Re: [ofa-general] Re: Demand paging for memory regions

[Christian Bell <christian.bell@qlogic.com>]

On Wed, 13 Feb 2008, Christoph Lameter wrote:

> Right. We (SGI) have done something like this for a long time with XPmem 
> and it scales ok.

I'd dispute this based on experience developing PGAS language support
on the Altix but more importantly (and less subjectively), I think
that "scales ok" refers to a very specific case.  Sure, pages (and/or
regions) can be large on some systems and the number of systems may
not always be in the thousands but you're still claiming scalability
for a mechanism that essentially logs who accesses the regions.  Then
there's the fact that reclaim becomes a collective communication
operation over all region accessors.  Makes me nervous.

> > When messages are sufficiently large, the control messaging necessary
> > to setup/teardown the regions is relatively small.  This is not
> > always the case however -- in programming models that employ smaller
> > messages, the one-sided nature of RDMA is the most attractive part of
> > it.  
> 
> The messaging would only be needed if a process comes under memory 
> pressure. As long as there is enough memory nothing like this will occur.
> 
> > Nothing any communication/runtime system can't already do today.  The
> > point of RDMA demand paging is enabling the possibility of using RDMA
> > without the implied synchronization -- the optimistic part.  Using
> > the notifiers to duplicate existing memory region handling for RDMA
> > hardware that doesn't have HW page tables is possible but undermines
> > the more important consumer of your patches in my opinion.
> 

> The notifier schemet should integrate into existing memory region 
> handling and not cause a duplication. If you already have library layers 
> that do this then it should be possible to integrate it.

I appreciate that you're trying to make a general case for the
applicability of notifiers on all types of existing RDMA hardware and
wire protocols.  Also, I'm not disagreeing whether a HW page table
is required or not: clearly it's not required to make *some* use of
the notifier scheme.

However, short of providing user-level notifications for pinned pages
that are inadvertently released to the O/S, I don't believe that the
patchset provides any significant added value for the HPC community
that can't optimistically do RDMA demand paging.


    . . christian