Simplify LWLock tranche machinery by removing array_base/array_stride.

array_base and array_stride were added so that we could identify the
offset of an LWLock within a tranche, but this facility is only very
marginally used apart from the main tranche.  So, give every lock in
the main tranche its own tranche ID and get rid of array_base,
array_stride, and all that's attached.  For debugging facilities
(Trace_lwlocks and LWLOCK_STATS) print the pointer address of the
LWLock using %p instead of the offset.  This is arguably more useful,
and certainly a lot cheaper.  Drop the offset-within-tranche from
the information reported to dtrace and from one can't-happen message
inside lwlock.c.

The main user-visible impact of this change is that pg_stat_activity
will now report all waits for LWLocks as "LWLock" rather than
reporting some as "LWLockTranche" and others as "LWLockNamed".

The main motivation for this change is that the need to specify an
array_base and an array_stride is awkward for parallel query.  There
is only a very limited supply of tranche IDs so we can't just keep
allocating new ones, and if we try to use the same tranche IDs every
time then we run into trouble when multiple parallel contexts are
use simultaneously.  So if we didn't get rid of this mechanism we'd
have to make it even more complicated.  By simplifying it in this
way, we instead reduce the size of the generated code for lwlock.c
by about 5%.

Discussion: http://postgr.es/m/CA+TgmoYsFn6NUW1x0AZtupJGUAs1UDY4dJtCN47_Q6D0sP80PA@mail.gmail.com

1 files changed, 19 insertions, 33 deletions

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 128ee13b5f7..5b58d2e84dc 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -646,18 +646,11 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
        <itemizedlist>
         <listitem>
          <para>
-          <literal>LWLockNamed</>: The backend is waiting for a specific named
-          lightweight lock.  Each such lock protects a particular data
-          structure in shared memory.  <literal>wait_event</> will contain
-          the name of the lightweight lock.
-         </para>
-        </listitem>
-        <listitem>
-         <para>
-          <literal>LWLockTranche</>: The backend is waiting for one of a
-          group of related lightweight locks.  All locks in the group perform
-          a similar function; <literal>wait_event</> will identify the general
-          purpose of locks in that group.
+          <literal>LWLock</>: The backend is waiting for a lightweight lock.
+          Each such lock protects a particular data structure in shared memory.
+          <literal>wait_event</> will contain a name identifying the purpose
+          of the lightweight lock.  (Some locks have specific names; others
+          are part of a group of locks each with a similar purpose.)
          </para>
         </listitem>
         <listitem>
@@ -825,7 +818,7 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
 
       <tbody>
        <row>
-        <entry morerows="41"><literal>LWLockNamed</></entry>
+        <entry morerows="57"><literal>LWLock</></entry>
         <entry><literal>ShmemIndexLock</></entry>
         <entry>Waiting to find or allocate space in shared memory.</entry>
        </row>
@@ -1011,7 +1004,6 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
          <entry>Waiting to read or update old snapshot control information.</entry>
         </row>
         <row>
-         <entry morerows="15"><literal>LWLockTranche</></entry>
          <entry><literal>clog</></entry>
          <entry>Waiting for I/O on a clog (transaction status) buffer.</entry>
         </row>
@@ -1279,7 +1271,7 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
  pid  | wait_event_type |  wait_event
 ------+-----------------+---------------
  2540 | Lock            | relation
- 6644 | LWLockNamed     | ProcArrayLock
+ 6644 | LWLock          | ProcArrayLock
 (2 rows)
 </programlisting>
    </para>
@@ -3347,55 +3339,49 @@ SELECT pg_stat_get_backend_pid(s.backendid) AS pid,
     </row>
     <row>
      <entry><literal>lwlock-acquire</literal></entry>
-     <entry><literal>(char *, int, LWLockMode)</literal></entry>
+     <entry><literal>(char *, LWLockMode)</literal></entry>
      <entry>Probe that fires when an LWLock has been acquired.
       arg0 is the LWLock's tranche.
-      arg1 is the LWLock's offset within its tranche.
-      arg2 is the requested lock mode, either exclusive or shared.</entry>
+      arg1 is the requested lock mode, either exclusive or shared.</entry>
     </row>
     <row>
      <entry><literal>lwlock-release</literal></entry>
-     <entry><literal>(char *, int)</literal></entry>
+     <entry><literal>(char *)</literal></entry>
      <entry>Probe that fires when an LWLock has been released (but note
       that any released waiters have not yet been awakened).
-      arg0 is the LWLock's tranche.
-      arg1 is the LWLock's offset within its tranche.</entry>
+      arg0 is the LWLock's tranche.</entry>
     </row>
     <row>
      <entry><literal>lwlock-wait-start</literal></entry>
-     <entry><literal>(char *, int, LWLockMode)</literal></entry>
+     <entry><literal>(char *, LWLockMode)</literal></entry>
      <entry>Probe that fires when an LWLock was not immediately available and
       a server process has begun to wait for the lock to become available.
       arg0 is the LWLock's tranche.
-      arg1 is the LWLock's offset within its tranche.
-      arg2 is the requested lock mode, either exclusive or shared.</entry>
+      arg1 is the requested lock mode, either exclusive or shared.</entry>
     </row>
     <row>
      <entry><literal>lwlock-wait-done</literal></entry>
-     <entry><literal>(char *, int, LWLockMode)</literal></entry>
+     <entry><literal>(char *, LWLockMode)</literal></entry>
      <entry>Probe that fires when a server process has been released from its
       wait for an LWLock (it does not actually have the lock yet).
       arg0 is the LWLock's tranche.
-      arg1 is the LWLock's offset within its tranche.
-      arg2 is the requested lock mode, either exclusive or shared.</entry>
+      arg1 is the requested lock mode, either exclusive or shared.</entry>
     </row>
     <row>
      <entry><literal>lwlock-condacquire</literal></entry>
-     <entry><literal>(char *, int, LWLockMode)</literal></entry>
+     <entry><literal>(char *, LWLockMode)</literal></entry>
      <entry>Probe that fires when an LWLock was successfully acquired when the
       caller specified no waiting.
       arg0 is the LWLock's tranche.
-      arg1 is the LWLock's offset within its tranche.
-      arg2 is the requested lock mode, either exclusive or shared.</entry>
+      arg1 is the requested lock mode, either exclusive or shared.</entry>
     </row>
     <row>
      <entry><literal>lwlock-condacquire-fail</literal></entry>
-     <entry><literal>(char *, int, LWLockMode)</literal></entry>
+     <entry><literal>(char *, LWLockMode)</literal></entry>
      <entry>Probe that fires when an LWLock was not successfully acquired when
       the caller specified no waiting.
       arg0 is the LWLock's tranche.
-      arg1 is the LWLock's offset within its tranche.
-      arg2 is the requested lock mode, either exclusive or shared.</entry>
+      arg1 is the requested lock mode, either exclusive or shared.</entry>
     </row>
     <row>
      <entry><literal>lock-wait-start</literal></entry>