Convert [autovacuum_]vacuum_cost_delay into floating-point GUCs.

This change makes it possible to specify sub-millisecond delays,
which work well on most modern platforms, though that was not true
when the cost-delay feature was designed.

To support this without breaking existing configuration entries,
improve guc.c to allow floating-point GUCs to have units.  Also,
allow "us" (microseconds) as an input/output unit for time-unit GUCs.
(It's not allowed as a base unit, at least not yet.)

Likewise change the autovacuum_vacuum_cost_delay reloption to be
floating-point; this forces a catversion bump because the layout of
StdRdOptions changes.

This patch doesn't in itself change the default values or allowed
ranges for these parameters, and it should not affect the behavior
for any already-allowed setting for them.

Discussion: https://postgr.es/m/1798.1552165479@sss.pgh.pa.us

2 files changed, 14 insertions, 11 deletions

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 7bbe8f590b6..c12170b4b63 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -91,7 +91,9 @@
 
         <listitem>
          <para>
-          Valid time units are <literal>ms</literal> (milliseconds),
+          Valid time units are
+          <literal>us</literal> (microseconds),
+          <literal>ms</literal> (milliseconds),
           <literal>s</literal> (seconds), <literal>min</literal> (minutes),
           <literal>h</literal> (hours), and <literal>d</literal> (days).
          </para>
@@ -1845,7 +1847,7 @@ include_dir 'conf.d'
 
      <variablelist>
       <varlistentry id="guc-vacuum-cost-delay" xreflabel="vacuum_cost_delay">
-       <term><varname>vacuum_cost_delay</varname> (<type>integer</type>)
+       <term><varname>vacuum_cost_delay</varname> (<type>floating point</type>)
        <indexterm>
         <primary><varname>vacuum_cost_delay</varname> configuration parameter</primary>
        </indexterm>
@@ -1856,18 +1858,19 @@ include_dir 'conf.d'
          when the cost limit has been exceeded.
          The default value is zero, which disables the cost-based vacuum
          delay feature.  Positive values enable cost-based vacuuming.
-         Note that on many systems, the effective resolution
-         of sleep delays is 10 milliseconds; setting
-         <varname>vacuum_cost_delay</varname> to a value that is
-         not a multiple of 10 might have the same results as setting it
-         to the next higher multiple of 10.
         </para>
 
         <para>
          When using cost-based vacuuming, appropriate values for
          <varname>vacuum_cost_delay</varname> are usually quite small, perhaps
-         10 or 20 milliseconds.  Adjusting vacuum's resource consumption
-         is best done by changing the other vacuum cost parameters.
+         less than 1 millisecond.  While <varname>vacuum_cost_delay</varname>
+         can be set to fractional-millisecond values, such delays may not be
+         measured accurately on older platforms.  On such platforms,
+         increasing <command>VACUUM</command>'s throttled resource consumption
+         above what you get at 1ms will require changing the other vacuum cost
+         parameters.  You should, nonetheless,
+         keep <varname>vacuum_cost_delay</varname> as small as your platform
+         will consistently measure; large delays are not helpful.
         </para>
        </listitem>
       </varlistentry>
@@ -7020,7 +7023,7 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
      </varlistentry>
 
      <varlistentry id="guc-autovacuum-vacuum-cost-delay" xreflabel="autovacuum_vacuum_cost_delay">
-      <term><varname>autovacuum_vacuum_cost_delay</varname> (<type>integer</type>)
+      <term><varname>autovacuum_vacuum_cost_delay</varname> (<type>floating point</type>)
       <indexterm>
        <primary><varname>autovacuum_vacuum_cost_delay</varname> configuration parameter</primary>
       </indexterm>
diff --git a/doc/src/sgml/ref/create_table.sgml b/doc/src/sgml/ref/create_table.sgml
index 22dbc07b238..e94fe2c3b67 100644
--- a/doc/src/sgml/ref/create_table.sgml
+++ b/doc/src/sgml/ref/create_table.sgml
@@ -1385,7 +1385,7 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
    </varlistentry>
 
    <varlistentry>
-    <term><literal>autovacuum_vacuum_cost_delay</literal>, <literal>toast.autovacuum_vacuum_cost_delay</literal> (<type>integer</type>)</term>
+    <term><literal>autovacuum_vacuum_cost_delay</literal>, <literal>toast.autovacuum_vacuum_cost_delay</literal> (<type>floating point</type>)</term>
     <listitem>
      <para>
       Per-table value for <xref linkend="guc-autovacuum-vacuum-cost-delay"/>