As it stands today, the problem is that we can inject things into the
BTL successfully that are not injected into the NIC (due to software
flow control). Once a message is injected into the BTL, the PML marks
completion on the MPI request. If it was a blocking send that got
marked as complete, but the message isn't injected into the NIC/NIC
library, and the user doesn't re-enter the MPI library for a
considerable amount of time, then we have a problem.
Personally, I'd rather just not mark MPI completion until a local
completion callback from the BTL. But others don't like that idea, so
we came up with a way for back pressure from the BTL to say "it's not
on the wire yet". This is more complicated than just not marking MPI
completion early, but why would we do something that helps real apps
at the expense of benchmarks? That would just be silly!
Brian
On Nov 7, 2007, at 7:56 PM, Richard Graham wrote:
Does this mean that we don’t have a queue to store btl level
descriptors that
are only partially complete ? Do we do an all or nothing with
respect to btl
level requests at this stage ?
Seems to me like we want to mark things complete at the MPI level
ASAP, and
that this proposal is not to do that – is this correct ?
Rich
On 11/7/07 11:26 PM, "Jeff Squyres" <jsquy...@cisco.com> wrote:
On Nov 7, 2007, at 9:33 PM, Patrick Geoffray wrote:
>> Remember that this is all in the context of Galen's proposal for
>> btl_send() to be able to return NOT_ON_WIRE -- meaning that the
send
>> was successful, but it has not yet been sent (e.g., openib BTL
>> buffered it because it ran out of credits).
>
> Sorry if I miss something obvious, but why does the PML has to be
> aware
> of the flow control situation of the BTL ? If the BTL cannot send
> something right away for any reason, it should be the
responsibility
> of
> the BTL to buffer it and to progress on it later.
That's currently the way it is. But the BTL currently only has the
option to say two things:
1. "ok, done!" -- then the PML will think that the request is
complete
2. "doh -- error!" -- then the PML thinks that Something Bad
Happened(tm)
What we really need is for the BTL to have a third option:
3. "not done yet!"
So that the PML knows that the request is not yet done, but will
allow
other things to progress while we're waiting for it to complete.
Without this, the openib BTL currently replies "ok, done!", even when
it has only buffered a message (rather than actually sending it out).
This optimization works great (yeah, I know...) except for apps that
don't dip into the MPI library frequently. :-\
--
Jeff Squyres
Cisco Systems
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