Use multi-threading in cache_labels (ultralytics#3505)

* Use multi threading in cache_labels

* PEP8 reformat

* Add num_threads

* changed ThreadPool.imap_unordered to Pool.imap_unordered

* Remove inplace additions

* Update datasets.py

refactor initial desc

Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>
(cherry picked from commit 28bff22)

utils/datasets.py

@@ -9,7 +9,7 @@
 import shutil
 import time
 from itertools import repeat
-from multiprocessing.pool import ThreadPool
+from multiprocessing.pool import ThreadPool, Pool
 from pathlib import Path
 from threading import Thread
 
@@ -29,6 +29,7 @@
 help_url = 'https://github.com/ultralytics/yolov5/wiki/Train-Custom-Data'
 img_formats = ['bmp', 'jpg', 'jpeg', 'png', 'tif', 'tiff', 'dng', 'webp', 'mpo']  # acceptable image suffixes
 vid_formats = ['mov', 'avi', 'mp4', 'mpg', 'mpeg', 'm4v', 'wmv', 'mkv']  # acceptable video suffixes
+num_threads = min(8, os.cpu_count())  # number of multiprocessing threads
 logger = logging.getLogger(__name__)
 
 # Get orientation exif tag
@@ -447,7 +448,7 @@ def __init__(self, path, img_size=640, batch_size=16, augment=False, hyp=None, r
         if cache_images:
             gb = 0  # Gigabytes of cached images
             self.img_hw0, self.img_hw = [None] * n, [None] * n
-            results = ThreadPool(8).imap(lambda x: load_image(*x), zip(repeat(self), range(n)))  # 8 threads
+            results = ThreadPool(num_threads).imap(lambda x: load_image(*x), zip(repeat(self), range(n)))
             pbar = tqdm(enumerate(results), total=n)
             for i, x in pbar:
                 self.imgs[i], self.img_hw0[i], self.img_hw[i] = x  # img, hw_original, hw_resized = load_image(self, i)
@@ -458,53 +459,24 @@ def __init__(self, path, img_size=640, batch_size=16, augment=False, hyp=None, r
     def cache_labels(self, path=Path('./labels.cache'), prefix=''):
         # Cache dataset labels, check images and read shapes
         x = {}  # dict
-        nm, nf, ne, nc = 0, 0, 0, 0  # number missing, found, empty, duplicate
-        pbar = tqdm(zip(self.img_files, self.label_files), desc='Scanning images', total=len(self.img_files))
-        for i, (im_file, lb_file) in enumerate(pbar):
-            try:
-                # verify images
-                im = Image.open(im_file)
-                im.verify()  # PIL verify
-                shape = exif_size(im)  # image size
-                segments = []  # instance segments
-                assert (shape[0] > 9) & (shape[1] > 9), f'image size {shape} <10 pixels'
-                assert im.format.lower() in img_formats, f'invalid image format {im.format}'
-
-                # verify labels
-                if os.path.isfile(lb_file):
-                    nf += 1  # label found
-                    with open(lb_file, 'r') as f:
-                        l = [x.split() for x in f.read().strip().splitlines() if len(x)]
-                        if any([len(x) > 8 for x in l]):  # is segment
-                            classes = np.array([x[0] for x in l], dtype=np.float32)
-                            segments = [np.array(x[1:], dtype=np.float32).reshape(-1, 2) for x in l]  # (cls, xy1...)
-                            l = np.concatenate((classes.reshape(-1, 1), segments2boxes(segments)), 1)  # (cls, xywh)
-                        l = np.array(l, dtype=np.float32)
-                    if len(l):
-                        assert l.shape[1] == 5, 'labels require 5 columns each'
-                        assert (l >= 0).all(), 'negative labels'
-                        assert (l[:, 1:] <= 1).all(), 'non-normalized or out of bounds coordinate labels'
-                        assert np.unique(l, axis=0).shape[0] == l.shape[0], 'duplicate labels'
-                    else:
-                        ne += 1  # label empty
-                        l = np.zeros((0, 5), dtype=np.float32)
-                else:
-                    nm += 1  # label missing
-                    l = np.zeros((0, 5), dtype=np.float32)
-                x[im_file] = [l, shape, segments]
-            except Exception as e:
-                nc += 1
-                logging.info(f'{prefix}WARNING: Ignoring corrupted image and/or label {im_file}: {e}')
-
-            pbar.desc = f"{prefix}Scanning '{path.parent / path.stem}' images and labels... " \
-                        f"{nf} found, {nm} missing, {ne} empty, {nc} corrupted"
+        nm, nf, ne, nc = 0, 0, 0, 0  # number missing, found, empty, corrupt
+        desc = f"{prefix}Scanning '{path.parent / path.stem}' images and labels..."
+        with Pool(num_threads) as pool:
+            pbar = tqdm(pool.imap_unordered(verify_image_label,
+                                            zip(self.img_files, self.label_files, repeat(prefix))),
+                        desc=desc, total=len(self.img_files))
+            for im_file, l, shape, segments, nm_f, nf_f, ne_f, nc_f in pbar:
+                if im_file:
+                    x[im_file] = [l, shape, segments]
+                nm, nf, ne, nc = nm + nm_f, nf + nf_f, ne + ne_f, nc + nc_f
+                pbar.desc = f"{desc}{nf} found, {nm} missing, {ne} empty, {nc} corrupted"
         pbar.close()
 
         if nf == 0:
             logging.info(f'{prefix}WARNING: No labels found in {path}. See {help_url}')
 
         x['hash'] = get_hash(self.label_files + self.img_files)
-        x['results'] = nf, nm, ne, nc, i + 1
+        x['results'] = nf, nm, ne, nc, len(self.img_files)
         x['version'] = 0.2  # cache version
         try:
             torch.save(x, path)  # save cache for next time
@@ -1069,3 +1041,44 @@ def autosplit(path='../coco128', weights=(0.9, 0.1, 0.0), annotated_only=False):
         if not annotated_only or Path(img2label_paths([str(img)])[0]).exists():  # check label
             with open(path / txt[i], 'a') as f:
                 f.write(str(img) + '\n')  # add image to txt file
+
+
+def verify_image_label(params):
+    # Verify one image-label pair
+    im_file, lb_file, prefix = params
+    nm, nf, ne, nc = 0, 0, 0, 0  # number missing, found, empty, corrupt
+    try:
+        # verify images
+        im = Image.open(im_file)
+        im.verify()  # PIL verify
+        shape = exif_size(im)  # image size
+        segments = []  # instance segments
+        assert (shape[0] > 9) & (shape[1] > 9), f'image size {shape} <10 pixels'
+        assert im.format.lower() in img_formats, f'invalid image format {im.format}'
+
+        # verify labels
+        if os.path.isfile(lb_file):
+            nf = 1  # label found
+            with open(lb_file, 'r') as f:
+                l = [x.split() for x in f.read().strip().splitlines() if len(x)]
+                if any([len(x) > 8 for x in l]):  # is segment
+                    classes = np.array([x[0] for x in l], dtype=np.float32)
+                    segments = [np.array(x[1:], dtype=np.float32).reshape(-1, 2) for x in l]  # (cls, xy1...)
+                    l = np.concatenate((classes.reshape(-1, 1), segments2boxes(segments)), 1)  # (cls, xywh)
+                l = np.array(l, dtype=np.float32)
+            if len(l):
+                assert l.shape[1] == 5, 'labels require 5 columns each'
+                assert (l >= 0).all(), 'negative labels'
+                assert (l[:, 1:] <= 1).all(), 'non-normalized or out of bounds coordinate labels'
+                assert np.unique(l, axis=0).shape[0] == l.shape[0], 'duplicate labels'
+            else:
+                ne = 1  # label empty
+                l = np.zeros((0, 5), dtype=np.float32)
+        else:
+            nm = 1  # label missing
+            l = np.zeros((0, 5), dtype=np.float32)
+        return im_file, l, shape, segments, nm, nf, ne, nc
+    except Exception as e:
+        nc = 1
+        logging.info(f'{prefix}WARNING: Ignoring corrupted image and/or label {im_file}: {e}')
+        return [None] * 4 + [nm, nf, ne, nc]