add warmup support back to cosine lr sched (important for mt)

fairseq/optim/lr_scheduler/cosine_lr_scheduler.py

@@ -13,16 +13,18 @@ from . import FairseqLRScheduler, register_lr_scheduler
 @register_lr_scheduler('cosine')
 class CosineSchedule(FairseqLRScheduler):
     """Assign LR based on a cyclical schedule that follows the cosine function.
-
     See https://arxiv.org/pdf/1608.03983.pdf for details
-
+    We also support a warmup phase where we linearly increase the learning rate
+    from some initial learning rate (`--warmup-init-lr`) until the configured
+    learning rate (`--lr`).
+    During warmup:
+      lrs = torch.linspace(args.warmup_init_lr, args.lr, args.warmup_updates)
+      lr = lrs[update_num]
+    After warmup:
       lr = lr_min + 0.5*(lr_max - lr_min)*(1 + cos(t_curr / t_i))
-
     where
-
       t_curr is current percentage of updates within the current period range
       t_i is the current period range, which is scaled by t_mul after every iteration
-
     """
 
     def __init__(self, args, optimizer):
@@ -33,6 +35,10 @@ class CosineSchedule(FairseqLRScheduler):
                 ' Consider --lr-scheduler=fixed instead.'
             )
 
+        warmup_end_lr = args.max_lr
+        if args.warmup_init_lr < 0:
+            args.warmup_init_lr = args.lr[0]
+
         self.min_lr =  args.lr[0]
         self.max_lr = args.max_lr
 
@@ -40,15 +46,27 @@ class CosineSchedule(FairseqLRScheduler):
 
         self.t_mult = args.t_mult
         self.period = args.lr_period_updates
+
+        if args.warmup_updates > 0:
+            # linearly warmup for the first args.warmup_updates
+            self.lr_step = (warmup_end_lr - args.warmup_init_lr) / args.warmup_updates
+        else:
+            self.lr_step = 1
+
+        self.warmup_updates = args.warmup_updates
         self.lr_shrink = args.lr_shrink
 
         # initial learning rate
-        self.lr = self.max_lr
+        self.lr = args.warmup_init_lr
         self.optimizer.set_lr(self.lr)
 
     @staticmethod
     def add_args(parser):
         """Add arguments to the parser for this LR scheduler."""
+        parser.add_argument('--warmup-updates', default=0, type=int, metavar='N',
+                            help='warmup the learning rate linearly for the first N updates')
+        parser.add_argument('--warmup-init-lr', default=-1, type=float, metavar='LR',
+                            help='initial learning rate during warmup phase; default is args.lr')
         parser.add_argument('--max-lr', required=True, type=float, metavar='LR',
                             help='max learning rate, must be more than args.lr')
         parser.add_argument('--t-mult', default=1, type=float, metavar='LR',
@@ -64,12 +82,16 @@ class CosineSchedule(FairseqLRScheduler):
 
     def step_update(self, num_updates):
         """Update the learning rate after each update."""
+        if num_updates < self.args.warmup_updates:
+            self.lr = self.args.warmup_init_lr + num_updates * self.lr_step
+        else:
+            curr_updates = num_updates - self.args.warmup_updates
             if self.t_mult != 1:
-            i = math.floor(math.log(1 - num_updates / self.period * (1 - self.t_mult), self.t_mult))
+                i = math.floor(math.log(1 - curr_updates / self.period * (1 - self.t_mult), self.t_mult))
                 t_i = self.t_mult ** i * self.period
-            t_curr = num_updates - (1 - self.t_mult ** i) / (1 - self.t_mult) * self.period
+                t_curr = curr_updates - (1 - self.t_mult ** i) / (1 - self.t_mult) * self.period
             else:
-            i = math.floor(num_updates / self.period)
+                i = math.floor(curr_updates / self.period)
                 t_i = self.period
                 t_curr = num_updates - (self.period * i)