Re: [PATCH/RFC 0/8] numa - Migrate-on-Fault

[Avi Kivity <avi@redhat.com>]

On 11/11/2010 09:44 PM, Lee Schermerhorn wrote:
> This series of patches implements page migration in the fault path.
>
> !!! N.B., Need to consider iteraction with KSM and Transparent Huge
> !!! Pages.
>
> The basic idea is that when a fault handler such as do_swap_page()
> finds a cached page with zero mappings that is otherwise "stable"--
> e.g., no I/O in progress--this is a good opportunity to check whether the
> page resides on the node indicated by the mempolicy in the current context.
>
> We only attempt to migrate when there are zero mappings because 1) we can
> easily migrate the page--don't have to go through the effort of removing
> all mappings and 2) default policy--a common case--can give different
> answers from different tasks running on different nodes.  Checking the
> policy when there are zero mappings effectively implements a "first touch"
> placement policy.

A couple of kvm-related notes:
- kvm page faults are significantly more expensive than ordinary page 
faults; this will affect the cost/benefit tradeoff
- kvm faults go through get_user_pages_fast(), not the ordinary fault 
path.  Will the code handle this?

-- 
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