linux-kernel - Re: [patch 2/2] cpusets: add interleave_over

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Date:	Fri, 26 Oct 2007 11:30:44 -0400
From:	Lee Schermerhorn <Lee.Schermerhorn@...com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Paul Jackson <pj@....com>, Christoph Lameter <clameter@....com>,
	akpm@...ux-foundation.org, ak@...e.de, linux-kernel@...r.kernel.org
Subject: Re: [patch 2/2] cpusets: add interleave_over_allowed option

On Thu, 2007-10-25 at 19:11 -0700, David Rientjes wrote:
> On Thu, 25 Oct 2007, Paul Jackson wrote:
> 
> > David - could you describe the real world situation in which you
> > are finding that this new 'interleave_over_allowed' option, aka
> > 'memory_spread_user', is useful?  I'm not always opposed to special
> > case solutions; but they do usually require special case needs to
> > justify them ;).
> > 
> 
> Yes, when a task with MPOL_INTERLEAVE has its cpuset mems_allowed expanded 
> to include more memory.  The task itself can't access all that memory with 
> the memory policy of its choice.
> 
> Since the cpuset has changed the mems_allowed of the task without its 
> knowledge, it would require a constant get_mempolicy() and set_mempolicy() 
> loop in the application to catch these changes.  That's obviously not in 
> the best interest of anyone.
> 
> So my change allows those tasks that have already expressed the desire to 
> interleave their memory with MPOL_INTERLEAVE to always use the full range 
> of memory available that is dynamically changing beneath them as a result 
> of cpusets.  Keep in mind that it is still possible to request an 
> interleave only over a subset of allowed mems: but you must do it when you 
> create the interleaved mempolicy after it has been attached to the cpuset.
> set_mempolicy() changes are always honored.
> 
> The only other way to support such a feature is through a modification to 
> mempolicies themselves, which Lee has already proposed.  The problem with 
> that is it requires mempolicy support for cpuset cases and modification to 
> the set_mempolicy() API.  My solution presents a cpuset fix for a cpuset 
> problem.

Actually, my patch doesn't change the set_mempolicy() API at all, it
just co-opts a currently unused/illegal value for the nodemask to
indicate "all allowed nodes".  Again, I need to provide a libnuma API to
request this.   Soon come, mon...

Here's a link the last posting of my patch, as Paul requested:

http://marc.info/?l=linux-mm&m=118849999128086&w=4

A bit out of date, but I'll fix that maybe next week.

Lee
<snip>

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