determine best checkpoint using mAP instead of val_loss

keypoint_detection/models/detector.py

@@ -159,6 +159,8 @@ def __init__(
         # this is for later reference (e.g. checkpoint loading) and consistency.
         self.save_hyperparameters(ignore=["**kwargs", "backbone"])
 
+        self._most_recent_val_mean_ap = 0.0 # used to store the most recent validation mean AP and log it in each epoch, so that checkpoint can be chosen based on this one.
+
     def forward(self, x: torch.Tensor):
         """
         x shape must be of shape (N,3,H,W)
@@ -386,6 +388,9 @@ def log_and_reset_mean_ap(self, mode: str):
         self.log(f"{mode}/meanAP", mean_ap)
         self.log(f"{mode}/meanAP/meanAP", mean_ap)
 
+        if mode== "validation":
+            self._most_recent_val_mean_ap = mean_ap
+
     def training_epoch_end(self, outputs):
         """
         Called on the end of a training epoch.
@@ -401,6 +406,7 @@ def validation_epoch_end(self, outputs):
         """
         if self.is_ap_epoch():
             self.log_and_reset_mean_ap("validation")
+        self.log("checkpointing_metrics/valmeanAP", self._most_recent_val_mean_ap)
 
     def test_epoch_end(self, outputs):
         """

keypoint_detection/tasks/train_utils.py

@@ -82,13 +82,13 @@ def create_pl_trainer(hparams: dict, wandb_logger: WandbLogger) -> Trainer:
     )
     # cf https://pytorch-lightning.readthedocs.io/en/latest/api/pytorch_lightning.loggers.wandb.html
 
-    # would be better to use mAP metric for checkpointing, but this is not calculated every epoch because it is rather expensive
-    # epoch_loss still correlates rather well though
+    # would be better to use mAP metric for checkpointing, but this is not calculated every epoch
+    # so I manually log the last known value to make the callback happy.
     # only store the best checkpoint and only the weights
     # so cannot be used to resume training but only for inference
     # saves storage though and training the detector is usually cheap enough to retrain it from scratch if you need specific weights etc.
     checkpoint_callback = ModelCheckpoint(
-        monitor="validation/epoch_loss", mode="min", save_weights_only=True, save_top_k=1
+        monitor="checkpointing_metrics/valmeanAP", mode="max", save_weights_only=True, save_top_k=1
     )
 
     trainer = pl.Trainer(**trainer_kwargs, callbacks=[early_stopping, checkpoint_callback])