v4.0.x: -cpu-set as a constraint rather than as a binding #9298
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The first category of issue I'm addressing is that recent code changes
seem to only consider -cpu-set as a binding option. Eg a command like
this
% mpirun -np 2 --report-bindings --use-hwthread-cpus \
--bind-to cpulist:ordered --map-by hwthread --cpu-set 6,7 hostname
which just round robins over the --cpu-set list.
Example output which seems fine to me:
> MCW rank 0: [..../..B./..../..../..../..../..../..../..../..../..../....][..../..../..../..../..../..../..../..../..../..../..../....]
> MCW rank 1: [..../...B/..../..../..../..../..../..../..../..../..../....][..../..../..../..../..../..../..../..../..../..../..../....]
It should also be possible though to pass a --cpu-set to most other
map/bind options and have it be a constraint on that binding. Eg
% mpirun -np 2 --report-bindings \
--bind-to hwthread --map-by hwthread --cpu-set 6,7 hostname
% mpirun -np 2 --report-bindings \
--bind-to hwthread --map-by ppr:2:node,pe=2 --cpu-set 6,7,12,13 hostname
The first command above errors that
> Conflicting directives for mapping policy are causing the policy
> to be redefined:
> New policy: RANK_FILE
> Prior policy: BYHWTHREAD
The error check in orte_rmaps_rank_file_open() is likely too aggressive.
The intent seems to be that any option like "--map-by whatever" will
check to see if a rankfile is in use, and report that mapping via rmaps
and using an explicit rankfile is a conflict.
But the check has been expanded to not just check
NULL != orte_rankfile
but also errors out if
(NULL != opal_hwloc_base_cpu_list &&
!OPAL_BIND_ORDERED_REQUESTED(opal_hwloc_binding_policy))
which seems to be only recognizing -cpu-set as a binding option and
ignoring -cpu-set as a constraint on other binding policies.
For now I've changed the
NULL != opal_hwloc_base_cpu_list
to
OPAL_BIND_TO_CPUSET == OPAL_GET_BINDING_POLICY(opal_hwloc_binding_policy)
so it hopefully only errors out if -cpu-set is being used as a binding
policy. Whether I did that right or not it's enough to get to the next
stage of testing the example commands I have above.
Another place similar logic is used is hwloc_base_frame.c where it has
/* did the user provide a slot list? */
if (NULL != opal_hwloc_base_cpu_list) {
OPAL_SET_BINDING_POLICY(opal_hwloc_binding_policy, OPAL_BIND_TO_CPUSET);
}
where it used to (long ago) only do that if
!OPAL_BINDING_POLICY_IS_SET(opal_hwloc_binding_policy)
I think the new code is making it impossible to use --cpu-set as anything
other than a binding policy.
That brings us past the error detection and into the real functionality, some of
which has been stripped out, probably in moving to hwloc-2:
% mpirun -np 2 --report-bindings \
--bind-to hwthread --map-by hwthread --cpu-set 6,7 hostname
> MCW rank 0: [B.../..../..../..../..../..../..../..../..../..../..../....][..../..../..../..../..../..../..../..../..../..../..../....]
> MCW rank 1: [.B../..../..../..../..../..../..../..../..../..../..../....][..../..../..../..../..../..../..../..../..../..../..../....]
The rank_by() function in rmaps_base_ranking.c makes an array out of objects
returned from
opal_hwloc_base_get_obj_by_type(,,,i,)
which uses df_search(). That function changed quite a bit from hwloc-1 to 2
but it used to include a check for
available = opal_hwloc_base_get_available_cpus(topo, start)
which is where the bitmask from --cpu-set goes. And it used to skip objs that
had hwloc_bitmap_iszero(available).
So I restored that behavior in ds_search() by adding a "constrained_cpuset" to
replace start->cpuset that it was otherwise processing. With that change in
place the first command works:
% mpirun -np 2 --report-bindings \
--bind-to hwthread --map-by hwthread --cpu-set 6,7 hostname
> MCW rank 0: [..../..B./..../..../..../..../..../..../..../..../..../....][..../..../..../..../..../..../..../..../..../..../..../....]
> MCW rank 1: [..../...B/..../..../..../..../..../..../..../..../..../....][..../..../..../..../..../..../..../..../..../..../..../....]
The other command uses a different path though that still ignored the
available mask:
% mpirun -np 2 --report-bindings \
--bind-to hwthread --map-by ppr:2:node:pe=2 --cpu-set 6,7,12,13 hostname
> MCW rank 0: [BB../..../..../..../..../..../..../..../..../..../..../....][..../..../..../..../..../..../..../..../..../..../..../....]
> MCW rank 1: [..BB/..../..../..../..../..../..../..../..../..../..../....][..../..../..../..../..../..../..../..../..../..../..../....]
In bind_generic() the code used to call
opal_hwloc_base_find_min_bound_target_under_obj() which used
opal_hwloc_base_get_ncpus(), and that's where it would
intersect objects with the available cpuset and skip over ones
that were't available. To match the old behavior I added a few
lines in bind_generic() to skip over objects that don't intersect
the available mask. After that we get
% mpirun -np 2 --report-bindings \
--bind-to hwthread --map-by ppr:2:node:pe=2 --cpu-set 6,7,12,13 hostname
> MCW rank 0: [..../..BB/..../..../..../..../..../..../..../..../..../....][..../..../..../..../..../..../..../..../..../..../..../....]
> MCW rank 1: [..../..../..../BB../..../..../..../..../..../..../..../....][..../..../..../..../..../..../..../..../..../..../..../....]
I think the above changes are improvements, but I don't feel like they're
comprehensive. I only traced through enough code to fix the two specific
bugs I was dealing with.
Signed-off-by: Mark Allen <markalle@us.ibm.com>
(cherry picked from commit bdd92a7)
|
This is a long forgotten commit that never got ported to a release branch. |
|
Note - there was another commit in the PR from this cherry-pick (bf3980d), but from my testing that doesn't need to go back as it was resolved in another patch. |
|
I hand verified that this fixes the referenced issue. |
|
It has been a long time, but IIRC, this PR may have fixed the cited problems, but introduced breakage in other use-cases. I seem to recall we wound up fixing this another way on the master branch that avoided the collateral damage. 🤷♂️ Your choice - not sure how much trouble it would be to backport the master fix, or how important this fix is to the release branch. |
|
I brought in these changes one-by-one, and can confirm that all changes are needed to make this option, --cpu-list, work. Even including the changes made to the hwloc files. If this patch is to not be taken in to fix this bug - I wonder if instead we should disable --cpu-list, since as is on the v4 series it seems broken. Thoughts? |
|
As of 9/21 ompi call, it was decided this is too risky to take this late in the v4 series. Instead we will print a warning saying that this option is broken, as well as update the man page. |
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The first category of issue I'm addressing is that recent code changes
seem to only consider -cpu-set as a binding option. Eg a command like
this
% mpirun -np 2 --report-bindings --use-hwthread-cpus
--bind-to cpulist:ordered --map-by hwthread --cpu-set 6,7 hostname
which just round robins over the --cpu-set list.
Example output which seems fine to me:
It should also be possible though to pass a --cpu-set to most other
map/bind options and have it be a constraint on that binding. Eg
% mpirun -np 2 --report-bindings
--bind-to hwthread --map-by hwthread --cpu-set 6,7 hostname
% mpirun -np 2 --report-bindings
--bind-to hwthread --map-by ppr:2:node,pe=2 --cpu-set 6,7,12,13 hostname
The first command above errors that
The error check in orte_rmaps_rank_file_open() is likely too aggressive.
The intent seems to be that any option like "--map-by whatever" will
check to see if a rankfile is in use, and report that mapping via rmaps
and using an explicit rankfile is a conflict.
But the check has been expanded to not just check
NULL != orte_rankfile
but also errors out if
(NULL != opal_hwloc_base_cpu_list &&
!OPAL_BIND_ORDERED_REQUESTED(opal_hwloc_binding_policy))
which seems to be only recognizing -cpu-set as a binding option and
ignoring -cpu-set as a constraint on other binding policies.
For now I've changed the
NULL != opal_hwloc_base_cpu_list
to
OPAL_BIND_TO_CPUSET == OPAL_GET_BINDING_POLICY(opal_hwloc_binding_policy)
so it hopefully only errors out if -cpu-set is being used as a binding
policy. Whether I did that right or not it's enough to get to the next
stage of testing the example commands I have above.
Another place similar logic is used is hwloc_base_frame.c where it has
/* did the user provide a slot list? */
if (NULL != opal_hwloc_base_cpu_list) {
OPAL_SET_BINDING_POLICY(opal_hwloc_binding_policy, OPAL_BIND_TO_CPUSET);
}
where it used to (long ago) only do that if
!OPAL_BINDING_POLICY_IS_SET(opal_hwloc_binding_policy)
I think the new code is making it impossible to use --cpu-set as anything
other than a binding policy.
That brings us past the error detection and into the real functionality, some of
which has been stripped out, probably in moving to hwloc-2:
% mpirun -np 2 --report-bindings
--bind-to hwthread --map-by hwthread --cpu-set 6,7 hostname
The rank_by() function in rmaps_base_ranking.c makes an array out of objects
returned from
opal_hwloc_base_get_obj_by_type(,,,i,)
which uses df_search(). That function changed quite a bit from hwloc-1 to 2
but it used to include a check for
available = opal_hwloc_base_get_available_cpus(topo, start)
which is where the bitmask from --cpu-set goes. And it used to skip objs that
had hwloc_bitmap_iszero(available).
So I restored that behavior in ds_search() by adding a "constrained_cpuset" to
replace start->cpuset that it was otherwise processing. With that change in
place the first command works:
% mpirun -np 2 --report-bindings
--bind-to hwthread --map-by hwthread --cpu-set 6,7 hostname
The other command uses a different path though that still ignored the
available mask:
% mpirun -np 2 --report-bindings
--bind-to hwthread --map-by ppr:2:node:pe=2 --cpu-set 6,7,12,13 hostname
I think the above changes are improvements, but I don't feel like they're
comprehensive. I only traced through enough code to fix the two specific
bugs I was dealing with.
Fixes #6540
Signed-off-by: Mark Allen markalle@us.ibm.com
(cherry picked from commit bdd92a7)