[Mpi-22] please review - Send Buffer Access (ticket #45)

[Barrett, Brian W]

Ok, now that I've gotten my user's point of view off my chest, back to my
view as an MPI implementer.  I've tried to respond to all Alexander's
objections.  If I missed one, I apologize.

> 1) The memory remapping scenario IO brought up a couple of days ago

Unless someone's done something fun with operating system and architecture
design I'm not aware of, remapping has one problem that always must be
considered...  Send buffers are not required to be (and frequently aren't)
page aligned or a multiple of page size.  Therefore, completely removing the
send buffer from the user's process has the problem of also taking legal
addresses with it (which  would violate the standard).  IBM's solution is
elegant in that it allows remapping without removing from the sender's
process space.  Sandia has a solution called SMARTMAP that is both not
patented and allows single copy transfers in shared memory environments.

Point number 2 was a procedural argument.  I believe others are in a better
position than I to comment on this.  My understanding, however, is that a
technical objection can cause the vote to fail, but is not grounds on
preventing a vote (particularly a second vote).  If it were, we'd never get
anything done.

> 3) Imagine send buffers have to pinned in the memory. To avoid doing this too
> often, these registrations will normally be cached. If more than one send can
> be used for a buffer or, for that matter, overlapping portions of the same
> buffer, say by different threads, access to the lookup-and-pin will have to be
> made atomic. This will further complicate implementation and introduce a
> potentially costly mutual exclusion primitive into the critical path.

The caching problem already exists.  Consider a case where a large send is
completed, then multiple small sends occur within that base and bound after
the first is completed.  This situation is perfectly legal, happens in codes
in the wild, and must be dealt with by MPI implementations.  If that's not
enough, consider a case where the buffer is part of an active Window (which
is legal, as long as the buffers in use for communication don't overlap).
All these cases certainly should be handled by an MPI today.

> 4) I wonder what a const modifier will do for a buffer identifies by
> MPI_BOTTOM and/or a derived data type, possibly with holes in it. How will
> this square up with the C language sequence association rules?

This sounds like an issue for the const proposal, which is different from
the send buffer access proposal.  I'm not sure I have enough data to form an
opinion on the const proposal, but I'm fairly sure we can discuss the send
buffer access proposal without considering this issue.

> 5) Note also if both #45 and #46 will be introduced, there will be no way to
> retract this, even with the help of the MPI_INIT_ASSERTED, should we later
> decide to introduce assertion like MPI_NO_SEND_BUFFER_READ_ACCESS. The const
> modifier from #46 will make that syntactically useless.

If both are passed, that might be true.  It could be argued the const
proposal depends on the access proposal.  However, it can not be rationally
argued that the access proposal in any way depends upon the const proposal.

The send buffer access proposal can certainly be passed and an assert added
later (at whatever point the init_assert proposal is integrated into the
standard) that allows MPI implementations to modify the send buffer.

You raise a good point about the const proposal.  But it has absolutely no
bearing on the send buffer access proposal.

> 6) Finally, what will happen in the Fortran interface? With the
> copy-in/copy-out possibly happening on the MPI subroutine boundary for array
> sections? If more than one send is allowed, the application can pretty easily
> exhaust any virtual memory with a couple of long enough vectors.

How does that change from today?  Today users send multiple buffers at the
same time, and seem to cope with memory exhaustion issues just fine.  So
soon they might be able to remove the data copy they've had to make at the
user level to work around the MPI access restriction, so there's actually
less virtual memory in use.  Seems like a win to me.

> 7) In-place compression and/or encryption of the messages. Compression in
> particular can work wonders on monotonous messages, and cost less time in
> total than the transmission of so many giga-zeroes, for example. Again, having
> send buffer access allowed and const modifier attached will kill this huge
> optimization opportunity. Too bad.

While I hope you're joking about the giga-zeros, you do raise a valid
concern, in that there are a number of optimizations regarding compression,
encryption, and endian-swapping that may be eliminated by this proposal.  On
the flip side, as I argued in a previous e-mail, the user gains quite a bit
in usability.  We have to balance these two factors.  Since users know where
my office is, I tend to lean towards making their lives easier, particularly
when it doesn't cause extra work for me.  But I already sent an e-mail on
that point...

Our experience with Open MPI was that the potential for performance in other
parts of the MPI (collectives, etc.) far outweighed any send-side tricks we
could think of (and you haven't brought up any we didn't think of).  So if
we wanted to do compression or encryption, it would be done with send-side
bounce buffers.  Since a software pipeline would practically be required to
get good performance, the bounce buffer would not have to scale with the
size of the communication buffer but instead with the properties of the
network pipeline.  Of course, my opinion would be that it would be much
simpler and much higher performance to support compression or encryption as
part of the NIC as the data is streamed to the network.  Otherwise, you're
burning memory bandwidth doing the extra copy (even in the modify the send
buffer case), and memory bandwidth is a precious resource for HPC
applications.

One other point to consider.  If I was a user, I'd expect that my one-sided
traffic also be compressed, encrypted, or endian-swapped.  The standard
already requires multiple accesses be legal for one-sided communication.  So
you're going to have a situation where some communication can use a
send-modify implementation and some can not.  I'm not familiar with how
Intel's MPI is architected, but Open MPI is architected such that decisions
such as compression, encryption, and endian-swapping would be made at a low
enough level that the code path is the same whether the message is a
point-to-point send or a one-sided put.  Since that's some of the most
complicated code in Open MPI, I can't foresee adding a second code path just
to get a (dubious) performance benefit.

Brian


--
   Brian W. Barrett
   Dept. 1422: Scalable Computer Architectures
   Sandia National Laboratories