Re: [Xen-ia64-devel] [PATCH] Use saner dom0 memory and vcpu defaults, don't panic on over-allocation

[Jarod Wilson <jwilson@xxxxxxxxxx>]

Jarod Wilson wrote:
> Jarod Wilson wrote:
> > Isaku Yamahata wrote:
> > > On Wed, Aug 01, 2007 at 02:49:19PM -0400, Jarod Wilson wrote:
> > >
> > > > > Rather than that approach, a simple 'max_dom0_pages =
> > > > > avail_domheap_pages()' is working just fine on both my 4G and 16G boxes,
> > > > > with the 4G box now getting ~260MB more memory for dom0 and the 16G box
> > > > > getting ~512MB more. Are there potential pitfalls here? 
> > > Hi Jarod. Sorry for delayed reply.
> > > Reviewing the Alex's mail, it might have used up xenheap at that time.
> > > However now that the p2m table is allocated from domheap, 
> > > memory for the p2m table would be counted.
> > > It can be calculated by very roughly dom0_pages / PTRS_PER_PTE.
> > > Here PTRS_PER_PTE = 2048 with 16kb page size, 1024 with 8KB page size...
> > >
> > > the p2m table needs about  2MB for  4GB of dom0 with 16KB page size.
> > >                     about  8MB for 16GB
> > > 		    about 43MB for 86GB 
> > > 		    about 48MB for 96GB 
> > >
> > > (It counts only PTE pages and it supposes that dom0 memory is contiguous.
> > > For more precise calculation it should count PMD, PGD and sparseness.
> > > But its memory size would be only KB order. Even for 1TB dom0,
> > > it would be about 1MB. So I ignored them.)
> > >
> > > With max_dom0_pages = avail_domheap_pages() as you proposed,
> > > we use xenheap for the p2m table, I suppose.
> > > Xenheap size is at most 64MB and so precious.
> > >
> > > How about this heurictic?
> > > max_dom0_pages = avail_domheap_pages() - avail_domheap_pages() / PTRS_PER_PTE;
> > Sounds quite reasonable to me. I'm build and boot testing an updated
> > patch, which assuming all goes well, I'll ship off to the list a bit
> > later today...
> >
> > Ah, one more thing I'm adding: if one specifies dom0_mem=0 on the xen
> > command line, that'll now allocate all available memory.
> ...and here it is. I shuffled a few things around in the max_dom0_size
> calculation a little bit for better readability and avoid multiple calls
> to avail_domheap_pages() (my assumption being that its increasingly
> costly on larger and larger systems).
>
> Indeed, on my 16GB system, only 8MB less from the v2 incantation, and
> the dom0_mem=0 option does properly allocate all available memory to
> dom0. I'm quite happy with this version if everyone else is...
Eep! I retract my happiness... Seems with all memory allocated like 
this, I can't get a guest to actually boot. I get this when I try to 
bring one up via xm create:
Error: (4, 'Out of memory', "xc_dom_boot_mem_init: can't allocate low 
memory for domain\n")
I *think* with the same fudge-factor swiped from x86, I actually could 
get guests to boot when allocating as much as possible, but I'm not 
certain anymore. I'll prod it some more tomorrow... :\
--
Jarod Wilson
jwilson@xxxxxxxxxx


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