Re: [RFC PATCH 00/14] Heterogeneous Memory System (HMS) and hbind()

[Logan Gunthorpe <logang@deltatee.com>]

On 2018-12-06 4:09 p.m., Dave Hansen wrote:
> This looks great.  But, we don't _have_ this kind of information for any
> system that I know about or any system available in the near future.
> 
> We basically have two different world views:
> 1. The system is described point-to-point.  A connects to B @
>    100GB/s.  B connects to C at 50GB/s.  Thus, C->A should be
>    50GB/s.
>    * Less information to convey
>    * Potentially less precise if the properties are not perfectly
>      additive.  If A->B=10ns and B->C=20ns, A->C might be >30ns.
>    * Costs must be calculated instead of being explicitly specified
> 2. The system is described endpoint-to-endpoint.  A->B @ 100GB/s
>    B->C @ 50GB/s, A->C @ 50GB/s.
>    * A *lot* more information to convey O(N^2)?
>    * Potentially more precise.
>    * Costs are explicitly specified, not calculated
> 
> These patches are really tied to world view #1.  But, the HMAT is really
> tied to world view #1.

I didn't think this was meant to describe actual real world performance
between all of the links. If that's the case all of this seems like a
pipe dream to me.

Attributes like cache coherency, atomics, etc should fit well in world
view #1... and, at best, some kind of flag saying whether or not to use
a particular link if you care about transfer speed. -- But we don't need
special "link" directories to describe the properties of existing buses.

You're not *really* going to know bandwidth or latency for any of this
unless you actually measure it on the system in question.

Logan