Add phase switch contention to build statistics

2 files changed, 14 insertions, 1 deletions

diff --git a/libbuild2/context.cxx b/libbuild2/context.cxx
index 80343bd..d460fa7 100644
--- a/libbuild2/context.cxx
+++ b/libbuild2/context.cxx
@@ -765,6 +765,8 @@ namespace build2
       }
       else if (ctx_.phase != n)
       {
+        ++contention;
+
         ctx_.sched.deactivate (false /* external */);
         for (; ctx_.phase != n; v->wait (l)) ;
         r = !fail_;
@@ -781,6 +783,8 @@ namespace build2
     {
       if (!lm_.try_lock ())
       {
+        ++contention;
+
         ctx_.sched.deactivate (false /* external */);
         lm_.lock ();
         ctx_.sched.activate (false /* external */);
@@ -901,6 +905,8 @@ namespace build2
       }
       else // phase != n
       {
+        ++contention;
+
         ctx_.sched.deactivate (false /* external */);
         for (; ctx_.phase != n; v->wait (l)) ;
         r = !fail_;
@@ -913,6 +919,8 @@ namespace build2
     {
       if (!lm_.try_lock ())
       {
+        ++contention;
+
         ctx_.sched.deactivate (false /* external */);
         lm_.lock ();
         ctx_.sched.activate (false /* external */);
diff --git a/libbuild2/context.hxx b/libbuild2/context.hxx
index ef628c9..0595413 100644
--- a/libbuild2/context.hxx
+++ b/libbuild2/context.hxx
@@ -43,6 +43,11 @@ namespace build2
     bool
     relock (run_phase unlock, run_phase lock);
 
+    // Statistics.
+    //
+  public:
+    size_t contention = 0; // # of contentious phase (re)locks.
+
   private:
     friend class context;
 
@@ -61,7 +66,7 @@ namespace build2
     // is exclusive so we have a separate mutex to serialize it (think of it
     // as a second level locking).
     //
-    // When the mutex is unlocked (all three counters become zero, the phase
+    // When the mutex is unlocked (all three counters become zero), the phase
     // is always changed to load (this is also the initial state).
     //
     context& ctx_;