pgbench: Comment on thread timing hazards.

Reviewed by Fabien COELHO.

diff --git a/contrib/pgbench/pgbench.c b/contrib/pgbench/pgbench.c
index 926246e67bacdbd498dee9bb295085e318178154..816400f6dd8277255ed4e1e7de3da8f77bd1e049 100644 (file)
--- a/contrib/pgbench/pgbench.c
+++ b/contrib/pgbench/pgbench.c
@@ -2896,7 +2896,16 @@ main(int argc, char **argv)
    }
    disconnect_all(state, nclients);
 
-   /* get end time */
+   /*
+    * XXX We compute results as though every client of every thread started
+    * and finished at the same time.  That model can diverge noticeably from
+    * reality for a short benchmark run involving relatively many threads.
+    * The first thread may process notably many transactions before the last
+    * thread begins.  Improving the model alone would bring limited benefit,
+    * because performance during those periods of partial thread count can
+    * easily exceed steady state performance.  This is one of the many ways
+    * short runs convey deceptive performance figures.
+    */
    INSTR_TIME_SET_CURRENT(total_time);
    INSTR_TIME_SUBTRACT(total_time, start_time);
    printResults(ttype, total_xacts, nclients, threads, nthreads,