Re: [RFC 2.6.11-rc2-mm2 7/7] mm: manual page migration -- sys_page_migrate

Ray Bryant
2005-02-16T00:38:19Z

Martin J. Bligh wrote:
>>>A possibly stupid suggestion:
>>>
>>>Can page migration be done lazily, instead of all at once? Move the
>>>process, mark its pages as candidates for migration, and when
>>>the page faults, decide whether to copy across or not...
>>>
>>>That way you only copy the pages the process is using, and only copy
>>>each page once. It makes copy for replication easier in some future
>>>incarnation, too, because the same basic infrastructure can be used.
>>>
>>
>>I think that part of the motivation here (e. g. the batch scheduler on
>>a large NUMA machine) is to push pages off of the old nodes so that
>>a new job running on the old nodes can allocate memory quickly and
>>efficiently (i. e. without having to swap out the old job's pages).
>
>
> If our VM code wasn't crap, we'd do that automatically. It seems somewhat
> excessive to do that from a manual interface?
>

SGI had code in IRIX to do that kind of thing (automatically move a page to
the node where most of the references were coming from). Never worked very
well, I have been told. So our bias is away from such "automatic" page
migration schemes and toward "manual" methods driven either by a user
command or a user-level program such as a batch scheduler.

>
>>True enough, we may move pages that are not currently being used.
>>But. on our large NUMA systems, we want the nodes where a new job
>>starts to be relatively clean so that local page allocations are
>>indeed satisfied by local pages and that these requests do not
>>spill off node.
>
>
> Yes. The objective was to kick the LRU page off this node onto some other
> node, or to disk ... at the moment, if one node is more heavily used, we
> will always allocate off node for all new pages. that's crap.
>

Tell me about it. I've spent most of the past couple of years wrassling
with VM system to get it to behave in this situation. :-)

> M.
>
>

--
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Ray Bryant
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raybry@sgi.com raybry@austin.rr.com
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