Re: [Xen-devel] [PATCH 6 of 8 [RFC]] libxc: introduce xc_domain_move_memory

[Tim Deegan <tim@xxxxxxx>]

At 16:07 +0100 on 02 May (1367510834), George Dunlap wrote:
> On 02/05/13 15:32, Tim Deegan wrote:
> >Hi,
> >
> >This looks like a promising start.  Two thoughts:
> >
> >1. You currently move memory into a bufferm free it, allocate new memory
> >    and restore the contents.  Copying directly from old to new would be
> >    significantly faster, and you could do it for _most_ batches:
> >    - copy old batch 0 to the backup buffer; free old batch 0;
> >    - allocate new batch 1; copy batch 1 directly; free old batch 1;
> >      ...
> >    - allocate new batch n; copy batch n directly; free old batch n;
> >    - allocate new batch 0; copy batch 0 from the backup buffer.
> 
> Hmm -- isn't it the case that if there is not *free* memory lying around 
> somewhere, then this operation is fairly pointless?  What will happen is 
> that after freeing batch 0, "allocate new batch 1" will get that 
> memory.  So copying it to a temporary buffer in dom0 seems like not a 
> particularly useful thing to do -- it should try to allocate a new 
> buffer to copy into directly, and if that fails, just say "No point 
> trying -- no empty memory to move into."

Sure, that's better, as long as the temporary bump in the VM's max_pages
is acceptable to the rest of the toolstack. :)

Tim.

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