Remove GPUClock Cost Function

Removing the GPUClock cost function due to the following reasons.

Incomplete Implementation:
The implementation is only added to selected kernels. The
implementation is not generalized to work with varying occupancy
of different kernels, even if it were used in all kernels. The
implementation is verbose.

Unused:
Our host-side timer implementation was in the last years extended
to synchronize kernels at minimal overhead cost. This and heuristic
is actually used.

Research scope shifted:
In the last years, we realized that we do not need more precise
scalar cost functions, but instead vector cost functions to build
better load balance performance models from.

Costly when used:
The implementation uses an atomic add of each kernel, instead
of, e.g., just using one per warp. This adds severe memory bandwidth
strain.

Costly, even if not used:
The implementation adds about 4 registers unnecessary to all
instrumented GPU kernels once compiled in (by default).

CMakeLists.txt

@@ -69,10 +69,6 @@ include(CMakeDependentOption)
 option(WarpX_APP           "Build the WarpX executable application"     ON)
 option(WarpX_ASCENT        "Ascent in situ diagnostics"                 OFF)
 option(WarpX_EB            "Embedded boundary support"                  OFF)
-cmake_dependent_option(WarpX_GPUCLOCK
-                           "Add GPU kernel timers (cost function)"      ON
-                           "WarpX_COMPUTE STREQUAL CUDA OR WarpX_COMPUTE STREQUAL HIP"
-                                                                        OFF)
 option(WarpX_LIB           "Build WarpX as a library"                   OFF)
 option(WarpX_MPI           "Multi-node support (message-passing)"       ON)
 option(WarpX_OPENPMD       "openPMD I/O (HDF5, ADIOS)"                  ON)
@@ -510,10 +506,6 @@ foreach(D IN LISTS WarpX_DIMS)
         target_compile_definitions(ablastr_${SD} PUBLIC WARPX_DIM_RZ WARPX_ZINDEX=1)
     endif()
 
-    if(WarpX_GPUCLOCK)
-        target_compile_definitions(ablastr_${SD} PUBLIC WARPX_USE_GPUCLOCK)
-    endif()
-
     if(WarpX_OPENPMD)
         target_compile_definitions(ablastr_${SD} PUBLIC WARPX_USE_OPENPMD)
     endif()

Docs/source/developers/testing.rst

@@ -90,7 +90,7 @@ The content of this directory will look like the following (possibly including b
     $ ls ./test_dir/rt-WarpX/WarpX-tests/2021-04-30/pml_x_yee/
     analysis_pml_yee.py     # Python analysis script
     inputs_2d               # input file
-    main2d.gnu.TEST.TPROF.MTMPI.OMP.QED.GPUCLOCK.ex  # executable
+    main2d.gnu.TEST.TPROF.MTMPI.OMP.QED.ex  # executable
     pml_x_yee.analysis.out  # Python analysis output
     pml_x_yee.err.out       # error output
     pml_x_yee.make.out      # build output

Docs/source/install/cmake.rst

@@ -89,7 +89,6 @@ CMake Option                  Default & Values                             Descr
 ``WarpX_COMPUTE``             NOACC/**OMP**/CUDA/SYCL/HIP                  On-node, accelerated computing backend
 ``WarpX_DIMS``                **3**/2/1/RZ                                 Simulation dimensionality. Use ``"1;2;RZ;3"`` for all.
 ``WarpX_EB``                  ON/**OFF**                                   Embedded boundary support (not supported in RZ yet)
-``WarpX_GPUCLOCK``            **ON**/OFF                                   Add GPU kernel timers (cost function, +4 registers/kernel)
 ``WarpX_IPO``                 ON/**OFF**                                   Compile WarpX with interprocedural optimization (aka LTO)
 ``WarpX_LIB``                 ON/**OFF**                                   Build WarpX as a library, e.g., for PICMI Python
 ``WarpX_MPI``                 **ON**/OFF                                   Multi-node support (message-passing)

Docs/source/usage/parameters.rst

@@ -557,7 +557,7 @@ Distribution across MPI ranks and parallelization
     For example, if there are 4 boxes per rank and `load_balance_knapsack_factor=2`,
     no more than 8 boxes can be assigned to any rank.
 
-* ``algo.load_balance_costs_update`` (`heuristic` or `timers` or `gpuclock`) optional (default `timers`)
+* ``algo.load_balance_costs_update`` (``heuristic`` or ``timers``) optional (default ``timers``)
     If this is `heuristic`: load balance costs are updated according to a measure of
     particles and cells assigned to each box of the domain.  The cost :math:`c` is
     computed as
@@ -574,10 +574,6 @@ Distribution across MPI ranks and parallelization
 
     If this is `timers`: costs are updated according to in-code timers.
 
-    If this is `gpuclock`: [**requires to compile with option** ``-DWarpX_GPUCLOCK=ON``]
-    costs are measured as (max-over-threads) time spent in current deposition
-    routine (only applies when running on GPUs).
-
 * ``algo.costs_heuristic_particles_wt`` (`float`) optional
     Particle weight factor used in `Heuristic` strategy for costs update; if running on GPU,
     the particle weight is set to a value determined from single-GPU tests on Summit,

GNUmakefile

@@ -29,7 +29,6 @@ USE_GPU   = FALSE
 
 EBASE     = main
 
-USE_GPUCLOCK = TRUE
 USE_PYTHON_MAIN = FALSE
 
 USE_SENSEI_INSITU = FALSE

Python/pywarpx/picmi.py

@@ -1825,7 +1825,7 @@ class Simulation(picmistandard.PICMI_Simulation):
     warpx_load_balance_knapsack_factor: float, default=1.24
         (See documentation)
 
-    warpx_load_balance_costs_update: {'heuristic' or 'timers' or 'gpuclock'}, optional
+    warpx_load_balance_costs_update: {'heuristic' or 'timers'}, optional
         (See documentation)
 
     warpx_costs_heuristic_particles_wt: float, optional

Source/Make.WarpX

@@ -214,11 +214,6 @@ ifeq ($(USE_HDF5),TRUE)
   DEFINES += -DWARPX_USE_HDF5
 endif
 
-ifeq ($(USE_GPUCLOCK),TRUE)
-  USERSuffix := $(USERSuffix).GPUCLOCK
-  DEFINES += -DWARPX_USE_GPUCLOCK
-endif
-
 # job_info support
 CEXE_sources += AMReX_buildInfo.cpp
 INCLUDE_LOCATIONS += $(AMREX_HOME)/Tools/C_scripts

Source/Particles/Deposition/ChargeDeposition.H

@@ -33,8 +33,6 @@
  * \param lo           Index lower bounds of domain.
  * \param q            species charge.
  * \param n_rz_azimuthal_modes Number of azimuthal modes when using RZ geometry.
- * \param cost: Pointer to (load balancing) cost corresponding to box where present particles deposit current.
- * \param load_balance_costs_update_algo Selected method for updating load balance costs.
  */
 template <int depos_order>
 void doChargeDepositionShapeN (const GetParticlePosition<PIdx>& GetPosition,
@@ -46,16 +44,10 @@ void doChargeDepositionShapeN (const GetParticlePosition<PIdx>& GetPosition,
                                const std::array<amrex::Real, 3> xyzmin,
                                amrex::Dim3 lo,
                                amrex::Real q,
-                               int n_rz_azimuthal_modes,
-                               amrex::Real* cost,
-                               long load_balance_costs_update_algo)
+                               int n_rz_azimuthal_modes)
 {
     using namespace amrex;
 
-#if !defined(AMREX_USE_GPU)
-    amrex::ignore_unused(cost, load_balance_costs_update_algo);
-#endif
-
     // Whether ion_lev is a null pointer (do_ionization=0) or a real pointer
     // (do_ionization=1)
     const bool do_ionization = ion_lev;
@@ -87,21 +79,9 @@ void doChargeDepositionShapeN (const GetParticlePosition<PIdx>& GetPosition,
     constexpr int CELL = amrex::IndexType::CELL;
 
     // Loop over particles and deposit into rho_fab
-#if defined(WARPX_USE_GPUCLOCK)
-    amrex::Real* cost_real = nullptr;
-    if( load_balance_costs_update_algo == LoadBalanceCostsUpdateAlgo::GpuClock) {
-        cost_real = (amrex::Real *) amrex::The_Managed_Arena()->alloc(sizeof(amrex::Real));
-        *cost_real = 0.;
-    }
-#endif
     amrex::ParallelFor(
             np_to_deposit,
             [=] AMREX_GPU_DEVICE (long ip) {
-#if defined(WARPX_USE_GPUCLOCK)
-            const auto KernelTimer = ablastr::parallelization::KernelTimer(
-                cost && (load_balance_costs_update_algo == LoadBalanceCostsUpdateAlgo::GpuClock),
-                cost_real);
-#endif
             // --- Get particle quantities
             amrex::Real wq = q*wp[ip]*invvol;
             if (do_ionization){
@@ -202,13 +182,6 @@ void doChargeDepositionShapeN (const GetParticlePosition<PIdx>& GetPosition,
 #endif
         }
         );
-#if defined(WARPX_USE_GPUCLOCK)
-    if (cost && load_balance_costs_update_algo == LoadBalanceCostsUpdateAlgo::GpuClock) {
-        amrex::Gpu::streamSynchronize();
-        *cost += *cost_real;
-        amrex::The_Managed_Arena()->free(cost_real);
-    }
-#endif
 
 #ifndef WARPX_DIM_RZ
     amrex::ignore_unused(n_rz_azimuthal_modes);
@@ -299,14 +272,6 @@ void doChargeDepositionSharedShapeN (const GetParticlePosition<PIdx>& GetPositio
     constexpr int CELL = amrex::IndexType::CELL;
 
     // Loop over particles and deposit into rho_fab
-#if defined(WARPX_USE_GPUCLOCK)
-    amrex::Real* cost_real = nullptr;
-    if( load_balance_costs_update_algo == LoadBalanceCostsUpdateAlgo::GpuClock) {
-        cost_real = (amrex::Real *) amrex::The_Managed_Arena()->alloc(sizeof(amrex::Real));
-        *cost_real = 0.;
-    }
-#endif
-
 #if defined(AMREX_USE_CUDA) || defined(AMREX_USE_HIP)
     const auto dxiarr = geom.InvCellSizeArray();
     const auto plo = geom.ProbLoArray();
@@ -394,11 +359,6 @@ void doChargeDepositionSharedShapeN (const GetParticlePosition<PIdx>& GetPositio
         {
             const unsigned int ip = permutation[ip_orig];
 
-#if defined(WARPX_USE_GPUCLOCK)
-            const auto KernelTimer = ablastr::parallelization::KernelTimer(
-                cost && (load_balance_costs_update_algo == LoadBalanceCostsUpdateAlgo::GpuClock),
-                cost_real);
-#endif
             // --- Get particle quantities
             amrex::Real wq = q*wp[ip]*invvol;
             if (do_ionization){
@@ -506,13 +466,6 @@ void doChargeDepositionSharedShapeN (const GetParticlePosition<PIdx>& GetPositio
 #endif // defined(AMREX_USE_CUDA) || defined(AMREX_USE_HIP)
       }
       );
-#if defined(WARPX_USE_GPUCLOCK)
-        if(cost && load_balance_costs_update_algo == LoadBalanceCostsUpdateAlgo::GpuClock) {
-            amrex::Gpu::streamSynchronize();
-            *cost += *cost_real;
-            amrex::The_Managed_Arena()->free(cost_real);
-        }
-#endif
 
 #ifndef WARPX_DIM_RZ
         amrex::ignore_unused(n_rz_azimuthal_modes);