[BUG]DisplayEpollWatch causes a 20% increase in CPU usage

[yxungh]

Describe the bug

When I enable the macro IF_HEAVY_LOGGING.
The string processing here will increase the usage by 20%, and I believe it’s not worth it.
IF_HEAVY_LOGGING(string tracking = " TRACKING: " + ed.DisplayEpollWatch());

[ethouris]

Why is that a bug? This log is exclusively for development support to track any problems, and as the name shows, it's a "heavy" logging and it is known that this will burden the performance, by definition.

If you want, apply this below patch - it changes this string into stringstream so that this is displayed directly buffer-to-buffer, without involving a string intermediately. Maybe this will improve the performance a bit.

diff --git a/srtcore/epoll.cpp b/srtcore/epoll.cpp
index 8cd8440c..a0b8a503 100644
--- a/srtcore/epoll.cpp
+++ b/srtcore/epoll.cpp
@@ -901,7 +901,8 @@ int srt::CEPoll::update_events(const SRTSOCKET& uid, std::set<int>& eids, const
             continue;
         }

-        IF_HEAVY_LOGGING(string tracking = " TRACKING: " + ed.DisplayEpollWatch());
+        //IF_HEAVY_LOGGING(string tracking = " TRACKING: " + ed.DisplayEpollWatch());
+        IF_HEAVY_LOGGING(std::ostringstream tracking; tracking << " TRACKING: "; ed.DisplayEpollWatch(tracking));
         // compute new states

         // New state to be set into the permanent state
@@ -912,7 +913,7 @@ int srt::CEPoll::update_events(const SRTSOCKET& uid, std::set<int>& eids, const
         int changes = pwait->state ^ newstate; // oldState XOR newState
         if (!changes)
         {
-            HLOGC(eilog.Debug, log << debug.str() << ": E" << (*i)
+            HLOGC(eilog.Debug, log << debug << ": E" << (*i)
                     << tracking << " NOT updated: no changes");
             continue; // no changes!
         }
@@ -967,6 +968,12 @@ string DisplayEpollResults(const std::map<SRTSOCKET, int>& sockset)
 string CEPollDesc::DisplayEpollWatch()
 {
     ostringstream os;
+    DisplayEpollWatch(os);
+    return os.str();
+}
+
+std::ostream& CEPollDesc::DisplayEpollWatch(std::ostream& os)
+{
     for (ewatch_t::const_iterator i = m_USockWatchState.begin(); i != m_USockWatchState.end(); ++i)
     {
         os << "@" << i->first << ":";
@@ -974,7 +981,7 @@ string CEPollDesc::DisplayEpollWatch()
         os << " ";
     }

-    return os.str();
+    return os;
 }

 } // namespace srt
diff --git a/srtcore/epoll.h b/srtcore/epoll.h
index 00d46ceb..0c4a9010 100644
--- a/srtcore/epoll.h
+++ b/srtcore/epoll.h
@@ -151,6 +151,7 @@ class CEPollDesc

 #if ENABLE_HEAVY_LOGGING
 std::string DisplayEpollWatch();
+std::ostream& DisplayEpollWatch(std::ostream& log);
 #endif

    /// Sockets that are subscribed for events in this eid.

[yxungh]

Why is that a bug?
I believe it’s somewhat unreasonable and not user-friendly for those who have high requirements for CPU usage.
The key point is to process the string first; that is, even if I don’t enable eilog, it will still waste CPU.