From 230e7cb0bab4582132de2bc411e355eacbacff4a Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Fri, 8 Apr 2022 08:10:32 +0200
Subject: Add phase switch contention to build statistics

---
 build2/b.cxx          | 36 ++++++++++++++++++++++++++----------
 libbuild2/context.cxx |  8 ++++++++
 libbuild2/context.hxx |  7 ++++++-
 3 files changed, 40 insertions(+), 11 deletions(-)

diff --git a/build2/b.cxx b/build2/b.cxx
index 556cf36..e615ef5 100644
--- a/build2/b.cxx
+++ b/build2/b.cxx
@@ -268,6 +268,10 @@ main (int argc, char* argv[])
   options ops;
   scheduler sched;
 
+  // Statistics.
+  //
+  size_t phase_switch_contention (0);
+
   try
   {
     // Parse the command line.
@@ -412,9 +416,17 @@ main (int argc, char* argv[])
     // below).
     //
     unique_ptr<context> pctx;
-    auto new_context = [&ops, &cmdl, &pctx, &sched, &mutexes, &fcache]
+    auto new_context = [&ops, &cmdl,
+                        &sched, &mutexes, &fcache,
+                        &phase_switch_contention,
+                        &pctx]
     {
-      pctx = nullptr; // Free first.
+      if (pctx != nullptr)
+      {
+        phase_switch_contention += pctx->phase_mutex.contention;
+        pctx = nullptr; // Free first to reuse memory.
+      }
+
       pctx.reset (new context (sched,
                                mutexes,
                                fcache,
@@ -1389,6 +1401,8 @@ main (int argc, char* argv[])
       cout << endl;
     }
 #endif
+
+    phase_switch_contention += pctx->phase_mutex.contention;
   }
   catch (const failed&)
   {
@@ -1410,16 +1424,18 @@ main (int argc, char* argv[])
   {
     text << '\n'
          << "build statistics:" << "\n\n"
-         << "  thread_max_active      " << st.thread_max_active     << '\n'
-         << "  thread_max_total       " << st.thread_max_total      << '\n'
-         << "  thread_helpers         " << st.thread_helpers        << '\n'
-         << "  thread_max_waiting     " << st.thread_max_waiting    << '\n'
+         << "  thread_max_active       " << st.thread_max_active     << '\n'
+         << "  thread_max_total        " << st.thread_max_total      << '\n'
+         << "  thread_helpers          " << st.thread_helpers        << '\n'
+         << "  thread_max_waiting      " << st.thread_max_waiting    << '\n'
+         << '\n'
+         << "  task_queue_depth        " << st.task_queue_depth      << '\n'
+         << "  task_queue_full         " << st.task_queue_full       << '\n'
          << '\n'
-         << "  task_queue_depth       " << st.task_queue_depth      << '\n'
-         << "  task_queue_full        " << st.task_queue_full       << '\n'
+         << "  wait_queue_slots        " << st.wait_queue_slots      << '\n'
+         << "  wait_queue_collisions   " << st.wait_queue_collisions << '\n'
          << '\n'
-         << "  wait_queue_slots       " << st.wait_queue_slots      << '\n'
-         << "  wait_queue_collisions  " << st.wait_queue_collisions << '\n';
+         << "  phase_switch_contention " << phase_switch_contention << '\n';
   }
 
   return r;
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_;
-- 
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