[Mpi3-rma] MPI3 RMA Design Goals

Fri Sep 4 10:40:56 CDT 2009

hence the word "similar" to MDBind. MDBind can potentially a lot more as far as preparing the network. Binding is for reducing latency, for no other reason. I agree that in your example of random target communication, it is not useful.

You are right and I have always agreed that when you do need such a table of mem-object data structures or a table of pointers under the hood of the implementation of collective mem-object data structures, random accesses will incur cache misses and latency. I get that. My points are(please let me know if you disagree to any of the bullets):

1) the problem doesn't exist if collective memobjs are used and the memobjs can internally allocate symmetric memory (same pointers and same "keys" across the system).

2) This is a more fundamental problem associated with if it is possible to allocate symmetric memory and corresponding symmetric key-ids on networks that require them. 

3) this problem is same to shmem and these interfaces. 

4) this problem of cache misses for random target communication call will occur if: a)the implementation of a collective object requires an array of keys or memory region identifies for sake of communication and creation of a single key is not possible, b) an array of pointers is required on this system instead of 1 symmetric pointer, and, c) if, say, ofed will have different keys on each node and the user cannot pass a number that he/she wants to be the key value -- there is nothing, you, me or any interface (the "users") can do to fix this.

To me it seems like we are discussing about a case that is a fundamental problem that we don’t have control over. I cannot see how you would define an interface that will not have cache misses for random target communication in case of 4).

So the question is are these interfaces causing cache misses when they can be avoided otherwise? I don’t think so. Do you? If you agree, we have concluded the discussion.

--
Vinod Tipparaju ^ http://ft.ornl.gov/~vinod ^ 1-865-241-1802

From: keith.d.underwood at intel.com
To: mpi3-rma at lists.mpi-forum.org
Date: Fri, 4 Sep 2009 08:51:12 -0600
Subject: Re: [Mpi3-rma] MPI3 RMA Design Goals

Bill’s proposal was for a persistent handle for transmit operations that included things like target rank.  I think there is some merit in that, though we need to evaluate the trade-offs.  Unfortunately, that does not really do anything for the random communication scenario.   In the non-random communication scenarios, I’m a lot less worried about things like random lookups of local data structures getting in the way.  

Bill’s proposal was nothing like MDBind in Portals, and MDBind does nothing to help the issue I was concerned about.  Specifically, MDBind only associates local options and a local memory region with a handle.  It says nothing about the target node or the target memory region.  It is the lookup of information associated with the target node and target memory region that I am worried about and that is addressed by Bill’s proposal for the non-random access case.

Keith

From: mpi3-rma-bounces at lists.mpi-forum.org [mailto:mpi3-rma-bounces at lists.mpi-forum.org] On Behalf Of Vinod tipparaju
Sent: Thursday, September 03, 2009 10:39 PM
To: MPI 3.0 Remote Memory Access working group
Subject: Re: [Mpi3-rma] MPI3 RMA Design Goals

I forgot to include an important parameter (communicator) in the psuedo interface below:
Create_memobj_collective(IN user_ptr, IN size, IN communicator, OUT mem_obj)

In addition to this Bill suggested Bind interface (something similar to MDBind in portals) that would help reduce latency for commonly re-used RMAs.

Vinod Tipparaju ^ http://ft.ornl.gov/~vinod ^ 1-865-241-1802

From: wgropp at illinois.edu
To: mpi3-rma at lists.mpi-forum.org
Date: Thu, 3 Sep 2009 13:20:38 -0500
Subject: Re: [Mpi3-rma] MPI3 RMA Design Goals

Design goal one allows collective creation of objects; its there because many important algorithms don't have collective (over MPI_COMM_WORLD) allocation semantics, and a design that *requires* collective creation of memory objects will also limit the use of the interface.

Bill

On Sep 3, 2009, at 10:49 AM, Underwood, Keith D wrote:

My commentary was on the design goals…  if we allow collective creation of memory objects, and design goal #1 simply says we don’t require it, that may be ok.  Design goal #1 could be interpreted to mean that you wouldn’t have collective creation in the semantic at all.  Do you really anticipate one data type for an object that is either collectively or non-collectively created? 

I strongly disagree with your assertion that you can communicate with no cache misses for the non-collectively allocated memory object.  In a non-collectively allocated case, you will have to keep an array of these on every process, right?  i.e. one for every process you are communicating with?  Randomly indexing that array is going to pound on your cache.

We need to make sure that we don’t ignore the overhead having multiple communicators and heterogeneity.  Yes, I think there are ways around this, but we should at least consider what is practical and likely rather than just what is possible.

Keith

From: mpi3-rma-bounces at lists.mpi-forum.org [mailto:mpi3-rma-bounces at lists.mpi-forum.org] On Behalf Of Vinod tipparaju
Sent: Tuesday, September 01, 2009 10:15 AM
To: MPI 3.0 Remote Memory Access working group
Subject: Re: [Mpi3-rma] MPI3 RMA Design Goals

You are correct in trying to look at the best possible case and estimating cache-misses/performance-bottlenecks. However, personally don't see any difference between this and shmem. When you cannot really allocate symmetric memory underneath, the amount of bookkeeping is same in both cases. When there is no heterogeneity, the check for this can be disabled at MPI startup. When there is heterogeneity we cannot compare with shmem.

I cannot imagine not having symmetric/collective memory object creation to support these RMA interfaces, I think it is a must-have. Sorry I have only been saying we should have these interfaces but haven't given any example for this yet. Given how many times this same issue is coming up, I will do it now.

Consider the creation interfaces:

Create_memobj(IN user_ptr, IN size, OUT mem_obj)

Create_memobj_collective(user_ptr, size, OUT mem_obj)

Assign_memobj(IN/OUT mem_obj, IN user_address, IN size) 

There will be more details on how a mem object which is a result of create_memobj on process A will be exchanged with process B. When it is exchanged explicitly, the heterogeneity information can be created at process B. 

Now take the example with symmetric object:

Process A                                                                       

myptr = allocate(mysize);

Create_memobj_collective(myptr,mysize, all_obj);

Do all kinds of RMA_Xfers

and an example without symmetric object:

myptr = allocate(mysize);

Create_memobj(myptr,mysize,my_obj);

 ----exchange objects here----

do all kinds of RAM_Xfers

In both cases, I can see being able to communicate without any cache misses for mem_obj.

Vinod Tipparaju ^ http://ft.ornl.gov/~vinod ^ 1-865-241-1802

From: keith.d.underwood at intel.com
To: mpi3-rma at lists.mpi-forum.org
Date: Tue, 1 Sep 2009 09:07:41 -0600
Subject: Re: [Mpi3-rma] MPI3 RMA Design Goals

If we take the SINGLE_RMA_INTERFACE_DRAFT_PROPOSAL as an example, and combine it with the draft design goal #1: In order to support RMA to arbitrary locations, no constraints on memory, such as symmetric allocation or collective window creation, can be required

We get an interesting view on how difficult it can be to get “close to the metal”.  So, for MPI_RMA_xfer, we have to assume that the user has some array of target_mem data items.  That means the sequence of steps in user space is:

target_mem = ranks[dest];

MPI_RMA_xfer(… target_mem, dest…);

If we assume that the message sizes are small and the destinations randomly selected and the machine is large… every access to ranks is a cache miss, and we cannot prevent that by providing fancy hardware.  This actually leads me to believe that we may need to reconsider design goal #1, or at least clarify what it means in a way that makes the access more efficient.

MPI_RMA_xfer itself is no picnic either.  If we take the draft design goal #5: The RMA model must support non-cache-coherent and heterogeneous environments, then MPI is required to maintain a data structure for every rank (ok, it has to do this anyway, but we are trying to get close to the metal) and do a lookup into that data structure with every MPI_RMA_xfer to find out if the target is heterogeneous relative to the target rank – another cache miss.  Now, nominally, since this is inside MPI, a lower layer could absorb that check… or, a given MPI could refuse to support heterogeneity or… but, you get the idea. 

So, we’ve got two cache line loads for every transfer.  One in the application and one in the MPI library.  One is impossible to move to the hardware and the other is simply very difficult to move. 

For a contrast, look at SHMEM.  Assume homogeneous, only one communicator context, and hardware mapping of ranks to physical locations.  A shmem_put() of a short item could literally be turned into a few instructions and a processor store (on machines that supported such things).  Personally, I think we will have done well if we can get to the point that a reasonable hardware implementation can get MPI RMA to within 2x of a reasonable SHMEM implementation.  I think we have a long way to go to get there.

Keith

From: mpi3-rma-bounces at lists.mpi-forum.org [mailto:mpi3-rma-bounces at lists.mpi-forum.org] On Behalf Of Vinod tipparaju
Sent: Tuesday, September 01, 2009 5:23 AM
To: MPI 3.0 Remote Memory Access working group
Subject: Re: [Mpi3-rma] MPI3 RMA Design Goals

Good points! RMA interfaces should do nothing to prevent utilizing a high message rate (or low overhead communication) that the underlying hardware may offer. To ensure this happens, there should always be a unrestricted path (lets call it this for now, people have called it a "thin layer", "direct access") to the network. 

This means, despite the fact the the RMA interface has features that abstract out complexity by providing useful characteristics such as ordering and atomicity, it (the RMA interface) should always have this unrestricted, close to the heart of the hardware, path. To achieve this, the unrestricted path should not require any book keeping (from implementation perspective) in relation to the feature-rich path or vice-versa.  

I believe this is what we have demonstrated with the example interfaces hence the null set isn't the case here :-). I will distribute an example implementation very soon so people can get a feel.

---

Vinod Tipparaju ^ http://ft.ornl.gov/~vinod ^ 1-865-241-1802

> From: keith.d.underwood at intel.com
> To: mpi3-rma at lists.mpi-forum.org
> Date: Mon, 31 Aug 2009 16:17:28 -0600
> Subject: Re: [Mpi3-rma] MPI3 RMA Design Goals
> 
> There has been stunning silence since this email, so I will go ahead and toss out a thought...
> 
> In the draft design goals, I don't see two issues that I see as key. The first is "support for high message rate/low overhead communications to random targets". As best I can tell, this is one of the key places were the existing one-sided operations are perceived as falling down for existing customers of SHMEM/PGAS. The second is "elimination of the access epoch requirement". This one may be, um, more controversial, but I believe it is part and parcel with the first one. That is, the first one is not that valuable if the programming model requires an excessive amount of access epoch opens and closes just to force the global visibility of the operations. Unfortunately, the intersection of this solution space with the solution space for the current draft design goal #5 (support non-cache-coherent and heterogeneous environments) may be the null set... I will hold out hope that this isn't the case ;-)
> 
> Keith 
> 
> > -----Original Message-----
> > From: mpi3-rma-bounces at lists.mpi-forum.org [mailto:mpi3-rma-
> > bounces at lists.mpi-forum.org] On Behalf Of William Gropp
> > Sent: Wednesday, August 05, 2009 12:37 PM
> > To: mpi3-rma at lists.mpi-forum.org
> > Subject: [Mpi3-rma] MPI3 RMA Design Goals
> > 
> > I've added versions of the RMA design goals that we discussed at the
> > Forum meeting last week to the wiki page for our group (
> > https://svn.mpi-forum.org/trac/mpi-forum-web/wiki/RmaWikiPage
> > ). This is a draft; lets discuss these. Also, feel free to add to
> > the discussion, particularly in the background section.
> > 
> > Bill
> > 
> > William Gropp
> > Deputy Director for Research
> > Institute for Advanced Computing Applications and Technologies
> > Paul and Cynthia Saylor Professor of Computer Science
> > University of Illinois Urbana-Champaign
> > 
> > 
> > 
> > 
> > _______________________________________________
> > mpi3-rma mailing list
> > mpi3-rma at lists.mpi-forum.org
> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma
> 
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> mpi3-rma mailing list
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<ATT00001.txt>

William Gropp

Deputy Director for Research

Institute for Advanced Computing Applications and Technologies

Paul and Cynthia Saylor Professor of Computer Science

University of Illinois Urbana-Champaign

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