libgo/runtime/parfor.c

   1 // Copyright 2012 The Go Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 // Parallel for algorithm.
   6
   7 #include "runtime.h"
   8 #include "arch.h"
   9
  10 struct ParForThread
  11 {
  12         // the thread's iteration space [32lsb, 32msb)
  13         uint64 pos;
  14         // stats
  15         uint64 nsteal;
  16         uint64 nstealcnt;
  17         uint64 nprocyield;
  18         uint64 nosyield;
  19         uint64 nsleep;
  20         byte pad[CacheLineSize];
  21 };
  22
  23 ParFor*
  24 runtime_parforalloc(uint32 nthrmax)
  25 {
  26         ParFor *desc;
  27
  28         // The ParFor object is followed by CacheLineSize padding
  29         // and then nthrmax ParForThread.
  30         desc = (ParFor*)runtime_malloc(sizeof(ParFor) + CacheLineSize + nthrmax * sizeof(ParForThread));
  31         desc->thr = (ParForThread*)((byte*)(desc+1) + CacheLineSize);
  32         desc->nthrmax = nthrmax;
  33         return desc;
  34 }
  35
  36 // For testing from Go
  37 // func parforalloc2(nthrmax uint32) *ParFor
  38
  39 ParFor *runtime_parforalloc2(uint32)
  40    asm("runtime.parforalloc2");
  41
  42 ParFor *
  43 runtime_parforalloc2(uint32 nthrmax)
  44 {
  45         return runtime_parforalloc(nthrmax);
  46 }
  47
  48 void
  49 runtime_parforsetup(ParFor *desc, uint32 nthr, uint32 n, void *ctx, bool wait, void (*body)(ParFor*, uint32))
  50 {
  51         uint32 i, begin, end;
  52
  53         if(desc == nil || nthr == 0 || nthr > desc->nthrmax || body == nil) {
  54                 runtime_printf("desc=%p nthr=%d count=%d body=%p\n", desc, nthr, n, body);
  55                 runtime_throw("parfor: invalid args");
  56         }
  57
  58         desc->body = body;
  59         desc->done = 0;
  60         desc->nthr = nthr;
  61         desc->thrseq = 0;
  62         desc->cnt = n;
  63         desc->ctx = ctx;
  64         desc->wait = wait;
  65         desc->nsteal = 0;
  66         desc->nstealcnt = 0;
  67         desc->nprocyield = 0;
  68         desc->nosyield = 0;
  69         desc->nsleep = 0;
  70         for(i=0; i<nthr; i++) {
  71                 begin = (uint64)n*i / nthr;
  72                 end = (uint64)n*(i+1) / nthr;
  73                 desc->thr[i].pos = (uint64)begin | (((uint64)end)<<32);
  74         }
  75 }
  76
  77 // For testing from Go
  78 // func parforsetup2(desc *ParFor, nthr, n uint32, ctx *byte, wait bool, body func(*ParFor, uint32))
  79
  80 void runtime_parforsetup2(ParFor *, uint32, uint32, void *, bool, void *)
  81   asm("runtime.parforsetup2");
  82
  83 void
  84 runtime_parforsetup2(ParFor *desc, uint32 nthr, uint32 n, void *ctx, bool wait, void *body)
  85 {
  86         runtime_parforsetup(desc, nthr, n, ctx, wait, (void(*)(ParFor*, uint32))body);
  87 }
  88
  89 void
  90 runtime_parfordo(ParFor *desc)
  91 {
  92         ParForThread *me;
  93         uint32 tid, begin, end, begin2, try, victim, i;
  94         uint64 *mypos, *victimpos, pos, newpos;
  95         void (*body)(ParFor*, uint32);
  96         bool idle;
  97
  98         // Obtain 0-based thread index.
  99         tid = runtime_xadd(&desc->thrseq, 1) - 1;
 100         if(tid >= desc->nthr) {
 101                 runtime_printf("tid=%d nthr=%d\n", tid, desc->nthr);
 102                 runtime_throw("parfor: invalid tid");
 103         }
 104
 105         // If single-threaded, just execute the for serially.
 106         if(desc->nthr==1) {
 107                 for(i=0; i<desc->cnt; i++)
 108                         desc->body(desc, i);
 109                 return;
 110         }
 111
 112         body = desc->body;
 113         me = &desc->thr[tid];
 114         mypos = &me->pos;
 115         for(;;) {
 116                 for(;;) {
 117                         // While there is local work,
 118                         // bump low index and execute the iteration.
 119                         pos = runtime_xadd64(mypos, 1);
 120                         begin = (uint32)pos-1;
 121                         end = (uint32)(pos>>32);
 122                         if(begin < end) {
 123                                 body(desc, begin);
 124                                 continue;
 125                         }
 126                         break;
 127                 }
 128
 129                 // Out of work, need to steal something.
 130                 idle = false;
 131                 for(try=0;; try++) {
 132                         // If we don't see any work for long enough,
 133                         // increment the done counter...
 134                         if(try > desc->nthr*4 && !idle) {
 135                                 idle = true;
 136                                 runtime_xadd(&desc->done, 1);
 137                         }
 138                         // ...if all threads have incremented the counter,
 139                         // we are done.
 140                         if(desc->done + !idle == desc->nthr) {
 141                                 if(!idle)
 142                                         runtime_xadd(&desc->done, 1);
 143                                 goto exit;
 144                         }
 145                         // Choose a random victim for stealing.
 146                         victim = runtime_fastrand1() % (desc->nthr-1);
 147                         if(victim >= tid)
 148                                 victim++;
 149                         victimpos = &desc->thr[victim].pos;
 150                         pos = runtime_atomicload64(victimpos);
 151                         for(;;) {
 152                                 // See if it has any work.
 153                                 begin = (uint32)pos;
 154                                 end = (uint32)(pos>>32);
 155                                 if(begin >= end-1) {
 156                                         begin = end = 0;
 157                                         break;
 158                                 }
 159                                 if(idle) {
 160                                         runtime_xadd(&desc->done, -1);
 161                                         idle = false;
 162                                 }
 163                                 begin2 = begin + (end-begin)/2;
 164                                 newpos = (uint64)begin | (uint64)begin2<<32;
 165                                 if(runtime_cas64(victimpos, &pos, newpos)) {
 166                                         begin = begin2;
 167                                         break;
 168                                 }
 169                         }
 170                         if(begin < end) {
 171                                 // Has successfully stolen some work.
 172                                 if(idle)
 173                                         runtime_throw("parfor: should not be idle");
 174                                 runtime_atomicstore64(mypos, (uint64)begin | (uint64)end<<32);
 175                                 me->nsteal++;
 176                                 me->nstealcnt += end-begin;
 177                                 break;
 178                         }
 179                         // Backoff.
 180                         if(try < desc->nthr) {
 181                                 // nothing
 182                         } else if (try < 4*desc->nthr) {
 183                                 me->nprocyield++;
 184                                 runtime_procyield(20);
 185                         // If a caller asked not to wait for the others, exit now
 186                         // (assume that most work is already done at this point).
 187                         } else if (!desc->wait) {
 188                                 if(!idle)
 189                                         runtime_xadd(&desc->done, 1);
 190                                 goto exit;
 191                         } else if (try < 6*desc->nthr) {
 192                                 me->nosyield++;
 193                                 runtime_osyield();
 194                         } else {
 195                                 me->nsleep++;
 196                                 runtime_usleep(1);
 197                         }
 198                 }
 199         }
 200 exit:
 201         runtime_xadd64(&desc->nsteal, me->nsteal);
 202         runtime_xadd64(&desc->nstealcnt, me->nstealcnt);
 203         runtime_xadd64(&desc->nprocyield, me->nprocyield);
 204         runtime_xadd64(&desc->nosyield, me->nosyield);
 205         runtime_xadd64(&desc->nsleep, me->nsleep);
 206         me->nsteal = 0;
 207         me->nstealcnt = 0;
 208         me->nprocyield = 0;
 209         me->nosyield = 0;
 210         me->nsleep = 0;
 211 }
 212
 213 // For testing from Go
 214 // func parforiters(desc *ParFor, tid uintptr) (uintptr, uintptr)
 215
 216 struct parforiters_ret {
 217   uintptr start;
 218   uintptr end;
 219 };
 220
 221 struct parforiters_ret runtime_parforiters(ParFor *, uintptr)
 222   asm("runtime.parforiters");
 223
 224 struct parforiters_ret
 225 runtime_parforiters(ParFor *desc, uintptr tid)
 226 {
 227         struct parforiters_ret ret;
 228
 229         ret.start = (uint32)desc->thr[tid].pos;
 230         ret.end = (uint32)(desc->thr[tid].pos>>32);
 231         return ret;
 232 }