Experimental patch for adapting adaptation to CPU count on Mac.

webrtc/video/overuse_frame_detector.h

Index: webrtc/video/overuse_frame_detector.h
diff --git a/webrtc/video/overuse_frame_detector.h b/webrtc/video/overuse_frame_detector.h
index 0ef2e43857d2601c800c48a09d0dc6e237692e53..832a9a4148dc6546d0d4abecfd5ea43713e63814 100644
--- a/webrtc/video/overuse_frame_detector.h
+++ b/webrtc/video/overuse_frame_detector.h
@@ -19,6 +19,10 @@
 #include "webrtc/base/thread_checker.h"
 #include "webrtc/modules/include/module.h"
 
+#if defined(WEBRTC_MAC)
+#include <mach/mach.h>
+#endif
+
 namespace webrtc {
 
 class Clock;
@@ -38,12 +42,34 @@ class CpuOveruseObserver {
 
 struct CpuOveruseOptions {
   CpuOveruseOptions()
-      : low_encode_usage_threshold_percent(55),
-        high_encode_usage_threshold_percent(85),
+      : low_encode_usage_threshold_percent(42),
         frame_timeout_interval_ms(1500),
         min_frame_samples(120),
         min_process_count(3),
-        high_threshold_consecutive_count(2) {}
+        high_threshold_consecutive_count(2) {
+#if defined  (WEBRTC_MAC)

[tommi, 2016/02/04 20:32:44]

#if defined(WEBRTC_MAC)

[torbjorng (webrtc), 2016/02/05 16:31:05]

Done.

+    // This is a proof-of-concept hack for letting the physical core count
+    // affect the interval into which we attempt to scale.  For now, the code
+    // is Mac OS specific, since that's the platform were we saw the problem.
+    // Note that we make the interval 2x+epsilon wide, since scaling steps are
+    // close to that.
+    struct host_basic_info hbi;
+    mach_msg_type_number_t datasize;
+    // TODO(torbjorng): Add syscall error checking, fallback code.
+    host_info(mach_host_self(), HOST_BASIC_INFO, (host_info_t)&hbi, &datasize);

[tommi, 2016/02/04 20:32:44]

Can you change the cast to a C++ cast?  As is it looks like the cast is casting
a pointer to an object... does host_info() accept a pointer or a by value? (is
host_info_t a pointer type?)

[Robert Sesek, 2016/02/04 22:37:15]

The result of mach_host_self must be deallocated. This is currently leaking a
system resource.

[tommi (sloooow) - chröme, 2016/02/04 22:44:40]

Ah, thanks for catching that!
Now this makes more sense to me. I'd prefer to have the variable of the expected
type too and only cast after error checking (and of course free too).

[torbjorng (webrtc), 2016/02/05 16:31:05]

Yes, host_info_t is a ptr (it is in fact int*).

Fixing cast.

[torbjorng (webrtc), 2016/02/05 16:31:05]

Robert: I changed the code to look more like in other places in WebRTC. This
means calling mach_port_deallocate with mach_task_self(). Since the latter just
like mach_host_self() returns a magic port number (an int) one might suspect
that the deallocation could leak. I cannot find proper documentation, but IIUC,
these port numbers are (small) refcounted kernel resources.

Tommi: This is a message passing API. You are supposed to cast things like this
to the generic message type.

[tommi, 2016/02/05 22:56:35]

Thanks, I'm familiar with APIs like these :)  I was thinking along these lines:

host_info_t host_info = nullptr;
host_info(mach_host_self(), HOST_BASIC_INFO, &host_info, &datasize);
if (data_size == sizeof(host_basic_info)) {
  struct host_basic_info* basic_info =
reintepret_cast<host_basic_info*>(host_info);
  // use |basic_info|
} else {
  NOTREACHED() << "host_info returned something unexpected.";
}


However, what you have now is fine too.

[Robert Sesek, 2016/02/05 22:59:58]

Thanks! This is now OK. mach_task_self() is actually a macro for an extern of
the task port cached in userspace, so you don't need to deallocate it.
mach_host_self() is a real kernel trap, so you do have to deallocate that.

(Yes, this is more confusing than it needs to be).

+
+    if (hbi.physical_cpu == 1) {
+      low_encode_usage_threshold_percent = 17;
+    } else if (hbi.physical_cpu == 2) {
+      low_encode_usage_threshold_percent = 27;
+    } else {
+      // Init list default.
+    }
+#endif

[tommi, 2016/02/04 20:32:44]

nit:
#endif  // WEBRTC_MAC

[torbjorng (webrtc), 2016/02/05 16:31:05]

Done.

+    // Make high = 2 * low + epsilon.
+    high_encode_usage_threshold_percent =
+      33 * low_encode_usage_threshold_percent / 16 + 1;

[tommi, 2016/02/04 20:32:44]

4 space indent.  (or just run git cl format?)

[torbjorng (webrtc), 2016/02/05 16:31:06]

Done.

+  }
 
   int low_encode_usage_threshold_percent;  // Threshold for triggering underuse.
   int high_encode_usage_threshold_percent;  // Threshold for triggering overuse.