Re: legality of interleaving untagged message segments

Michael Krause said:
>
> Within a given LLP stream, you are correct that the optimization benefits
> may be minimal if I/O device is somewhat "distant" (read as relatively
> high
> latency) from the memory controller.  However, as we see the industry move
> towards PCI Express (PCIe) and with the potential to create more
> integrated
> memory controller chipsets, there may be greater benefit over time
> (especially with some architectures moving to be more NUMA oriented).
>
> Given this may take time to occur and validate, would you be willing to
> make this a SHOULD NOT instead of a MUST NOT thus allowing leeway for
> future development while providing strong guidance that such interleaving
> is strongly discouraged?
>
> Mike
>

There is already a SHOULD that segment SHOULD be submitted in ascending
order. So no further SHOULD clause is required.

Or for that matter useful.

 A "SHOULD NOT 'X'" clause does not truly allow the receiver to optimize on
the assumption that senders will not do X. It just discourages testing of
'X',
so in a sense it's worse than doing nothing. Receivers are not free to
optimize,
and senders are discouraged from whatever optimization there might be.

My assumption is that the MUST NOT I am proposing will not preclude any
achievable optimization on the sender side.  If that assumption is correct
then benefit to the receiver is clearly preferable. But if anyone had an
algorithm that really would optimize across message boundaries and gain
actual benefits from it then the tradeoff is indeed more complex.

The benefit at the receiver side, for an implementation that defers placement
processing of DDP segments until they can be processed in order, is that
there would be at most one active untagged buffer per DDP Stream. This
is a major benefit, which I believe outweighs the theoretical benefits that
some unidentified sender algorithm *might* achieve in the future.

Of course I am willing to be convinced to the contrary if someone were to
present a contemplated optimization that this restriction would interfere
with.
Or for that matter, even assert that they are researching such an algorithm
that they are unwilling to disclose.