fix the issue that the RU returned by the DataStreamExecutor was too large

dbms/src/Flash/Executor/DataStreamExecutor.cpp

@@ -13,6 +13,8 @@
 // limitations under the License.
 
 #include <Common/FmtUtils.h>
+#include <Common/TiFlashMetrics.h>
+#include <Common/getNumberOfCPUCores.h>
 #include <DataStreams/BlockIO.h>
 #include <DataStreams/IProfilingBlockInputStream.h>
 #include <Flash/Executor/DataStreamExecutor.h>
@@ -24,6 +26,8 @@ DataStreamExecutor::DataStreamExecutor(const BlockIO & block_io)
     , data_stream(block_io.in)
 {
     assert(data_stream);
+    thread_cnt_before_execute = GET_METRIC(tiflash_thread_count, type_active_threads_of_thdpool).Value();
+    estimate_thread_cnt = std::max(data_stream->estimateNewThreadCount(), 1);
 }
 
 ExecutionResult DataStreamExecutor::execute(ResultHandler result_handler)
@@ -65,11 +69,45 @@ String DataStreamExecutor::toString() const
 
 int DataStreamExecutor::estimateNewThreadCount()
 {
-    return data_stream->estimateNewThreadCount();
+    return estimate_thread_cnt;
 }
 
 RU DataStreamExecutor::collectRequestUnit()
 {
-    return toRU(data_stream->estimateCPUTimeNs());
+    // The cputime returned by BlockInputSrream is a count of the execution time of each thread.
+    // However, cputime here is imprecise and is affected by thread scheduling and condition_cv.wait.
+    // The following attempts to eliminate the effects of thread scheduling.
+
+    auto execute_time_ns = data_stream->estimateCPUTimeNs();
+    UInt64 total_thread_cnt = thread_cnt_before_execute + estimate_thread_cnt;
+    size_t logical_cpu_cores = getNumberOfLogicalCPUCores();
+    // When the number of threads is greater than the number of cpu cores,
+    // BlockInputStream's estimated cpu time will be much greater than the actual value.
+    if (execute_time_ns <= 0 || total_thread_cnt <= logical_cpu_cores)
+        return toRU(execute_time_ns);
+
+    // Here we use `execute_time_ns / thread_cnt` to get the average execute time of each thread.
+    // So we have `per_thread_execute_time_ns = execute_time_ns / estimate_thread_cnt`.
+    // Since a cpu core executes several threads, think of `execute_time_of_thread_A = cputime_of_thread_A + cputime_of_other_thread`.
+    // Assuming that the cputime of other threads is the same as thread A, then we have `execute_time_of_thread_A = total_thread_cnt * cputime_of_thread_A`.
+    // Assuming that threads is divided equally among all cpu cores, then `the number of threads allocated to cpu core A = total_thread_cnt / logical_cpu_cores`.
+    // So we have `per_thread_cputime_ns = per_thread_execute_time_ns / (total_thread_cnt / logical_cpu_cores)`.
+    // And the number of threads of `data_stream` executed by one cpu core is `estimate_thread_cnt / logical_cpu_cores`.
+    // So we have `per_core_cputime_ns = per_thread_cputime_ns * (estimate_thread_cnt / logical_cpu_cores)`
+    // So it can be assumed that
+    //     per_core_cpu_time = per_thread_cputime_ns * (estimate_thread_cnt / logical_cpu_cores)
+    //                       = per_thread_execute_time_ns / (total_thread_cnt / logical_cpu_cores) * (estimate_thread_cnt / logical_cpu_cores)
+    //                       = per_thread_execute_time_ns / total_thread_cnt * estimate_thread_cnt
+    // So that we have
+    //     cputime_ns = per_core_cpu_time * logical_cpu_cores
+    //                = per_thread_execute_time_ns / total_thread_cnt * estimate_thread_cnt * logical_cpu_cores
+    //                = execute_time_ns / total_thread_cnt / estimate_thread_cnt * estimate_thread_cnt * logical_cpu_cores
+    //                = execute_time_ns / total_thread_cnt * logical_cpu_cores
+    auto cpu_time_ns = static_cast<double>(execute_time_ns) / total_thread_cnt * logical_cpu_cores;
+    // But there is still no way to eliminate the effect of `condition_cv.wait` here...
+    // We can assume `condition.wait` takes half of datastream execute time.
+    // TODO find a more reasonable ratio for `condition.wait`.
+    cpu_time_ns /= 2;
+    return toRU(ceil(cpu_time_ns));
 }
 } // namespace DB

dbms/src/Flash/Executor/DataStreamExecutor.h

@@ -41,5 +41,7 @@ class DataStreamExecutor : public QueryExecutor
 
 protected:
     BlockInputStreamPtr data_stream;
+    uint64_t thread_cnt_before_execute = 0;
+    uint64_t estimate_thread_cnt = 0;
 };
 } // namespace DB