videoio(ffmpeg): limit number of default threads

modules/videoio/src/cap_ffmpeg_impl.hpp

@@ -373,42 +373,6 @@ inline double get_monotonic_time_diff_ms(timespec time1, timespec time2)
 }
 #endif // USE_AV_INTERRUPT_CALLBACK
 
-static int get_number_of_cpus(void)
-{
-#if defined _WIN32
-    SYSTEM_INFO sysinfo;
-    GetSystemInfo( &sysinfo );
-
-    return (int)sysinfo.dwNumberOfProcessors;
-#elif defined __linux__ || defined __HAIKU__
-    return (int)sysconf( _SC_NPROCESSORS_ONLN );
-#elif defined __APPLE__
-    int numCPU=0;
-    int mib[4];
-    size_t len = sizeof(numCPU);
-
-    // set the mib for hw.ncpu
-    mib[0] = CTL_HW;
-    mib[1] = HW_AVAILCPU;  // alternatively, try HW_NCPU;
-
-    // get the number of CPUs from the system
-    sysctl(mib, 2, &numCPU, &len, NULL, 0);
-
-    if( numCPU < 1 )
-    {
-        mib[1] = HW_NCPU;
-        sysctl( mib, 2, &numCPU, &len, NULL, 0 );
-
-        if( numCPU < 1 )
-            numCPU = 1;
-    }
-
-    return (int)numCPU;
-#else
-    return 1;
-#endif
-}
-
 
 struct Image_FFMPEG
 {
@@ -998,12 +962,17 @@ public:
 
 inline void fill_codec_context(AVCodecContext * enc, AVDictionary * dict)
 {
-//#ifdef FF_API_THREAD_INIT
-//  avcodec_thread_init(enc, get_number_of_cpus());
-//#else
-    const int nCpus = get_number_of_cpus();
-    enc->thread_count = enc->thread_count ? enc->thread_count: nCpus;
-//#endif
+    if (!enc->thread_count)
+    {
+        int nCpus = cv::getNumberOfCPUs();
+        int requestedThreads = std::min(nCpus, 16);  // [OPENCV:FFMPEG:24] Application has requested XX threads. Using a thread count greater than 16 is not recommended.
+        char* threads_option = getenv("OPENCV_FFMPEG_THREADS");
+        if (threads_option != NULL)
+        {
+            requestedThreads = atoi(threads_option);
+        }
+        enc->thread_count = requestedThreads;
+    }
 
     AVDictionaryEntry* avdiscard_entry = av_dict_get(dict, "avdiscard", NULL, 0);