Re: eliminating 166G limit (was Re: Problem with nr_nodes on large memory NUMA machine)

[beth kon <eak@us.ibm.com>]

Has there been any more thought on this subject? The discussion seems to 
have stalled, and we're hoping to find a way past this 166G limit...

Jan Beulich wrote:

>>>>Keir Fraser <Keir.Fraser@cl.cam.ac.uk> 27.11.07 10:21 >>>
>>>>        
>>>>
>>On 27/11/07 09:00, "Jan Beulich" <jbeulich@novell.com> wrote:
>>
>>    
>>
>>>>I don't get how your netback approach works. The pages we transfer do not
>>>>originate from netback, so it has little control over them. And, even if it
>>>>did, when we allocate pages for network receive we do not know which
>>>>domain's packet will end up in each buffer.
>>>>        
>>>>
>>>Oh, right, I mixed up old_mfn and new_mfn in netbk_gop_frag(). Nevertheless
>>>netback could take care of this by doing the copying there, as at that point i
>>>already knows the destination domain.
>>>      
>>>
>>You may not know constraints on that domain's max_mfn though. We could add
>>an interface to Xen to interrogate that, but generally it's not something we
>>probably want to expose outside of Xen and the guest itself.
>>    
>>
>
>What constraints other than the guest's address size influence its max_mfn?
>Of course, if there's anything beyond the address size, then having a way to
>obtain the constraint explicitly would be desirable. But otherwise (and as
>fallback) using 37 bits (128G) seems quite reasonable.
>
>  
>
>>>>Personally I think doing it in Xen is perfectly good enough for supporting
>>>>this very out-of-date network receive mechanism.
>>>>        
>>>>
>>>I'm not just concerned about netback here. The interface exists, and other
>>>users might show up and/or exist already. Whether it would be acceptable
>>>for them to do allocation and copying is unknown. You'd therefore either
>>>need a way to prevent future users of the transfer mechanism, or set proper
>>>requirements on its use. I think that placing extra requirements on the user
>>>of the interface is better than introducing extra (possibly hard to reproduce/
>>>recognize/debug) possibilities of failure.
>>>      
>>>
>>The interface is obsolete.
>>    
>>
>
>Then it should be clearly indicated as such, e.g. by a mechanism similar to
>deprecated_irq_flag() in Linux 2.6.22. And as a result, its use in netback should
>then probably be conditional upon an extra config option, which could at once
>be used to provide a note to Xen that the feature isn't being used so that the
>function could return -ENOSYS and the clipping could be avoided/reverted.
>
>Jan
>
>
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>


-- 
Elizabeth Kon (Beth)
IBM Linux Technology Center
Open Hypervisor Team
email: eak@us.ibm.com