3.4.1
   if ( dom0_nrpages == 0 )
   {
       dom0_nrpages = avail;
       dom0_nrpages = min(dom0_nrpages / 16, 128L << (20 - PAGE_SHIFT));
       dom0_nrpages = -dom0_nrpages;
   }

However, RHEL >= 5.3 uses this:

   /*
    * If domain 0 allocation isn't specified, reserve 1/16th of available
    * memory for things like DMA buffers. This reservation is clamped to
    * a maximum of 384MB.
    */
   if ( dom0_nrpages == 0 )
   {
       dom0_nrpages = avail;
       dom0_nrpages = min(dom0_nrpages / 8, 384L << (20 - PAGE_SHIFT));
       dom0_nrpages = -dom0_nrpages;
   } else {
       /* User specified a dom0_size.  Do not clamp the maximum. */
       dom0_max_nrpages = LONG_MAX;
   }

I do understand that they like the idea of reserving more ram, but 
additionally /8 would make 1/8th of memory instead of 1/16th?
So this might be intended behavior, just not advertised anywhere, and as 
a kind of side effect, specifying dom0_mem would altogether skip this 
funny allocation scheme - at least in theory [ I've just put 
dom0_mem=64G (but I have 8G only) ] and it is not coming up, and I will 
not be able to t see the console for at least next couple of hours.
Vladimir Zidar wrote:
> Chris,
>
> good that you pointed to 5.2 vs 5.3 vs 5.4,
> the difference in number of pages is noticed between these:
>
>       xen.gz-2.6.18-92.1.22.el5  - last 5.2 update - all pages are ok,
>       xen.gz-2.6.18-128.el5 - first 5.3 release - ~80000 pages missing 
> on 8GB ram setup.
> Chris Lalancette wrote:
> > Vladimir Zidar wrote:
> >  
> > > Sounds possible. However it would be great if there was switch to 
> > > disable that feature in case hardware is not capable of VT-d, as I'd 
> > > rather use those 300mb than have software support for something that 
> > > I can't actually use.
> > >     
> > In point of fact, VT-d is disabled by default; you need to explicitly 
> > enable it
> > for it to use memory.  However, it's possible that there's a bug, or 
> > some other
> > change caused the memory difference, so it's worthwhile to try and 
> > track it down
> > a little better.  In particular, you jumped from the 5.2 kernel to 
> > the 5.4, so
> > it would be worthwhile to try the 5.3 kernel and see what you get.
> >
> >   
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