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From: Richard Graham (rlgraham_at_[hidden])
Date: 2007-07-25 10:00:47


This is good work, so I am happy to see it come over. My initial
understanding was that
 there would be compile time protection for this. In the absence of this, I
think we need
 to see performance data on a variety of communication substrates. It seems
like a latency
 measurement is, perhaps, the most sensitive measurement, and should be
sufficient to
 see the impact on the critical path.

Rich

On 7/25/07 9:04 AM, "Jeff Squyres" <jsquyres_at_[hidden]> wrote:

> WHAT: Merge the sparse groups work to the trunk; get the community's
> opinion on one remaining issue.
> WHY: For large MPI jobs, it can be memory-prohibitive to fully
> represent dense groups; you can save a lot of space by having
> "sparse" representations of groups that are (for example)
> derived from MPI_COMM_WORLD.
> WHERE: Main changes are (might have missed a few in this analysis,
> but this is 99% of it):
> - Big changes in ompi/group
> - Moderate changes in ompi/comm
> - Trivial changes in ompi/mpi/c, ompi/mca/pml/[dr|ob1],
> ompi/mca/comm/sm
> WHEN: The code is ready now in /tmp/sparse-groups (it is passing
> all Intel and IBM tests; see below).
> TIMEOUT: We'll merge all the work to the trunk and enable the
> possibility of using sparse groups (dense will still be the
> default, of course) if no one objects by COB Tuesday, 31 Aug
> 2007.
>
> ========================================================================
> ===
>
> The sparse groups work from U. Houston is ready to be brought into the
> trunk. It is built on the premise that for very large MPI jobs, you
> don't want to fully represent MPI groups in memory if you don't have
> to. Specifically, you can save memory for communicators/groups that
> are derived from MPI_COMM_WORLD by representing them in a sparse
> storage format.
>
> The sparse groups work introduces 3 new ompi_group_t storage formats:
>
> * dense (i.e., what it is today -- an array of ompi_proc_t pointers)
> * sparse, where the current group's contents are based on the group
> from which the child was derived:
> 1. range: a series of (offset,length) tuples
> 2. stride: a single (first,stride,last) tuple
> 3. bitmap: a bitmap
>
> Currently, all the sparse groups code must be enabled by configuring
> with --enable-sparse-groups. If sparse groups are enabled, each MPI
> group that is created will automatically use the storage format that
> takes the least amount of space.
>
> The Big Issue with the sparse groups is that getting a pointer to an
> ompi_proc_t may no longer be an O(1) operation -- you can't just
> access it via comm->group->procs[i]. Instead, you have to call a
> macro. If sparse groups are enabled, this will call a function to do
> the resolution and return the proc pointer. If sparse groups are not
> enabled, the macro currently resolves to group->procs[i].
>
> When sparse groups are enabled, looking up a proc pointer is an
> iterative process; you have to traverse up through one or more parent
> groups until you reach a "dense" group to get the pointer. So the
> time to lookup the proc pointer (essentially) depends on the group and
> how many times it has been derived from a parent group (there are
> corner cases where the lookup time is shorter). Lookup times in
> MPI_COMM_WORLD are O(1) because it is dense, but it now requires an
> inline function call rather than directly accessing the data
> structure (see below).
>
> Note that the code in /tmp/sparse-groups is currently out-of-date with
> respect to the orte and opal trees due to SVN merge mistakes and
> problems. Testing has occurred by copying full orte/opal branches
> from a trunk checkout into the sparse group tree, so we're confident
> that it's compatible with the trunk. Full integration will occur
> before commiting to the trunk, of course.
>
> The proposal we have for the community is as follows:
>
> 1. Remove the --enable-sparse-groups configure option
> 2. Default to use only dense groups (i.e., same as today)
> 3. If the new MCA parameter "mpi_use_sparse_groups" is enabled, enable
> the use of sparse groups
> 4. Eliminate the current macro used for group proc lookups and instead
> use an inline function of the form:
>
> static inline ompi_proc_t lookup_group(ompi_group_t *group, int
> index) {
> if (group_is_dense(group)) {
> return group->procs[index];
> } else {
> return sparse_group_lookup(group, index);
> }
> }
>
> *** NOTE: This design adds a single "if" in some
> performance-critical paths. If the group is sparse, it will
> add a function call and the overhead to do the lookup.
> If the group is dense (which will be the default), the proc
> will be returned directly from the inline function.
>
> The rationale is that adding a single "if" (perhaps with
> OPAL_[UN]LIKELY?) in a few code paths will not be a big deal.
>
> 5. Bring all these changes into the OMPI trunk and therefore into
> v1.3.
>
> Comments?
>
> --
> Jeff Squyres
> Cisco Systems
>
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