Add assertion for <=2 gpus DDP

train.py

@@ -413,7 +413,7 @@ def train(hyp, tb_writer, opt, device):
     parser.add_argument('--data', type=str, default='data/coco128.yaml', help='data.yaml path')
     parser.add_argument('--hyp', type=str, default='', help='hyp.yaml path (optional)')
     parser.add_argument('--epochs', type=int, default=300)
-    parser.add_argument('--batch-size', type=int, default=16, help="batch size for all gpus.")
+    parser.add_argument('--batch-size', type=int, default=16, help="Total batch size for all gpus.")
     parser.add_argument('--img-size', nargs='+', type=int, default=[640, 640], help='train,test sizes')
     parser.add_argument('--rect', action='store_true', help='rectangular training')
     parser.add_argument('--resume', nargs='?', const='get_last', default=False,
@@ -460,6 +460,8 @@ def train(hyp, tb_writer, opt, device):
         dist.init_process_group(backend='nccl', init_method='env://')  # distributed backend
 
         opt.world_size = dist.get_world_size()
+        assert opt.world_size <= 2, \
+            "DDP mode with > 2 gpus will suffer from performance deterioration. The reason remains unknown!"
         assert opt.batch_size % opt.world_size == 0, "Batch size is not a multiple of the number of devices given!"
         opt.batch_size = opt.total_batch_size // opt.world_size
     print(opt)