Re: Patch to show individual statement latencies in pgbench output - Mailing list pgsql-hackers

Florian Pflug wrote:
> Attached is an updated version (v4).
>

I've attached a v5.  No real code changes from Florian's version, just
some wording/style fixes and rework on the documentation.  The user side
is now consistent about calling these statement latencies for example,
even though the internals still call them command latencies most places.

Since using this new feature will introduce a whole stack of new calls
to get the system time, I also added a warning about that impacting results:

    Note that collecting the additional timing information needed for
    detailed latency computation does add some overhead.  This will slow
    average execution speed and lower the computed TPS.  The exact amount
    of slowdown varies significantly based on platform and hardware.
    Comparing average TPS values with and without latency reporting enabled
    is a good way to measure if the timing overhead is significant.

I wasn't able to see any significant slowdown on my modern Linux systems
doing such a test:

$ ./pgbench -T 10 -S -c 8 -j 4 pgbench
tps = 6716.039813 (including connections establishing)
tps = 6720.238878 (excluding connections establishing)
$ ./pgbench -T 10 -S -c 8 -j 4 -r pgbench
tps = 6708.544618 (including connections establishing)
tps = 6712.728526 (excluding connections establishing)

But I know gettimeofday is fast here.  Worth including a warning for
though I think.

I'm out of things to check here, marking this one ready for a committer
review.  The patch hasn't had a committer assigned yet, so whoever wants
to claim it should mark the CF app.

--
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com   www.2ndQuadrant.us

diff --git a/contrib/pgbench/pgbench.c b/contrib/pgbench/pgbench.c
index 53cc4b6..90d11f1 100644
*** a/contrib/pgbench/pgbench.c
--- b/contrib/pgbench/pgbench.c
*************** int            fillfactor = 100;
*** 133,138 ****
--- 133,139 ----

  bool        use_log;            /* log transaction latencies to a file */
  bool        is_connect;            /* establish connection for each transaction */
+ bool        is_latencies;        /* report per-command latencies */
  int            main_pid;            /* main process id used in log filename */

  char       *pghost = "";
*************** typedef struct
*** 171,177 ****
      int64        until;            /* napping until (usec) */
      Variable   *variables;        /* array of variable definitions */
      int            nvariables;
!     instr_time    txn_begin;        /* used for measuring latencies */
      int            use_file;        /* index in sql_files for this client */
      bool        prepared[MAX_FILES];
  } CState;
--- 172,179 ----
      int64        until;            /* napping until (usec) */
      Variable   *variables;        /* array of variable definitions */
      int            nvariables;
!     instr_time    txn_begin;        /* used for measuring transaction latencies */
!     instr_time    stmt_begin;        /* used for measuring statement latencies */
      int            use_file;        /* index in sql_files for this client */
      bool        prepared[MAX_FILES];
  } CState;
*************** typedef struct
*** 199,204 ****
--- 201,207 ----
  /*
   * queries read from files
   */
+ #define COMMENT_COMMAND    0
  #define SQL_COMMAND        1
  #define META_COMMAND    2
  #define MAX_ARGS        10
*************** static const char *QUERYMODE[] = {"simpl
*** 216,224 ****
--- 219,230 ----

  typedef struct
  {
+     char       *line;            /* line containing the command */
      int            type;            /* command type (SQL_COMMAND or META_COMMAND) */
      int            argc;            /* number of commands */
      char       *argv[MAX_ARGS]; /* command list */
+     instr_time *exec_elapsed;    /* time spent executing this command (per thread) */
+     int           *exec_count;        /* number of executions (per thread) */
  } Command;

  static Command **sql_files[MAX_FILES];    /* SQL script files */
*************** usage(const char *progname)
*** 287,292 ****
--- 293,299 ----
             "               define variable for use by custom script\n"
             "  -f FILENAME  read transaction script from FILENAME\n"
             "  -j NUM       number of threads (default: 1)\n"
+            "  -r           report average latency per command\n"
             "  -l           write transaction times to log file\n"
             "  -M {simple|extended|prepared}\n"
             "               protocol for submitting queries to server (default: simple)\n"
*************** clientDone(CState *st, bool ok)
*** 740,746 ****

  /* return false iff client should be disconnected */
  static bool
! doCustom(CState *st, instr_time *conn_time, FILE *logfile)
  {
      PGresult   *res;
      Command   **commands;
--- 747,753 ----

  /* return false iff client should be disconnected */
  static bool
! doCustom(TState* thread, CState *st, instr_time *conn_time, FILE *logfile)
  {
      PGresult   *res;
      Command   **commands;
*************** top:
*** 800,805 ****
--- 807,824 ----
  #endif
          }

+         /* Accumulate per-command execution times if per-command latencies
+            are requested */
+         if (is_latencies && (commands[st->state]->type != COMMENT_COMMAND))
+         {
+             instr_time now;
+
+             INSTR_TIME_SET_CURRENT(now);
+             INSTR_TIME_ACCUM_DIFF(commands[st->state]->exec_elapsed[thread->tid],
+                                   now, st->stmt_begin);
+             ++(commands[st->state]->exec_count[thread->tid]);
+         }
+
          if (commands[st->state]->type == SQL_COMMAND)
          {
              res = PQgetResult(st->con);
*************** top:
*** 840,845 ****
--- 859,871 ----
              commands = sql_files[st->use_file];
          }
      }
+
+     /* Skip comments */
+     if (commands[st->state]->type == COMMENT_COMMAND)
+     {
+         st->listen = 1;
+         goto top;
+     }

      if (st->con == NULL)
      {
*************** top:
*** 856,864 ****
--- 882,895 ----
          INSTR_TIME_ACCUM_DIFF(*conn_time, end, start);
      }

+     /* Record transaction start time if logging is enabled */
      if (logfile && st->state == 0)
          INSTR_TIME_SET_CURRENT(st->txn_begin);

+     /* Record statement start time if per-command latencies are requested */
+     if (is_latencies)
+         INSTR_TIME_SET_CURRENT(st->stmt_begin);
+
      if (commands[st->state]->type == SQL_COMMAND)
      {
          const Command *command = commands[st->state];
*************** process_commands(char *buf)
*** 1361,1385 ****
      char       *p,
                 *tok;

      if ((p = strchr(buf, '\n')) != NULL)
          *p = '\0';

!     p = buf;
!     while (isspace((unsigned char) *p))
!         p++;
!
!     if (*p == '\0' || strncmp(p, "--", 2) == 0)
      {
          return NULL;
      }

!     my_commands = (Command *) malloc(sizeof(Command));
!     if (my_commands == NULL)
      {
!         return NULL;
      }

!     my_commands->argc = 0;

      if (*p == '\\')
      {
--- 1392,1428 ----
      char       *p,
                 *tok;

+     /* Make the string buf end at the next newline */
      if ((p = strchr(buf, '\n')) != NULL)
          *p = '\0';

!     /* Allocate and initialize Command structure, initially
!      * assuming the line is a comment
!      */
!     my_commands = (Command *) malloc(sizeof(Command));
!     if (my_commands == NULL)
      {
          return NULL;
      }
+     my_commands->type = COMMENT_COMMAND;
+     my_commands->line = strdup(buf);
+     my_commands->argc = 0;
+     my_commands->exec_elapsed = NULL;
+     my_commands->exec_count = NULL;

!     p = buf;
!
!     /* Skip leading whitespace */
!     while (isspace((unsigned char) *p))
      {
!         p++;
      }

!     /* If the line is empty or actually a comment, we're done */
!     if (*p == '\0' || strncmp(p, "--", 2) == 0)
!     {
!         return my_commands;
!     }

      if (*p == '\\')
      {
*************** process_file(char *filename)
*** 1548,1570 ****
      while (fgets(buf, sizeof(buf), fd) != NULL)
      {
          Command    *commands;
-         int            i;

!         i = 0;
!         while (isspace((unsigned char) buf[i]))
!             i++;
!
!         if (buf[i] != '\0' && strncmp(&buf[i], "--", 2) != 0)
          {
!             commands = process_commands(&buf[i]);
!             if (commands == NULL)
!             {
!                 fclose(fd);
!                 return false;
!             }
          }
-         else
-             continue;

          my_commands[lineno] = commands;
          lineno++;
--- 1591,1603 ----
      while (fgets(buf, sizeof(buf), fd) != NULL)
      {
          Command    *commands;

!         commands = process_commands(buf);
!         if (commands == NULL)
          {
!             fclose(fd);
!             return false;
          }

          my_commands[lineno] = commands;
          lineno++;
*************** printResults(int ttype, int normal_xacts
*** 1660,1665 ****
--- 1693,1699 ----
                  tps_include,
                  tps_exclude;
      char       *s;
+     int            i;

      time_include = INSTR_TIME_GET_DOUBLE(total_time);
      tps_include = normal_xacts / time_include;
*************** printResults(int ttype, int normal_xacts
*** 1694,1699 ****
--- 1728,1771 ----
      }
      printf("tps = %f (including connections establishing)\n", tps_include);
      printf("tps = %f (excluding connections establishing)\n", tps_exclude);
+
+     /* Report per-command latencies */
+     if (is_latencies)
+     {
+         for (i = 0; i < num_files; ++i) {
+             Command** commands;
+
+             if (num_files > 1)
+                 printf("statement latencies in milliseconds, file %d\n:", i+1);
+             else
+                 printf("statement latencies in milliseconds:\n");
+
+             for (commands = sql_files[i]; *commands != NULL; ++commands)
+             {
+                 Command* command = *commands;
+                 instr_time total_exec_elapsed;
+                 int total_exec_count;
+                 int t;
+
+                 /* Accumulate per-thread data if available */
+                 INSTR_TIME_SET_ZERO(total_exec_elapsed);
+                 total_exec_count = 0;
+                 if ((command->exec_elapsed != NULL) && (command->exec_count != NULL))
+                 {
+                     for (t = 0; t < nthreads; ++t)
+                     {
+                         INSTR_TIME_ADD(total_exec_elapsed, command->exec_elapsed[t]);
+                         total_exec_count += command->exec_count[t];
+                     }
+                 }
+
+                 if (total_exec_count > 0)
+                     printf("\t%f\t%s\n", INSTR_TIME_GET_MILLISEC(total_exec_elapsed) / (double)total_exec_count,
command->line);
+                 else
+                     printf("\t%f\t%s\n", 0.0, command->line);
+             }
+         }
+     }
  }


*************** main(int argc, char **argv)
*** 1770,1776 ****

      memset(state, 0, sizeof(*state));

!     while ((c = getopt(argc, argv, "ih:nvp:dSNc:Cs:t:T:U:lf:D:F:M:j:")) != -1)
      {
          switch (c)
          {
--- 1842,1848 ----

      memset(state, 0, sizeof(*state));

!     while ((c = getopt(argc, argv, "ih:nvp:dSNc:Cs:t:T:U:rlf:D:F:M:j:")) != -1)
      {
          switch (c)
          {
*************** main(int argc, char **argv)
*** 1834,1839 ****
--- 1906,1914 ----
              case 'C':
                  is_connect = true;
                  break;
+             case 'r':
+                 is_latencies = true;
+                 break;
              case 's':
                  scale_given = true;
                  scale = atoi(optarg);
*************** main(int argc, char **argv)
*** 2090,2095 ****
--- 2165,2193 ----
          default:
              break;
      }
+
+     /* Reserve memory for the threads to store per-command latencies if requested */
+     if (is_latencies)
+     {
+         for (i = 0; i < num_files; ++i)
+         {
+             Command** commands;
+             for (commands = sql_files[i]; *commands != NULL; ++commands)
+             {
+                 Command* command = *commands;
+                 int t;
+
+                 command->exec_elapsed = (instr_time*) malloc(sizeof(instr_time) * nthreads);
+                 command->exec_count = (int*) malloc(sizeof(int) * nthreads);
+
+                 for (t = 0; t < nthreads; ++t)
+                 {
+                     INSTR_TIME_SET_ZERO(command->exec_elapsed[t]);
+                     command->exec_count[t] = 0;
+                 }
+             }
+         }
+     }

      /* get start up time */
      INSTR_TIME_SET_CURRENT(start_time);
*************** threadRun(void *arg)
*** 2211,2217 ****
          int            prev_ecnt = st->ecnt;

          st->use_file = getrand(0, num_files - 1);
!         if (!doCustom(st, &result->conn_time, logfile))
              remains--;            /* I've aborted */

          if (st->ecnt > prev_ecnt && commands[st->state]->type == META_COMMAND)
--- 2309,2315 ----
          int            prev_ecnt = st->ecnt;

          st->use_file = getrand(0, num_files - 1);
!         if (!doCustom(thread, st, &result->conn_time, logfile))
              remains--;            /* I've aborted */

          if (st->ecnt > prev_ecnt && commands[st->state]->type == META_COMMAND)
*************** threadRun(void *arg)
*** 2260,2266 ****
              {
                  continue;
              }
!             else if (commands[st->state]->type == META_COMMAND)
              {
                  min_usec = 0;    /* the connection is ready to run */
                  break;
--- 2358,2364 ----
              {
                  continue;
              }
!             else if ((commands[st->state]->type == META_COMMAND) || (commands[st->state]->type == COMMENT_COMMAND))
              {
                  min_usec = 0;    /* the connection is ready to run */
                  break;
*************** threadRun(void *arg)
*** 2313,2319 ****
              if (st->con && (FD_ISSET(PQsocket(st->con), &input_mask)
                              || commands[st->state]->type == META_COMMAND))
              {
!                 if (!doCustom(st, &result->conn_time, logfile))
                      remains--;    /* I've aborted */
              }

--- 2411,2417 ----
              if (st->con && (FD_ISSET(PQsocket(st->con), &input_mask)
                              || commands[st->state]->type == META_COMMAND))
              {
!                 if (!doCustom(thread, st, &result->conn_time, logfile))
                      remains--;    /* I've aborted */
              }

diff --git a/doc/src/sgml/pgbench.sgml b/doc/src/sgml/pgbench.sgml
index cdad212..6877bec 100644
*** a/doc/src/sgml/pgbench.sgml
--- b/doc/src/sgml/pgbench.sgml
*************** pgbench <optional> <replaceable>options<
*** 247,252 ****
--- 247,263 ----
       </varlistentry>

       <varlistentry>
+       <term><option>-r</option></term>
+       <listitem>
+        <para>
+         Report the average per-statement latency (execution time from the
+         perspective of the client) of each command after the benchmark
+         finishes.  See below for details.
+        </para>
+       </listitem>
+      </varlistentry>
+
+      <varlistentry>
        <term><option>-M</option> <replaceable>querymode</></term>
        <listitem>
         <para>
*************** END;
*** 632,638 ****
  <replaceable>client_id</> <replaceable>transaction_no</> <replaceable>time</> <replaceable>file_no</>
<replaceable>time_epoch</><replaceable>time_us</> 
  </synopsis>

!    where <replaceable>time</> is the elapsed transaction time in microseconds,
     <replaceable>file_no</> identifies which script file was used
     (useful when multiple scripts were specified with <literal>-f</>),
     and <replaceable>time_epoch</>/<replaceable>time_us</> are a
--- 643,649 ----
  <replaceable>client_id</> <replaceable>transaction_no</> <replaceable>time</> <replaceable>file_no</>
<replaceable>time_epoch</><replaceable>time_us</> 
  </synopsis>

!    where <replaceable>time</> is the total elapsed transaction time in microseconds,
     <replaceable>file_no</> identifies which script file was used
     (useful when multiple scripts were specified with <literal>-f</>),
     and <replaceable>time_epoch</>/<replaceable>time_us</> are a
*************** END;
*** 654,659 ****
--- 665,742 ----
   </sect2>

   <sect2>
+   <title>Per-statement latencies</title>
+   <para>
+    With the <literal>-r</> option, <application>pgbench</> collects
+    the elapsed transaction time of each statement executed by every
+    client.  It then reports an average of those values, referred to
+    as the latency for each statement, after the benchmark has finished.
+   </para>
+
+   <para>
+    For the default script, the output will look similar to this:
+ <screen>
+ starting vacuum...end.
+ transaction type: TPC-B (sort of)
+ scaling factor: 1
+ query mode: simple
+ number of clients: 10
+ number of threads: 1
+ number of transactions per client: 1000
+ number of transactions actually processed: 10000/10000
+ tps = 618.764555 (including connections establishing)
+ tps = 622.977698 (excluding connections establishing)
+ statement latencies in milliseconds:
+     0.004386    \set nbranches 1 * :scale
+     0.001343    \set ntellers 10 * :scale
+     0.001212    \set naccounts 100000 * :scale
+     0.001310    \setrandom aid 1 :naccounts
+     0.001073    \setrandom bid 1 :nbranches
+     0.001005    \setrandom tid 1 :ntellers
+     0.001078    \setrandom delta -5000 5000
+     0.326152    BEGIN;
+     0.603376    UPDATE pgbench_accounts SET abalance = abalance + :delta WHERE aid = :aid;
+     0.454643    SELECT abalance FROM pgbench_accounts WHERE aid = :aid;
+     5.528491    UPDATE pgbench_tellers SET tbalance = tbalance + :delta WHERE tid = :tid;
+     7.335435    UPDATE pgbench_branches SET bbalance = bbalance + :delta WHERE bid = :bid;
+     0.371851    INSERT INTO pgbench_history (tid, bid, aid, delta, mtime) VALUES (:tid, :bid, :aid, :delta,
CURRENT_TIMESTAMP);
+     1.212976    END;
+ </screen>
+   </para>
+
+   <para>
+    If multiple script files are specified, the averages are reported
+    separately for each individual script file:
+ <screen>
+ starting vacuum...end.
+ transaction type: Custom query
+ scaling factor: 1
+ query mode: simple
+ number of clients: 10
+ number of threads: 1
+ number of transactions per client: 1000
+ number of transactions actually processed: 10000/10000
+ tps = 162.902251 (including connections establishing)
+ tps = 163.166833 (excluding connections establishing)
+ statement latencies in milliseconds, file 1:
+     2.534675    SELECT * FROM pgbench_branches LIMIT 1;
+     1.168928    SELECT COUNT(*) FROM pgbench_branches;
+ statement latencies in milliseconds, file 2:
+     2.394322    SELECT * FROM pgbench_accounts LIMIT 1;
+     113.624613    SELECT COUNT(*) FROM pgbench_accounts;
+ </screen>
+   </para>
+   <para>
+    Note that collecting the additional timing information needed for
+    detailed latency computation does add some overhead.  This will slow
+    average execution speed and lower the computed TPS.  The exact amount
+    of slowdown varies significantly based on platform and hardware.
+    Comparing average TPS values with and without latency reporting enabled
+    is a good way to measure if the timing overhead is significant.
+   </para>
+  </sect2>
+
+  <sect2>
    <title>Good Practices</title>

    <para>