Reduce hash calculations of SampledContention

src/brpc/builtin/index_service.cpp

@@ -140,7 +140,9 @@ void IndexService::default_method(::google::protobuf::RpcController* controller,
            << (!IsHeapProfilerEnabled() ? " (disabled)" : "") << NL
            << Path("/hotspots/growth", html_addr)
            << " : Profiling growth of heap"
-           << (!IsHeapProfilerEnabled() ? " (disabled)" : "") << NL;
+           << (!IsHeapProfilerEnabled() ? " (disabled)" : "") << NL
+           << Path("/hotspots/contention", html_addr)
+           << " : Profiling contention of lock" << NL;
     }
     os << "curl -H 'Content-Type: application/json' -d 'JSON' ";
     if (butil::is_endpoint_extended(server->listen_address())) {

src/bthread/mutex.cpp

@@ -64,24 +64,32 @@ struct SampledContention : public bvar::Collected {
     int64_t duration_ns;
     // number of samples, normalized according to to sampling_range
     double count;
-    int nframes;          // #elements in stack
     void* stack[26];      // backtrace.
+    int nframes;          // #elements in stack
 
     // Implement bvar::Collected
     void dump_and_destroy(size_t round) override;
     void destroy() override;
     bvar::CollectorSpeedLimit* speed_limit() override { return &g_cp_sl; }
 
-    // For combining samples with hashmap.
     size_t hash_code() const {
         if (nframes == 0) {
             return 0;
         }
-        uint32_t code = 1;
-        uint32_t seed = nframes;
-        butil::MurmurHash3_x86_32(stack, sizeof(void*) * nframes, seed, &code);
-        return code;
+        if (_hash_code == 0) {
+            _hash_code = 1;
+            uint32_t seed = nframes;
+            butil::MurmurHash3_x86_32(stack, sizeof(void*) * nframes, seed, &_hash_code);
+        }
+        return _hash_code;
     }
+private:
+friend butil::ObjectPool<SampledContention>;
+    SampledContention()
+        : duration_ns(0), count(0), stack{NULL}, nframes(0), _hash_code(0) {}
+    ~SampledContention() override = default;
+
+    mutable uint32_t _hash_code; // For combining samples with hashmap.
 };
 
 BAIDU_CASSERT(sizeof(SampledContention) == 256, be_friendly_to_allocator);
@@ -294,6 +302,7 @@ void SampledContention::dump_and_destroy(size_t /*round*/) {
 }
 
 void SampledContention::destroy() {
+    _hash_code = 0;
     butil::return_object(this);
 }