alsa: Fix excessive I/O causing higher CPU usage

"On GCW Zero jz4770 platform, I saw higher than usual CPU usage when running a more recent kernel (4.xx series versus 3.xx). Upon investigation, it was found that the ALSA pcm file was not blocking as it should. This resulted in ~30-50,000 system calls a second that were unnecesary. After adjusting the order in which SDL requests its pcm blocking mode, the number of syscalls a second has dropped to a much smaller figure, < 1,000/sec if I recall correctly. CPU usage also dropped by ~5%." (This patch was written by Daniel Silsby.) Fixes Bugzilla #4941.
libsdl-org · Mar 23, 2020 · 5d44141 · 5d44141
1 parent e87e1ac
commit 5d44141
Showing 1 changed file with 4 additions and 3 deletions.
diff --git a/src/audio/alsa/SDL_alsa_audio.c b/src/audio/alsa/SDL_alsa_audio.c
@@ -496,6 +496,10 @@ static int ALSA_OpenAudio(_THIS, SDL_AudioSpec *spec)
 		return(-1);
 	}
 
+	/* Switch to blocking mode for playback */
+	/* Note: this must happen before hw/sw params are set. */
+	SDL_NAME(snd_pcm_nonblock)(pcm_handle, 0);
+
 	/* Figure out what the hardware is capable of */
 	snd_pcm_hw_params_alloca(&hwparams);
 	status = SDL_NAME(snd_pcm_hw_params_any)(pcm_handle, hwparams);
@@ -628,9 +632,6 @@ static int ALSA_OpenAudio(_THIS, SDL_AudioSpec *spec)
 	}
 	SDL_memset(mixbuf, spec->silence, spec->size);
 
-	/* Switch to blocking mode for playback */
-	SDL_NAME(snd_pcm_nonblock)(pcm_handle, 0);
-
 	/* We're ready to rock and roll. :-) */
 	return(0);
 }