[pulseaudio-discuss] [PATCH] echo-cancel: Limit the maximum sink/source latency

[Arun Raghavan]

On systems with constrained CPUs, we might run into a situation where
the master source/sink is configured to have too high a latency.

On the source side, this would cause us to wake up with a large chunk of
data to process, which might cause us to exhust our RT limit and thus be
killed.

So it makes sense to limit the overall latency that we request from the
source (and correspondingly, the sink, so we don't starve for playback
data on the source side).

The 10 blocks maximum is somewhat arbitrary (I'm assuming the system has
enough headroom to process 10 chunks through the canceller without
getting close to the RT limit). This might make sense to make tunable in
the future.
---
 src/modules/echo-cancel/module-echo-cancel.c | 32 +++++++++++++++++++++-------
 1 file changed, 24 insertions(+), 8 deletions(-)

diff --git a/src/modules/echo-cancel/module-echo-cancel.c b/src/modules/echo-cancel/module-echo-cancel.c
index ed75e0c..0a9f290 100644
--- a/src/modules/echo-cancel/module-echo-cancel.c
+++ b/src/modules/echo-cancel/module-echo-cancel.c
@@ -145,6 +145,8 @@ static const pa_echo_canceller ec_table[] = {
 
 #define MEMBLOCKQ_MAXLENGTH (16*1024*1024)
 
+#define MAX_LATENCY_BLOCKS 10
+
 /* Can only be used in main context */
 #define IS_ACTIVE(u) ((pa_source_get_state((u)->source) == PA_SOURCE_RUNNING) && \
                       (pa_sink_get_state((u)->sink) == PA_SINK_RUNNING))
@@ -515,6 +517,7 @@ static int sink_set_state_cb(pa_sink *s, pa_sink_state_t state) {
 /* Called from source I/O thread context */
 static void source_update_requested_latency_cb(pa_source *s) {
     struct userdata *u;
+    pa_usec_t latency;
 
     pa_source_assert_ref(s);
     pa_assert_se(u = s->userdata);
@@ -525,15 +528,17 @@ static void source_update_requested_latency_cb(pa_source *s) {
 
     pa_log_debug("Source update requested latency");
 
-    /* Just hand this one over to the master source */
-    pa_source_output_set_requested_latency_within_thread(
-            u->source_output,
-            pa_source_get_requested_latency_within_thread(s));
+    /* Cap the maximum latency so we don't have to process too large chunks */
+    latency = PA_MIN(pa_source_get_requested_latency_within_thread(s),
+                     pa_bytes_to_usec(u->source_blocksize, &s->sample_spec) * MAX_LATENCY_BLOCKS);
+
+    pa_source_output_set_requested_latency_within_thread(u->source_output, latency);
 }
 
 /* Called from sink I/O thread context */
 static void sink_update_requested_latency_cb(pa_sink *s) {
     struct userdata *u;
+    pa_usec_t latency;
 
     pa_sink_assert_ref(s);
     pa_assert_se(u = s->userdata);
@@ -544,10 +549,11 @@ static void sink_update_requested_latency_cb(pa_sink *s) {
 
     pa_log_debug("Sink update requested latency");
 
-    /* Just hand this one over to the master sink */
-    pa_sink_input_set_requested_latency_within_thread(
-            u->sink_input,
-            pa_sink_get_requested_latency_within_thread(s));
+    /* Cap the maximum latency so we don't have to process too large chunks */
+    latency = PA_MIN(pa_sink_get_requested_latency_within_thread(s),
+                     pa_bytes_to_usec(u->sink_blocksize, &s->sample_spec) * MAX_LATENCY_BLOCKS);
+
+    pa_sink_input_set_requested_latency_within_thread(u->sink_input, latency);
 }
 
 /* Called from sink I/O thread context */
@@ -1658,6 +1664,7 @@ int pa__init(pa_module*m) {
     uint32_t temp;
     uint32_t nframes = 0;
     bool use_master_format;
+    pa_usec_t blocksize_usec;
 
     pa_assert(m);
 
@@ -2020,6 +2027,15 @@ int pa__init(pa_module*m) {
 
     u->thread_info.current_volume = u->source->reference_volume;
 
+    /* We don't want to deal with too many chunks at a time */
+    blocksize_usec = pa_bytes_to_usec(u->source_blocksize, &u->source->sample_spec);
+    pa_source_set_latency_range(u->source, blocksize_usec, blocksize_usec * MAX_LATENCY_BLOCKS);
+    pa_source_output_set_requested_latency(u->source_output, blocksize_usec * MAX_LATENCY_BLOCKS);
+
+    blocksize_usec = pa_bytes_to_usec(u->sink_blocksize, &u->sink->sample_spec);
+    pa_sink_set_latency_range(u->sink, blocksize_usec, blocksize_usec * MAX_LATENCY_BLOCKS);
+    pa_sink_input_set_requested_latency(u->sink_input, blocksize_usec * MAX_LATENCY_BLOCKS);
+
     pa_sink_put(u->sink);
     pa_source_put(u->source);
 
-- 
2.9.3