Limit the number of threads for VP8 encoding

When we switched to using PyAV for VP8 encoding we dropped the logic
which defined the number of threads to use for encoding. This results in
using as many threads as there are CPUs:

https://github.com/FFmpeg/FFmpeg/blob/b6f84cd72acd84ba725922bd6a8967b416b2230a/libavcodec/libvpxenc.c#L1014

Restore the previous behaviour.

Author: jlaine

src/aiortc/codecs/vpx.py

@@ -1,3 +1,4 @@
+import multiprocessing
 import random
 from struct import pack, unpack_from
 from typing import Optional, Type, TypeVar, cast
@@ -22,6 +23,17 @@
 DESCRIPTOR_T = TypeVar("DESCRIPTOR_T", bound="VpxPayloadDescriptor")
 
 
+def number_of_threads(pixels: int, cpus: int) -> int:
+    if pixels >= 1920 * 1080 and cpus > 8:
+        return 8
+    elif pixels > 1280 * 960 and cpus >= 6:
+        return 3
+    elif pixels > 640 * 480 and cpus >= 3:
+        return 2
+    else:
+        return 1
+
+
 class VpxPayloadDescriptor:
     def __init__(
         self,
@@ -213,6 +225,9 @@ def encode(
                 "static-thresh": "1",
                 "undershoot-pct": "100",
             }
+            self.codec.thread_count = number_of_threads(
+                frame.width * frame.height, multiprocessing.cpu_count()
+            )
 
         data_to_send = b""
         for package in self.codec.encode(frame):

tests/test_vpx.py

@@ -2,7 +2,12 @@
 from unittest import TestCase
 
 from aiortc.codecs import get_decoder, get_encoder
-from aiortc.codecs.vpx import Vp8Decoder, Vp8Encoder, VpxPayloadDescriptor
+from aiortc.codecs.vpx import (
+    Vp8Decoder,
+    Vp8Encoder,
+    VpxPayloadDescriptor,
+    number_of_threads,
+)
 from aiortc.rtcrtpparameters import RTCRtpCodecParameters
 
 from .codecs import CodecTestCase
@@ -234,6 +239,12 @@ def test_encoder_target_bitrate(self) -> None:
         self.assertTrue(len(payloads[0]) < 1300)
         self.assertAlmostEqual(timestamp, 3000, delta=1)
 
+    def test_number_of_threads(self) -> None:
+        self.assertEqual(number_of_threads(1920 * 1080, 16), 8)
+        self.assertEqual(number_of_threads(1920 * 1080, 8), 3)
+        self.assertEqual(number_of_threads(1920 * 1080, 4), 2)
+        self.assertEqual(number_of_threads(1920 * 1080, 2), 1)
+
     def test_roundtrip_1280_720(self) -> None:
         self.roundtrip_video(VP8_CODEC, 1280, 720)