libexpr: Track and show GC time and cycle number

src/libexpr/eval-gc.cc

@@ -155,6 +155,10 @@ static inline void initGCReal()
        there. */
     GC_set_no_dls(1);
 
+    /* Enable perf measurements. This is just a setting; not much of a
+       start of something. */
+    GC_start_performance_measurement();
+
     GC_INIT();
 
     GC_set_oom_fn(oomHandler);
@@ -205,6 +209,7 @@ static inline void initGCReal()
 #endif
 
 static bool gcInitialised = false;
+static GC_word gcCyclesAfterInit = 0;
 
 void initGC()
 {
@@ -216,6 +221,7 @@ void initGC()
 #endif
 
     gcInitialised = true;
+    gcCyclesAfterInit = GC_get_gc_no();
 }
 
 void assertGCInitialized()
@@ -223,4 +229,10 @@ void assertGCInitialized()
     assert(gcInitialised);
 }
 
+size_t getGCCycles()
+{
+    assertGCInitialized();
+    return GC_get_gc_no() - gcCyclesAfterInit;
 }
+
+} // namespace nix

src/libexpr/eval-gc.hh

@@ -1,6 +1,8 @@
 #pragma once
 ///@file
 
+#include <cstddef>
+
 namespace nix {
 
 /**
@@ -13,4 +15,9 @@ void initGC();
  */
 void assertGCInitialized();
 
+/**
+ * The number of GC cycles since initGC().
+ */
+size_t getGCCycles();
+
 }

src/libexpr/eval.cc

@@ -2610,6 +2610,11 @@ void EvalState::printStatistics()
 #if HAVE_BOEHMGC
     GC_word heapSize, totalBytes;
     GC_get_heap_usage_safe(&heapSize, 0, 0, 0, &totalBytes);
+    double gcFullOnlyTime = ({
+        auto ms = GC_get_full_gc_total_time();
+        ms * 0.001;
+    });
+    auto gcCycles = getGCCycles();
 #endif
 
     auto outPath = getEnv("NIX_SHOW_STATS_PATH").value_or("-");
@@ -2620,6 +2625,13 @@ void EvalState::printStatistics()
 #ifndef _WIN32 // TODO implement
     topObj["cpuTime"] = cpuTime;
 #endif
+    topObj["time"] = {
+        {"cpu", cpuTime},
+#ifdef HAVE_BOEHMGC
+        {GC_is_incremental_mode() ? "gcNonIncremental" : "gc", gcFullOnlyTime},
+        {GC_is_incremental_mode() ? "gcNonIncrementalFraction" : "gcFraction", gcFullOnlyTime / cpuTime},
+#endif
+    };
     topObj["envs"] = {
         {"number", nrEnvs},
         {"elements", nrValuesInEnvs},
@@ -2661,6 +2673,7 @@ void EvalState::printStatistics()
     topObj["gc"] = {
         {"heapSize", heapSize},
         {"totalBytes", totalBytes},
+        {"cycles", gcCycles},
     };
 #endif