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

[Jarod Wilson <jwilson@xxxxxxxxxx>]

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.

-- 
Jarod Wilson
jwilson@xxxxxxxxxx

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