Hybrid parallel coloring fallback strategies (better strong scaling and user friendliness) #908

pcarruscag · 2020-03-11T11:07:40Z

Proposed Changes

The hybrid parallel implementation uses grid coloring (edge and element) to avoid race conditions when building the residual vector and Jacobian matrix.
Depending on grid size, type, and ordering, it may be difficult to find a good coloring, i.e. providing locality and enough parallelism (all threads getting almost the same number of work chunks to work on per color).

This PR extends the use of the "reducer strategy" to fine grids (introduced and described in #861 for coarse grids, which before #894 I thought would be the only problematic scenarios) as a fallback strategy that is automatically selected when the coloring efficiency (highlighted below) drops below a threshold value. The reducer strategy can be forced by setting EDGE_COLORING_GROUP_SIZE to 0 (for some cases it seems to give better performance, contrary to the initial assessment from #789).
For element loops (CFEASolver and CMeshSolver, modified in #843) an automatic fallback was also introduced, but using "locks" (explained below) as a "reducer strategy" would use too much memory, and element loops are more compute intensive (and therefore less affected by the overhead of using these locks).

Related Work

Part of #789, namely:
Improves scaling of the work from #861 #843 (should help with #894)
Fixes a bug from #830

PR Checklist

I am submitting my contribution to the develop branch.
My contribution generates no new compiler warnings (try with the '-Wall -Wextra -Wno-unused-parameter -Wno-empty-body' compiler flags).
My contribution is commented and consistent with SU2 style.
I have added a test case that demonstrates my contribution, if necessary.
I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp) , if necessary.

…d_SIMD

…arallel_and_SIMD

…d_SIMD

…arallel_and_SIMD

…ffness, cleanup unused vars

…d also on the fine grid

…ut' into feature_hybrid_parallel_and_SIMD

…arallel_and_SIMD

…ation in CConfig.cpp

…loring fails

…arallel_and_SIMD

pcarruscag

General comments:

pcarruscag · 2020-03-11T11:11:20Z

Common/include/toolboxes/C1DInterpolation.hpp

-  /*!
-   * \brief Destructor of the CAkimaInterpolation class.
-   */
-  ~CAkimaInterpolation(){}
-


This was removed for old compiler compatibility.

pcarruscag · 2020-03-11T11:15:00Z

Common/include/toolboxes/graph_toolbox.hpp

+  /*--- Ideally compute time is proportional to total work over number of threads. ---*/
+  su2double ideal = coloring.getNumNonZeros() / su2double(numThreads);
+
+  /*--- In practice the total work is quantized first by colors and then by chunks. ---*/
+  Index_t real = 0;
+  for(Index_t color = 0; color < coloring.getOuterSize(); ++color)
+    real += chunkSize * roundUpDiv(roundUpDiv(coloring.getNumNonZeros(color), chunkSize), numThreads);


The computation of coloring efficiency is described here, it is just a simple heuristic.

pcarruscag · 2020-03-11T11:16:07Z

Common/src/CConfig.cpp

-    }
-  }
+        }
+      }


A few indentation fixes here and there.

pcarruscag · 2020-03-11T11:34:07Z

Common/src/linear_algebra/CSysSolve.cpp

  if (!smooth_ready) {
+    SU2_OMP_BARRIER
    SU2_OMP_MASTER
    {
      auto nVar = b.GetNVar();


Bug fix relating to #830, sometimes the code would hang on the first iteration because of this:

if (!smooth_ready) { // if working variables have not been allocated we need to allocate... SU2_OMP_BARRIER // <- This is the fix. SU2_OMP_MASTER // ...only the master thread can do allocation as those variables are shared { // allocate variables ... smooth_ready = true; // set this flag so we don't allocate on the next call. } SU2_OMP_BARRIER // now we need a barrier so all threads "see" the new state of the variables. // but because we are "smart" the barrier is inside the "if" so that we don't hit it every time. // But if some threads arrive (very) late to the party they might see smooth_ready == true already // and they miss the barrier which results in a deadlock (all threads must arrive before execution // can continue). One solution is to make sure all threads are inside the "if" (via another barrier) // before changing the condition deciding whether that statement is executed. }

Oh! I think I did see this happen a couple of times when I briefly tested the multi-threading. I built SU2 with -Dwith-omp=true

It happened when I didn't specify a number of threads with the -t option. But it ran okay if I specified even -t 1. Does this fix that? Or do you have to specify a thread count if you build with OMP?

When you don't use option -t (--threads) SU2 picks up the number of threads from the environment, which often is the maximum. If you use MPI at the same time like this the system will probably be oversubscribed, I noticed that OpenMPI 3.1.4 on Ubuntu 16 deadlocks when this happens (oversubscribing with just threads does not seem to deadlock).
This fix was for a more subtle problem.

Another aspect of using MPI+threads is that you need to pay attention to the binding mode, some mpi will "--bind-to core" when the number of ranks is small, this then locks all threads spawn by each rank to the core. Always use "--bind-to socket/numa" as appropriate.

pcarruscag · 2020-03-11T11:36:44Z

Common/src/linear_algebra/CSysSolve.cpp

        unsigned long IterLimit = min(RestartIter, MaxIter-IterLinSol);
        IterLinSol += FGMRES_LinSolver(*LinSysRes_ptr, *LinSysSol_ptr, mat_vec, *precond, SolverTol, IterLimit, residual, ScreenOutput, config);
-        if ( residual < SolverTol*norm0 ) break;
+        if ( residual <= SolverTol*norm0 ) break;


Small fix, restarted FGMRES was going into an infinite loop when the RHS was zero (happens in mesh deformation cases).

pcarruscag · 2020-03-11T11:46:04Z

SU2_CFD/src/solvers/CEulerSolver.cpp

+void CEulerSolver::CommonPreprocessing(CGeometry *geometry, CSolver **solver_container, CConfig *config, unsigned short iMesh,
+                                       unsigned short iRKStep, unsigned short RunTime_EqSystem, bool Output) {


I've done some cleanup by factoring out common (to both Euler and NS solvers) preprocessing steps.

pcarruscag · 2020-03-11T11:47:11Z

SU2_CFD/src/solvers/CEulerSolver.cpp

+  if(!ReducerStrategy && !Output) {
+    LinSysRes.SetValZero();
+    if (implicit && !disc_adjoint) Jacobian.SetValZero();
+    else {SU2_OMP_BARRIER} // because of "nowait" in LinSysRes
+  }


The Jacobian.SetValZero(); bit was previously not guarded by !Output which was a bit wasteful.

pcarruscag · 2020-03-11T11:49:21Z

SU2_CFD/src/solvers/CEulerSolver.cpp

-void CEulerSolver::SetUndivided_Laplacian(CGeometry *geometry, CConfig *config) {
+void CEulerSolver::SetUndivided_Laplacian_And_Centered_Dissipation_Sensor(CGeometry *geometry, CConfig *config) {


These routines were transformed into point loops (no need for grid coloring) and fused since they are always called together. (The code is actually much simpler especially w.r.t. the boundary_i, boundary_j conditionals)

pcarruscag · 2020-03-11T11:52:29Z

SU2_CFD/src/solvers/CFEASolver.cpp

-      SU2_OMP_FOR_DYN(roundUpDiv(OMP_MIN_SIZE, ColorGroupSize)*ColorGroupSize)
+      SU2_OMP_FOR_DYN(nextMultiple(OMP_MIN_SIZE, color.groupSize))


In a number of places I did some replacing to make this operation more expressive.

pcarruscag · 2020-03-11T11:56:09Z

config_template.cfg

+% --------------------- HYBRID PARALLEL (MPI+OpenMP) OPTIONS ---------------------%
+%
+% An advanced performance parameter for FVM solvers, a large-ish value should be best
+% when relatively few threads per MPI rank are in use (~4). However, maximum parallelism
+% is obtained with EDGE_COLORING_GROUP_SIZE=1, consider using this value only if SU2
+% warns about low coloring efficiency during preprocessing (performance is usually worse).
+% Setting the option to 0 disables coloring and a different strategy is used instead,
+% that strategy is automatically used when the coloring efficiency is less than 0.875.
+% The optimum value/strategy is case-dependent.
+EDGE_COLORING_GROUP_SIZE= 512
+%
+% Independent "threads per MPI rank" setting for LU-SGS and ILU preconditioners.
+% For problems where time is spend mostly in the solution of linear systems (e.g. elasticity,
+% very high CFL central schemes), AND, if the memory bandwidth of the machine is saturated
+% (4 or more cores per memory channel) better performance (via a reduction in linear iterations)
+% may be possible by using a smaller value than that defined by the system or in the call to
+% SU2_CFD (via the -t/--threads option).
+% The default (0) means "same number of threads as for all else".
+LINEAR_SOLVER_PREC_THREADS= 0
+%


New options introduced in #853 and #861 added to config_template.

pcarruscag · 2020-03-11T12:06:11Z

SU2_CFD/src/solvers/CMeshSolver.cpp

+  /*--- Use the config option as an upper bound on elasticity modulus.
+   *    For RANS meshes the range of element volume or wall distance is
+   *    very large and leads to an ill-conditioned stiffness matrix.
+   *    Absolute values of elasticity modulus are not important for
+   *    mesh deformation, since linear elasticity is used and all
+   *    boundary conditions are essential (Dirichlet). ---*/
+  const su2double maxE = config->GetDeform_ElasticityMod();


@rsanfer I introduced a small change to the mesh elasticity, the config option above is used as an upper bound for elasticity modulus when using wall-distance or volume based mesh stiffness (for the reason in the comment).
When using constant stiffness the option is ignored and E is 1 everywhere, this change made no difference to the existing testcases.

pcarruscag

On the subject of the lock fallback strategy for element loops (which I still have to stress test a bit).
A lock is a kind of stop light, that can be used to protect against concurrent access to a shared resource (e.g. memory location, stream object, etc.).
The usage pattern is to set them (omp_set_lock) before using the resource and unset them (omp_unset_lock) after. If the lock was already set when a thread calls omp_set_lock it will wait there until the thread who set it first unsets it. The OpenMP runtime make the set/unset process itself thread safe (as it involves more than setting some flag as true or false).

pcarruscag · 2020-03-11T12:11:22Z

SU2_CFD/src/solvers/CFEASolver.cpp

        for (iNode = 0; iNode < nNodes; iNode++) {

+          if (LockStrategy) omp_set_lock(&UpdateLocks[indexNode[iNode]]);


In the loops over the points associated with an element, that follow the computation of element stiffness matrices (for example) we protected against concurrent accesses by setting (or acquiring) a lock for the point before writing to shared memory locations. Once done we unset the lock.
One downside of this strategy is that, contrary to coloring, the operations no longer have deterministic behaviour as which thread adds its contribution first depends on order of arrival, and so machine accuracy differences between otherwise identical runs should be expected.

pcarruscag · 2020-03-11T12:13:39Z

Common/include/omp_structure.hpp

+/*!
+ * \brief Dummy lock type and associated functions.
+ */
+struct omp_lock_t {};
+struct DummyVectorOfLocks {
+  omp_lock_t l;
+  inline omp_lock_t& operator[](int) {return l;}
+};
+inline void omp_init_lock(omp_lock_t*){}
+inline void omp_set_lock(omp_lock_t*){}
+inline void omp_unset_lock(omp_lock_t*){}
+inline void omp_destroy_lock(omp_lock_t*){}


As usual we define "do-nothing" types and functions when compiling without hybrid parallel support to make compilation compatible without having to throw #ifdefs everywhere.

… loop

pcarruscag · 2020-03-24T09:23:06Z

SU2_CFD/src/solvers/CTurbSolver.cpp

  const bool muscl = config->GetMUSCL_Turb();
  const bool limiter = (config->GetKind_SlopeLimit_Turb() != NO_LIMITER);

+  /*--- Only reconstruct flow variables if MUSCL is on for flow (requires upwind) and turbulence. ---*/
+  const bool musclFlow = config->GetMUSCL_Flow() && muscl &&
+                        (config->GetKind_ConvNumScheme_Flow() == SPACE_UPWIND);
+  /*--- Only consider flow limiters for cell-based limiters, edge-based would need to be recomputed. ---*/
+  const bool limiterFlow = (config->GetKind_SlopeLimit_Flow() != NO_LIMITER) &&
+                           (config->GetKind_SlopeLimit_Flow() != VAN_ALBADA_EDGE);


Since we are introducing turbulence-related fixes in #905, I wonder if it is a good time to revisit the reconstruction logic in the turbulent solver (which currently is not adequate for centered schemes), this only affects a couple of regressions.
@jayantmukho , @economon , @clarkpede what do you think?

Seems like you have revisited some of the logic here already?

I haven't looked at the reconstructions before. Before these changes the MUSCL reconstruction for all (flow and turbulent) variables was only done when MUSCL_TURB= YES (when MUSCL reconstruction was requested for the turbulence solver as well).

Here you have changed it such that the flow variables get reconstructed if MUSCL_FLOW = YES but the turbulent variables are treated as before, i.e. they get reconstructed only when MUSCL_TURB= YES.

Are you suggesting changing the reconstruction itself? Or having an alternate reconstruction when centered schemes are used? Either way, I am for it, but don't know any better reconstruction schemes. Do you have any references I could check out? CFD wiki is best I've got so far 😛

actually, I am surprised this didn't change more regression tests.

Well this is all I think the logic needs. Central schemes never use MUSCL so it did not make sense to reconstruct the flow variables using gradients.
This only affects cases with central schemes + MUSCL turbulence (we seem to have two :) )

In theory what we need here is not a reconstruction, but a recomputation of the mass flux that is consistent with the main solver. I played with that once here #721 #726, by storing the mass flux from the convective residual loop. It made convergence worse since it un-staggers the solution (block Gauss-Seidel vs block Jacobi) so I did not pursue it further...

Oh nevermind, I didn't see the && muscl in the declaration of musclFlow.

I am not sure why you would need a recomputation of the mass flux as opposed to flow variables. The reconstructions in the EulerSolver are identical to that in TurbSolver (except for things like the low mach corrections and non-physical points). I can see an argument made for copying all those corrections in the TurbSolver as well.

If you want the mass fluxes to respect the discretized continuity equation then they should be consistent with what is computed by the flow solver (otherwise you are introducing a source term that is proportional to the mass imbalance). In any case this did not seem to make a huge difference... But it is the recommended approach especially for some incompressible methods (where the mass flux is kept separate anyway for other reasons).

talbring

I just looked through everything. Nothing to complain. Thanks @pcarruscag Also for the small (unrelated) improvements in between!

talbring · 2020-03-25T16:45:32Z

SU2_CFD/src/solvers/CEulerSolver.cpp

-void CEulerSolver::SetUndivided_Laplacian(CGeometry *geometry, CConfig *config) {
+void CEulerSolver::SetUndivided_Laplacian_And_Centered_Dissipation_Sensor(CGeometry *geometry, CConfig *config) {


…fallback_strategies

jayantmukho

LGTM. We can continue the conversation on changing reconstruction on #905 if you want to pull this PR in.

jayantmukho · 2020-03-25T18:17:23Z

Common/src/linear_algebra/CSysSolve.cpp

  if (!smooth_ready) {
+    SU2_OMP_BARRIER
    SU2_OMP_MASTER
    {
      auto nVar = b.GetNVar();


Oh! I think I did see this happen a couple of times when I briefly tested the multi-threading. I built SU2 with -Dwith-omp=true

It happened when I didn't specify a number of threads with the -t option. But it ran okay if I specified even -t 1. Does this fix that? Or do you have to specify a thread count if you build with OMP?

jayantmukho · 2020-03-25T18:53:27Z

SU2_CFD/src/solvers/CTurbSolver.cpp

  const bool muscl = config->GetMUSCL_Turb();
  const bool limiter = (config->GetKind_SlopeLimit_Turb() != NO_LIMITER);

+  /*--- Only reconstruct flow variables if MUSCL is on for flow (requires upwind) and turbulence. ---*/
+  const bool musclFlow = config->GetMUSCL_Flow() && muscl &&
+                        (config->GetKind_ConvNumScheme_Flow() == SPACE_UPWIND);
+  /*--- Only consider flow limiters for cell-based limiters, edge-based would need to be recomputed. ---*/
+  const bool limiterFlow = (config->GetKind_SlopeLimit_Flow() != NO_LIMITER) &&
+                           (config->GetKind_SlopeLimit_Flow() != VAN_ALBADA_EDGE);


Seems like you have revisited some of the logic here already?

I haven't looked at the reconstructions before. Before these changes the MUSCL reconstruction for all (flow and turbulent) variables was only done when MUSCL_TURB= YES (when MUSCL reconstruction was requested for the turbulence solver as well).

Here you have changed it such that the flow variables get reconstructed if MUSCL_FLOW = YES but the turbulent variables are treated as before, i.e. they get reconstructed only when MUSCL_TURB= YES.

Are you suggesting changing the reconstruction itself? Or having an alternate reconstruction when centered schemes are used? Either way, I am for it, but don't know any better reconstruction schemes. Do you have any references I could check out? CFD wiki is best I've got so far 😛

jayantmukho · 2020-03-25T18:54:43Z

config_template.cfg

+% --------------------- HYBRID PARALLEL (MPI+OpenMP) OPTIONS ---------------------%
+%
+% An advanced performance parameter for FVM solvers, a large-ish value should be best
+% when relatively few threads per MPI rank are in use (~4). However, maximum parallelism
+% is obtained with EDGE_COLORING_GROUP_SIZE=1, consider using this value only if SU2
+% warns about low coloring efficiency during preprocessing (performance is usually worse).
+% Setting the option to 0 disables coloring and a different strategy is used instead,
+% that strategy is automatically used when the coloring efficiency is less than 0.875.
+% The optimum value/strategy is case-dependent.
+EDGE_COLORING_GROUP_SIZE= 512
+%
+% Independent "threads per MPI rank" setting for LU-SGS and ILU preconditioners.
+% For problems where time is spend mostly in the solution of linear systems (e.g. elasticity,
+% very high CFL central schemes), AND, if the memory bandwidth of the machine is saturated
+% (4 or more cores per memory channel) better performance (via a reduction in linear iterations)
+% may be possible by using a smaller value than that defined by the system or in the call to
+% SU2_CFD (via the -t/--threads option).
+% The default (0) means "same number of threads as for all else".
+LINEAR_SOLVER_PREC_THREADS= 0
+%


jayantmukho · 2020-03-25T18:57:10Z

SU2_CFD/src/solvers/CTurbSolver.cpp

  const bool muscl = config->GetMUSCL_Turb();
  const bool limiter = (config->GetKind_SlopeLimit_Turb() != NO_LIMITER);

+  /*--- Only reconstruct flow variables if MUSCL is on for flow (requires upwind) and turbulence. ---*/
+  const bool musclFlow = config->GetMUSCL_Flow() && muscl &&
+                        (config->GetKind_ConvNumScheme_Flow() == SPACE_UPWIND);
+  /*--- Only consider flow limiters for cell-based limiters, edge-based would need to be recomputed. ---*/
+  const bool limiterFlow = (config->GetKind_SlopeLimit_Flow() != NO_LIMITER) &&
+                           (config->GetKind_SlopeLimit_Flow() != VAN_ALBADA_EDGE);


actually, I am surprised this didn't change more regression tests.

pcarruscag added 30 commits February 27, 2020 19:33

Merge branch 'small_blas_docs_update' into feature_hybrid_parallel_an…

b1f667a

…d_SIMD

Merge remote-tracking branch 'upstream/develop' into feature_hybrid_p…

3611c10

…arallel_and_SIMD

Merge branch 'small_blas_docs_update' into feature_hybrid_parallel_an…

e428ef0

…d_SIMD

Merge remote-tracking branch 'upstream/develop' into feature_hybrid_p…

66f4ac5

…arallel_and_SIMD

prevent restarted FGMRES from going into infinite loop when RHS is zero

148d948

fix for "old compiler" compatibility and legacy build system

4dddf6d

Merge remote-tracking branch 'upstream/develop' into feature_hybrid_p…

a4c1cf7

…arallel_and_SIMD

update old build system

9344f88

unnecessary initialization of stiffness matrix in CMeshSolver

f4e8185

fix OpenMP bug in SetMesh_Stiffness, allow upper bound on element sti…

101b52d

…ffness, cleanup unused vars

potential fix for potential cause of observed deadlock

aba265f

add dummy locks and functions to omp_structure

998777d

bad coloring fallback strategy for CFEASolver

7e25a93

use the "reducer strategy" as a fallback for when grid coloring is ba…

32ef194

…d also on the fine grid

build diagonal and transpose map in parallel

14ed268

Merge remote-tracking branch 'upstream/feature_update_elasticity_outp…

9cd52b6

…ut' into feature_hybrid_parallel_and_SIMD

use a more expressive function to round up to next multiple

d7bb841

use reducer strategy for turbulence solvers

2fc6f30

polish up the reducer strategy, avoid unnecessary resets of CSysMatrix

c88ea7a

Merge remote-tracking branch 'upstream/develop' into feature_hybrid_p…

12c12b1

…arallel_and_SIMD

small regression changes

bcf6af4

write Undivided Laplacian as point loop

19f2fe5

write Centered_Dissipation_Sensor as a point loop

13e9572

allow forcing of "reducer strategy" without warnings, fix some indent…

733aef1

…ation in CConfig.cpp

update fixedcl regression

debb952

methods to set natural colorings

b406fa3

fix FEA solver lock strategy

fb500f2

make overhead of reducer strategy due to bad coloring same as when co…

db8d771

…loring fails

fuse "JST dissipation" loops, cleanup Euler/NS preprocessing

f4dd41a

fix virtual bug in SetPrimitiveVariables

775980e

pcarruscag added 3 commits March 5, 2020 15:32

fix some indentation in CDriver

45e3cbc

Merge remote-tracking branch 'upstream/develop' into feature_hybrid_p…

82e2d6d

…arallel_and_SIMD

add hybrid options to config_template

7fb78f8

pr-triage bot added the PR: unreviewed label Mar 11, 2020

pcarruscag changed the base branch from master to develop March 11, 2020 11:08

pcarruscag added changelog:fix enhancement labels Mar 11, 2020

pcarruscag commented Mar 11, 2020

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pcarruscag changed the title ~~Hybrid parallel fallback strategies~~ Hybrid parallel coloring fallback strategies for better strong scaling and user friendliness Mar 11, 2020

pcarruscag changed the title ~~Hybrid parallel coloring fallback strategies for better strong scaling and user friendliness~~ Hybrid parallel coloring fallback strategies (better strong scaling and user friendliness) Mar 11, 2020

pcarruscag commented Mar 11, 2020

View reviewed changes

convert moving mesh part of turb solver's dual time residual to point…

2aaf89f

… loop

pcarruscag mentioned this pull request Mar 20, 2020

Multiphysics interpolation new features and improvements #914

Closed

5 tasks

revise logic for flow variable reconstruction with MUSCL turbulence

fea625e

pcarruscag commented Mar 24, 2020

View reviewed changes

talbring approved these changes Mar 25, 2020

View reviewed changes

pr-triage bot added PR: reviewed-approved and removed PR: unreviewed labels Mar 25, 2020

Merge remote-tracking branch 'upstream/develop' into hybrid_parallel_…

cdf6b2c

…fallback_strategies

jayantmukho reviewed Mar 25, 2020

View reviewed changes

update some testcases

b6361f1

pr-triage bot added PR: unreviewed and removed PR: reviewed-approved labels Mar 25, 2020

pcarruscag merged commit 61a3efd into develop Mar 26, 2020

pcarruscag deleted the hybrid_parallel_fallback_strategies branch March 26, 2020 08:26

pr-triage bot added PR: merged and removed PR: unreviewed labels Mar 26, 2020

		void CEulerSolver::CommonPreprocessing(CGeometry geometry, CSolver solver_container, CConfig config, unsigned short iMesh,
		unsigned short iRKStep, unsigned short RunTime_EqSystem, bool Output) {

		void CEulerSolver::SetUndivided_Laplacian(CGeometry geometry, CConfig config) {
		void CEulerSolver::SetUndivided_Laplacian_And_Centered_Dissipation_Sensor(CGeometry geometry, CConfig config) {

		SU2_OMP_FOR_DYN(roundUpDiv(OMP_MIN_SIZE, ColorGroupSize)*ColorGroupSize)
		SU2_OMP_FOR_DYN(nextMultiple(OMP_MIN_SIZE, color.groupSize))

		for (iNode = 0; iNode < nNodes; iNode++) {

		if (LockStrategy) omp_set_lock(&UpdateLocks[indexNode[iNode]]);

Hybrid parallel coloring fallback strategies (better strong scaling and user friendliness) #908

Hybrid parallel coloring fallback strategies (better strong scaling and user friendliness) #908

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pcarruscag commented Mar 11, 2020

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pcarruscag Mar 11, 2020 •

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