Actual source code: mpishm.c

petsc-master 2020-10-26
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  1: #include <petscsys.h>
  2: #include <petsc/private/petscimpl.h>

  4: struct _n_PetscShmComm {
  5:   PetscMPIInt *globranks;       /* global ranks of each rank in the shared memory communicator */
  6:   PetscMPIInt shmsize;          /* size of the shared memory communicator */
  7:   MPI_Comm    globcomm,shmcomm; /* global communicator and shared memory communicator (a sub-communicator of the former) */
  8: };

 10: /*
 11:    Private routine to delete internal shared memory communicator when a communicator is freed.

 13:    This is called by MPI, not by users. This is called by MPI_Comm_free() when the communicator that has this  data as an attribute is freed.

 15:    Note: this is declared extern "C" because it is passed to MPI_Comm_create_keyval()

 17: */
 18: PETSC_EXTERN PetscMPIInt MPIAPI Petsc_ShmComm_Attr_Delete_Fn(MPI_Comm comm,PetscMPIInt keyval,void *val,void *extra_state)
 19: {
 20:   PetscErrorCode  ierr;
 21:   PetscShmComm p = (PetscShmComm)val;

 24:   PetscInfo1(NULL,"Deleting shared memory subcommunicator in a MPI_Comm %ld\n",(long)comm);CHKERRMPI(ierr);
 25:   MPI_Comm_free(&p->shmcomm);CHKERRMPI(ierr);
 26:   PetscFree(p->globranks);CHKERRMPI(ierr);
 27:   PetscFree(val);CHKERRMPI(ierr);
 28:   PetscFunctionReturn(MPI_SUCCESS);
 29: }

 31: /*@C
 32:     PetscShmCommGet - Given a PETSc communicator returns a communicator of all ranks that share a common memory


 35:     Collective.

 37:     Input Parameter:
 38: .   globcomm - MPI_Comm

 40:     Output Parameter:
 41: .   pshmcomm - the PETSc shared memory communicator object

 43:     Level: developer

 45:     Notes:
 46:     This should be called only with an PetscCommDuplicate() communictor

 48:            When used with MPICH, MPICH must be configured with --download-mpich-device=ch3:nemesis

 50: @*/
 51: PetscErrorCode PetscShmCommGet(MPI_Comm globcomm,PetscShmComm *pshmcomm)
 52: {
 53: #ifdef PETSC_HAVE_MPI_PROCESS_SHARED_MEMORY
 54:   PetscErrorCode   ierr;
 55:   MPI_Group        globgroup,shmgroup;
 56:   PetscMPIInt      *shmranks,i,flg;
 57:   PetscCommCounter *counter;

 60:   MPI_Comm_get_attr(globcomm,Petsc_Counter_keyval,&counter,&flg);
 61:   if (!flg) SETERRQ(globcomm,PETSC_ERR_ARG_CORRUPT,"Bad MPI communicator supplied; must be a PETSc communicator");

 63:   MPI_Comm_get_attr(globcomm,Petsc_ShmComm_keyval,pshmcomm,&flg);
 64:   if (flg) return(0);

 66:   PetscNew(pshmcomm);
 67:   (*pshmcomm)->globcomm = globcomm;

 69:   MPI_Comm_split_type(globcomm, MPI_COMM_TYPE_SHARED,0, MPI_INFO_NULL,&(*pshmcomm)->shmcomm);

 71:   MPI_Comm_size((*pshmcomm)->shmcomm,&(*pshmcomm)->shmsize);
 72:   MPI_Comm_group(globcomm, &globgroup);
 73:   MPI_Comm_group((*pshmcomm)->shmcomm, &shmgroup);
 74:   PetscMalloc1((*pshmcomm)->shmsize,&shmranks);
 75:   PetscMalloc1((*pshmcomm)->shmsize,&(*pshmcomm)->globranks);
 76:   for (i=0; i<(*pshmcomm)->shmsize; i++) shmranks[i] = i;
 77:   MPI_Group_translate_ranks(shmgroup, (*pshmcomm)->shmsize, shmranks, globgroup, (*pshmcomm)->globranks);
 78:   PetscFree(shmranks);
 79:   MPI_Group_free(&globgroup);
 80:   MPI_Group_free(&shmgroup);

 82:   for (i=0; i<(*pshmcomm)->shmsize; i++) {
 83:     PetscInfo2(NULL,"Shared memory rank %d global rank %d\n",i,(*pshmcomm)->globranks[i]);
 84:   }
 85:   MPI_Comm_set_attr(globcomm,Petsc_ShmComm_keyval,*pshmcomm);
 86:   return(0);
 87: #else
 88:   SETERRQ(globcomm, PETSC_ERR_SUP, "Shared memory communicators need MPI-3 package support.\nPlease upgrade your MPI or reconfigure with --download-mpich.");
 89: #endif
 90: }

 92: /*@C
 93:     PetscShmCommGlobalToLocal - Given a global rank returns the local rank in the shared memory communicator

 95:     Input Parameters:
 96: +   pshmcomm - the shared memory communicator object
 97: -   grank    - the global rank

 99:     Output Parameter:
100: .   lrank - the local rank, or MPI_PROC_NULL if it does not exist

102:     Level: developer

104:     Developer Notes:
105:     Assumes the pshmcomm->globranks[] is sorted

107:     It may be better to rewrite this to map multiple global ranks to local in the same function call

109: @*/
110: PetscErrorCode PetscShmCommGlobalToLocal(PetscShmComm pshmcomm,PetscMPIInt grank,PetscMPIInt *lrank)
111: {
112:   PetscMPIInt    low,high,t,i;
113:   PetscBool      flg = PETSC_FALSE;

117:   *lrank = MPI_PROC_NULL;
118:   if (grank < pshmcomm->globranks[0]) return(0);
119:   if (grank > pshmcomm->globranks[pshmcomm->shmsize-1]) return(0);
120:   PetscOptionsGetBool(NULL,NULL,"-noshared",&flg,NULL);
121:   if (flg) return(0);
122:   low  = 0;
123:   high = pshmcomm->shmsize;
124:   while (high-low > 5) {
125:     t = (low+high)/2;
126:     if (pshmcomm->globranks[t] > grank) high = t;
127:     else low = t;
128:   }
129:   for (i=low; i<high; i++) {
130:     if (pshmcomm->globranks[i] > grank) return(0);
131:     if (pshmcomm->globranks[i] == grank) {
132:       *lrank = i;
133:       return(0);
134:     }
135:   }
136:   return(0);
137: }

139: /*@C
140:     PetscShmCommLocalToGlobal - Given a local rank in the shared memory communicator returns the global rank

142:     Input Parameters:
143: +   pshmcomm - the shared memory communicator object
144: -   lrank    - the local rank in the shared memory communicator

146:     Output Parameter:
147: .   grank - the global rank in the global communicator where the shared memory communicator is built

149:     Level: developer

151: @*/
152: PetscErrorCode PetscShmCommLocalToGlobal(PetscShmComm pshmcomm,PetscMPIInt lrank,PetscMPIInt *grank)
153: {
155:   if (lrank < 0 || lrank >= pshmcomm->shmsize) SETERRQ1(PETSC_COMM_SELF,PETSC_ERR_ARG_OUTOFRANGE,"No rank %D in the shared memory communicator",lrank);
156:   *grank = pshmcomm->globranks[lrank];
157:   return(0);
158: }

160: /*@C
161:     PetscShmCommGetMpiShmComm - Returns the MPI communicator that represents all processes with common shared memory

163:     Input Parameter:
164: .   pshmcomm - PetscShmComm object obtained with PetscShmCommGet()

166:     Output Parameter:
167: .   comm     - the MPI communicator

169:     Level: developer

171: @*/
172: PetscErrorCode PetscShmCommGetMpiShmComm(PetscShmComm pshmcomm,MPI_Comm *comm)
173: {
175:   *comm = pshmcomm->shmcomm;
176:   return(0);
177: }

179: #if defined(PETSC_HAVE_OPENMP_SUPPORT)
180: #include <pthread.h>
181: #include <hwloc.h>
182: #include <omp.h>

184: /* Use mmap() to allocate shared mmeory (for the pthread_barrier_t object) if it is available,
185:    otherwise use MPI_Win_allocate_shared. They should have the same effect except MPI-3 is much
186:    simpler to use. However, on a Cori Haswell node with Cray MPI, MPI-3 worsened a test's performance
187:    by 50%. Until the reason is found out, we use mmap() instead.
188: */
189: #define USE_MMAP_ALLOCATE_SHARED_MEMORY

191: #if defined(USE_MMAP_ALLOCATE_SHARED_MEMORY) && defined(PETSC_HAVE_MMAP)
192: #include <sys/mman.h>
193: #include <sys/types.h>
194: #include <sys/stat.h>
195: #include <fcntl.h>
196: #endif

198: struct _n_PetscOmpCtrl {
199:   MPI_Comm          omp_comm;        /* a shared memory communicator to spawn omp threads */
200:   MPI_Comm          omp_master_comm; /* a communicator to give to third party libraries */
201:   PetscMPIInt       omp_comm_size;   /* size of omp_comm, a kind of OMP_NUM_THREADS */
202:   PetscBool         is_omp_master;   /* rank 0's in omp_comm */
203:   MPI_Win           omp_win;         /* a shared memory window containing a barrier */
204:   pthread_barrier_t *barrier;        /* pointer to the barrier */
205:   hwloc_topology_t  topology;
206:   hwloc_cpuset_t    cpuset;          /* cpu bindings of omp master */
207:   hwloc_cpuset_t    omp_cpuset;      /* union of cpu bindings of ranks in omp_comm */
208: };


211: /* Allocate and initialize a pthread_barrier_t object in memory shared by processes in omp_comm
212:    contained by the controller.

214:    PETSc OpenMP controller users do not call this function directly. This function exists
215:    only because we want to separate shared memory allocation methods from other code.
216:  */
217: PETSC_STATIC_INLINE PetscErrorCode PetscOmpCtrlCreateBarrier(PetscOmpCtrl ctrl)
218: {
219:   PetscErrorCode        ierr;
220:   MPI_Aint              size;
221:   void                  *baseptr;
222:   pthread_barrierattr_t  attr;

224: #if defined(USE_MMAP_ALLOCATE_SHARED_MEMORY) && defined(PETSC_HAVE_MMAP)
225:   PetscInt              fd;
226:   PetscChar             pathname[PETSC_MAX_PATH_LEN];
227: #else
228:   PetscMPIInt           disp_unit;
229: #endif

232: #if defined(USE_MMAP_ALLOCATE_SHARED_MEMORY) && defined(PETSC_HAVE_MMAP)
233:   size = sizeof(pthread_barrier_t);
234:   if (ctrl->is_omp_master) {
235:     /* use PETSC_COMM_SELF in PetscGetTmp, since it is a collective call. Using omp_comm would otherwise bcast the partially populated pathname to slaves */
236:     PetscGetTmp(PETSC_COMM_SELF,pathname,PETSC_MAX_PATH_LEN);
237:     PetscStrlcat(pathname,"/petsc-shm-XXXXXX",PETSC_MAX_PATH_LEN);
238:     /* mkstemp replaces XXXXXX with a unique file name and opens the file for us */
239:     fd      = mkstemp(pathname); if (fd == -1) SETERRQ1(PETSC_COMM_SELF,PETSC_ERR_LIB,"Could not create tmp file %s with mkstemp\n", pathname);
240:     ftruncate(fd,size);
241:     baseptr = mmap(NULL,size,PROT_READ | PROT_WRITE, MAP_SHARED,fd,0); if (baseptr == MAP_FAILED) SETERRQ(PETSC_COMM_SELF,PETSC_ERR_LIB,"mmap() failed\n");
242:     close(fd);
243:     MPI_Bcast(pathname,PETSC_MAX_PATH_LEN,MPI_CHAR,0,ctrl->omp_comm);
244:     /* this MPI_Barrier is to wait slaves to open the file before master unlinks it */
245:     MPI_Barrier(ctrl->omp_comm);
246:     unlink(pathname);
247:   } else {
248:     MPI_Bcast(pathname,PETSC_MAX_PATH_LEN,MPI_CHAR,0,ctrl->omp_comm);
249:     fd      = open(pathname,O_RDWR); if (fd == -1) SETERRQ1(PETSC_COMM_SELF,PETSC_ERR_LIB,"Could not open tmp file %s\n", pathname);
250:     baseptr = mmap(NULL,size,PROT_READ | PROT_WRITE, MAP_SHARED,fd,0); if (baseptr == MAP_FAILED) SETERRQ(PETSC_COMM_SELF,PETSC_ERR_LIB,"mmap() failed\n");
251:     close(fd);
252:     MPI_Barrier(ctrl->omp_comm);
253:   }
254: #else
255:   size = ctrl->is_omp_master ? sizeof(pthread_barrier_t) : 0;
256:   MPI_Win_allocate_shared(size,1,MPI_INFO_NULL,ctrl->omp_comm,&baseptr,&ctrl->omp_win);
257:   MPI_Win_shared_query(ctrl->omp_win,0,&size,&disp_unit,&baseptr);
258: #endif
259:   ctrl->barrier = (pthread_barrier_t*)baseptr;

261:   /* omp master initializes the barrier */
262:   if (ctrl->is_omp_master) {
263:     MPI_Comm_size(ctrl->omp_comm,&ctrl->omp_comm_size);
264:     pthread_barrierattr_init(&attr);
265:     pthread_barrierattr_setpshared(&attr,PTHREAD_PROCESS_SHARED); /* make the barrier also work for processes */
266:     pthread_barrier_init(ctrl->barrier,&attr,(unsigned int)ctrl->omp_comm_size);
267:     pthread_barrierattr_destroy(&attr);
268:   }

270:   /* this MPI_Barrier is to make sure the omp barrier is initialized before slaves use it */
271:   MPI_Barrier(ctrl->omp_comm);
272:   return(0);
273: }

275: /* Destroy the pthread barrier in the PETSc OpenMP controller */
276: PETSC_STATIC_INLINE PetscErrorCode PetscOmpCtrlDestroyBarrier(PetscOmpCtrl ctrl)
277: {

281:   /* this MPI_Barrier is to make sure slaves have finished using the omp barrier before master destroys it */
282:   MPI_Barrier(ctrl->omp_comm);
283:   if (ctrl->is_omp_master) { pthread_barrier_destroy(ctrl->barrier); }

285: #if defined(USE_MMAP_ALLOCATE_SHARED_MEMORY) && defined(PETSC_HAVE_MMAP)
286:   munmap(ctrl->barrier,sizeof(pthread_barrier_t));
287: #else
288:   MPI_Win_free(&ctrl->omp_win);
289: #endif
290:   return(0);
291: }

293: /*@C
294:     PetscOmpCtrlCreate - create a PETSc OpenMP controller, which manages PETSc's interaction with third party libraries using OpenMP

296:     Input Parameter:
297: +   petsc_comm - a communicator some PETSc object (for example, a matrix) lives in
298: -   nthreads   - number of threads per MPI rank to spawn in a library using OpenMP. If nthreads = -1, let PETSc decide a suitable value

300:     Output Parameter:
301: .   pctrl      - a PETSc OpenMP controller

303:     Level: developer

305:     TODO: Possibly use the variable PetscNumOMPThreads to determine the number for threads to use

307: .seealso PetscOmpCtrlDestroy()
308: @*/
309: PetscErrorCode PetscOmpCtrlCreate(MPI_Comm petsc_comm,PetscInt nthreads,PetscOmpCtrl *pctrl)
310: {
311:   PetscErrorCode        ierr;
312:   PetscOmpCtrl          ctrl;
313:   unsigned long         *cpu_ulongs=NULL;
314:   PetscInt              i,nr_cpu_ulongs;
315:   PetscShmComm          pshmcomm;
316:   MPI_Comm              shm_comm;
317:   PetscMPIInt           shm_rank,shm_comm_size,omp_rank,color;
318:   PetscInt              num_packages,num_cores;

321:   PetscNew(&ctrl);

323:   /*=================================================================================
324:     Init hwloc
325:    ==================================================================================*/
326:   hwloc_topology_init(&ctrl->topology);
327: #if HWLOC_API_VERSION >= 0x00020000
328:   /* to filter out unneeded info and have faster hwloc_topology_load */
329:   hwloc_topology_set_all_types_filter(ctrl->topology,HWLOC_TYPE_FILTER_KEEP_NONE);
330:   hwloc_topology_set_type_filter(ctrl->topology,HWLOC_OBJ_CORE,HWLOC_TYPE_FILTER_KEEP_ALL);
331: #endif
332:   hwloc_topology_load(ctrl->topology);

334:   /*=================================================================================
335:     Split petsc_comm into multiple omp_comms. Ranks in an omp_comm have access to
336:     physically shared memory. Rank 0 of each omp_comm is called an OMP master, and
337:     others are called slaves. OMP Masters make up a new comm called omp_master_comm,
338:     which is usually passed to third party libraries.
339:    ==================================================================================*/

341:   /* fetch the stored shared memory communicator */
342:   PetscShmCommGet(petsc_comm,&pshmcomm);
343:   PetscShmCommGetMpiShmComm(pshmcomm,&shm_comm);

345:   MPI_Comm_rank(shm_comm,&shm_rank);
346:   MPI_Comm_size(shm_comm,&shm_comm_size);

348:   /* PETSc decides nthreads, which is the smaller of shm_comm_size or cores per package(socket) */
349:   if (nthreads == -1) {
350:     num_packages = hwloc_get_nbobjs_by_type(ctrl->topology,HWLOC_OBJ_PACKAGE) <= 0 ? 1 : hwloc_get_nbobjs_by_type(ctrl->topology,HWLOC_OBJ_PACKAGE);
351:     num_cores    = hwloc_get_nbobjs_by_type(ctrl->topology,HWLOC_OBJ_CORE) <= 0 ? 1 :  hwloc_get_nbobjs_by_type(ctrl->topology,HWLOC_OBJ_CORE);
352:     nthreads     = num_cores/num_packages;
353:     if (nthreads > shm_comm_size) nthreads = shm_comm_size;
354:   }

356:   if (nthreads < 1 || nthreads > shm_comm_size) SETERRQ2(petsc_comm,PETSC_ERR_ARG_OUTOFRANGE,"number of OpenMP threads %D can not be < 1 or > the MPI shared memory communicator size %d\n",nthreads,shm_comm_size);
357:   if (shm_comm_size % nthreads) { PetscPrintf(petsc_comm,"Warning: number of OpenMP threads %D is not a factor of the MPI shared memory communicator size %d, which may cause load-imbalance!\n",nthreads,shm_comm_size); }

359:   /* split shm_comm into a set of omp_comms with each of size nthreads. Ex., if
360:      shm_comm_size=16, nthreads=8, then ranks 0~7 get color 0 and ranks 8~15 get
361:      color 1. They are put in two omp_comms. Note that petsc_ranks may or may not
362:      be consecutive in a shm_comm, but shm_ranks always run from 0 to shm_comm_size-1.
363:      Use 0 as key so that rank ordering wont change in new comm.
364:    */
365:   color = shm_rank / nthreads;
366:   MPI_Comm_split(shm_comm,color,0/*key*/,&ctrl->omp_comm);

368:   /* put rank 0's in omp_comms (i.e., master ranks) into a new comm - omp_master_comm */
369:   MPI_Comm_rank(ctrl->omp_comm,&omp_rank);
370:   if (!omp_rank) {
371:     ctrl->is_omp_master = PETSC_TRUE;  /* master */
372:     color = 0;
373:   } else {
374:     ctrl->is_omp_master = PETSC_FALSE; /* slave */
375:     color = MPI_UNDEFINED; /* to make slaves get omp_master_comm = MPI_COMM_NULL in MPI_Comm_split */
376:   }
377:   MPI_Comm_split(petsc_comm,color,0/*key*/,&ctrl->omp_master_comm); /* rank 0 in omp_master_comm is rank 0 in petsc_comm */

379:   /*=================================================================================
380:     Each omp_comm has a pthread_barrier_t in its shared memory, which is used to put
381:     slave ranks in sleep and idle their CPU, so that the master can fork OMP threads
382:     and run them on the idle CPUs.
383:    ==================================================================================*/
384:   PetscOmpCtrlCreateBarrier(ctrl);

386:   /*=================================================================================
387:     omp master logs its cpu binding (i.e., cpu set) and computes a new binding that
388:     is the union of the bindings of all ranks in the omp_comm
389:     =================================================================================*/

391:   ctrl->cpuset = hwloc_bitmap_alloc(); if (!ctrl->cpuset) SETERRQ(PETSC_COMM_SELF,PETSC_ERR_LIB,"hwloc_bitmap_alloc() failed\n");
392:   hwloc_get_cpubind(ctrl->topology,ctrl->cpuset, HWLOC_CPUBIND_PROCESS);

394:   /* hwloc main developer said they will add new APIs hwloc_bitmap_{nr,to,from}_ulongs in 2.1 to help us simplify the following bitmap pack/unpack code */
395:   nr_cpu_ulongs = (hwloc_bitmap_last(hwloc_topology_get_topology_cpuset (ctrl->topology))+sizeof(unsigned long)*8)/sizeof(unsigned long)/8;
396:   PetscMalloc1(nr_cpu_ulongs,&cpu_ulongs);
397:   if (nr_cpu_ulongs == 1) {
398:     cpu_ulongs[0] = hwloc_bitmap_to_ulong(ctrl->cpuset);
399:   } else {
400:     for (i=0; i<nr_cpu_ulongs; i++) cpu_ulongs[i] = hwloc_bitmap_to_ith_ulong(ctrl->cpuset,(unsigned)i);
401:   }

403:   MPI_Reduce(ctrl->is_omp_master ? MPI_IN_PLACE : cpu_ulongs, cpu_ulongs,nr_cpu_ulongs, MPI_UNSIGNED_LONG,MPI_BOR,0,ctrl->omp_comm);

405:   if (ctrl->is_omp_master) {
406:     ctrl->omp_cpuset = hwloc_bitmap_alloc(); if (!ctrl->omp_cpuset) SETERRQ(PETSC_COMM_SELF,PETSC_ERR_LIB,"hwloc_bitmap_alloc() failed\n");
407:     if (nr_cpu_ulongs == 1) {
408: #if HWLOC_API_VERSION >= 0x00020000
409:       hwloc_bitmap_from_ulong(ctrl->omp_cpuset,cpu_ulongs[0]);
410: #else
411:       hwloc_bitmap_from_ulong(ctrl->omp_cpuset,cpu_ulongs[0]);
412: #endif
413:     } else {
414:       for (i=0; i<nr_cpu_ulongs; i++)  {
415: #if HWLOC_API_VERSION >= 0x00020000
416:         hwloc_bitmap_set_ith_ulong(ctrl->omp_cpuset,(unsigned)i,cpu_ulongs[i]);
417: #else
418:         hwloc_bitmap_set_ith_ulong(ctrl->omp_cpuset,(unsigned)i,cpu_ulongs[i]);
419: #endif
420:       }
421:     }
422:   }

424:   PetscFree(cpu_ulongs);
425:   *pctrl = ctrl;
426:   return(0);
427: }

429: /*@C
430:     PetscOmpCtrlDestroy - destroy the PETSc OpenMP controller

432:     Input Parameter:
433: .   pctrl  - a PETSc OpenMP controller

435:     Level: developer

437: .seealso PetscOmpCtrlCreate()
438: @*/
439: PetscErrorCode PetscOmpCtrlDestroy(PetscOmpCtrl *pctrl)
440: {
441:   PetscErrorCode  ierr;
442:   PetscOmpCtrl    ctrl = *pctrl;

445:   hwloc_bitmap_free(ctrl->cpuset);
446:   hwloc_topology_destroy(ctrl->topology);
447:   PetscOmpCtrlDestroyBarrier(ctrl);
448:   MPI_Comm_free(&ctrl->omp_comm);
449:   if (ctrl->is_omp_master) {
450:     hwloc_bitmap_free(ctrl->omp_cpuset);
451:     MPI_Comm_free(&ctrl->omp_master_comm);
452:   }
453:   PetscFree(ctrl);
454:   return(0);
455: }

457: /*@C
458:     PetscOmpCtrlGetOmpComms - Get MPI communicators from a PETSc OMP controller

460:     Input Parameter:
461: .   ctrl - a PETSc OMP controller

463:     Output Parameter:
464: +   omp_comm         - a communicator that includes a master rank and slave ranks where master spawns threads
465: .   omp_master_comm  - on master ranks, return a communicator that include master ranks of each omp_comm;
466:                        on slave ranks, MPI_COMM_NULL will be return in reality.
467: -   is_omp_master    - true if the calling process is an OMP master rank.

469:     Notes: any output parameter can be NULL. The parameter is just ignored.

471:     Level: developer
472: @*/
473: PetscErrorCode PetscOmpCtrlGetOmpComms(PetscOmpCtrl ctrl,MPI_Comm *omp_comm,MPI_Comm *omp_master_comm,PetscBool *is_omp_master)
474: {
476:   if (omp_comm)        *omp_comm        = ctrl->omp_comm;
477:   if (omp_master_comm) *omp_master_comm = ctrl->omp_master_comm;
478:   if (is_omp_master)   *is_omp_master   = ctrl->is_omp_master;
479:   return(0);
480: }

482: /*@C
483:     PetscOmpCtrlBarrier - Do barrier on MPI ranks in omp_comm contained by the PETSc OMP controller (to let slave ranks free their CPU)

485:     Input Parameter:
486: .   ctrl - a PETSc OMP controller

488:     Notes:
489:     this is a pthread barrier on MPI processes. Using MPI_Barrier instead is conceptually correct. But MPI standard does not
490:     require processes blocked by MPI_Barrier free their CPUs to let other processes progress. In practice, to minilize latency,
491:     MPI processes stuck in MPI_Barrier keep polling and do not free CPUs. In contrast, pthread_barrier has this requirement.

493:     A code using PetscOmpCtrlBarrier() would be like this,

495:     if (is_omp_master) {
496:       PetscOmpCtrlOmpRegionOnMasterBegin(ctrl);
497:       Call the library using OpenMP
498:       PetscOmpCtrlOmpRegionOnMasterEnd(ctrl);
499:     }
500:     PetscOmpCtrlBarrier(ctrl);

502:     Level: developer

504: .seealso PetscOmpCtrlOmpRegionOnMasterBegin(), PetscOmpCtrlOmpRegionOnMasterEnd()
505: @*/
506: PetscErrorCode PetscOmpCtrlBarrier(PetscOmpCtrl ctrl)
507: {
508:   int err;

511:   err = pthread_barrier_wait(ctrl->barrier);
512:   if (err && err != PTHREAD_BARRIER_SERIAL_THREAD) SETERRQ1(PETSC_COMM_SELF,PETSC_ERR_LIB,"pthread_barrier_wait failed within PetscOmpCtrlBarrier with return code %D\n", err);
513:   return(0);
514: }

516: /*@C
517:     PetscOmpCtrlOmpRegionOnMasterBegin - Mark the beginning of an OpenMP library call on master ranks

519:     Input Parameter:
520: .   ctrl - a PETSc OMP controller

522:     Notes:
523:     Only master ranks can call this function. Call PetscOmpCtrlGetOmpComms() to know if this is a master rank.
524:     This function changes CPU binding of master ranks and nthreads-var of OpenMP runtime

526:     Level: developer

528: .seealso: PetscOmpCtrlOmpRegionOnMasterEnd()
529: @*/
530: PetscErrorCode PetscOmpCtrlOmpRegionOnMasterBegin(PetscOmpCtrl ctrl)
531: {

535:   hwloc_set_cpubind(ctrl->topology,ctrl->omp_cpuset,HWLOC_CPUBIND_PROCESS);
536:   omp_set_num_threads(ctrl->omp_comm_size); /* may override the OMP_NUM_THREAD env var */
537:   return(0);
538: }

540: /*@C
541:    PetscOmpCtrlOmpRegionOnMasterEnd - Mark the end of an OpenMP library call on master ranks

543:    Input Parameter:
544: .  ctrl - a PETSc OMP controller

546:    Notes:
547:    Only master ranks can call this function. Call PetscOmpCtrlGetOmpComms() to know if this is a master rank.
548:    This function restores the CPU binding of master ranks and set and nthreads-var of OpenMP runtime to 1.

550:    Level: developer

552: .seealso: PetscOmpCtrlOmpRegionOnMasterBegin()
553: @*/
554: PetscErrorCode PetscOmpCtrlOmpRegionOnMasterEnd(PetscOmpCtrl ctrl)
555: {

559:   hwloc_set_cpubind(ctrl->topology,ctrl->cpuset,HWLOC_CPUBIND_PROCESS);
560:   omp_set_num_threads(1);
561:   return(0);
562: }

564: #undef USE_MMAP_ALLOCATE_SHARED_MEMORY
565: #endif /* defined(PETSC_HAVE_OPENMP_SUPPORT) */