From d1f822e58597cac5000bf69b893cc236c5ef5fcb Mon Sep 17 00:00:00 2001
From: Robert Haas <rhaas@postgresql.org>
Date: Thu, 7 Jul 2016 11:18:51 -0400
Subject: [PATCH] Clarify resource utilization of parallel query.
temp_file_limit is a per-process limit, not a per-session limit across
all cooperating parallel processes; change wording accordingly, per a
suggestion from Tom Lane.
Also, document under max_parallel_workers_per_gather the fact that each
process involved in a parallel query may use as many resources as a
separate session. Caveat emptor.
Per a complaint from Peter Geoghegan.
---
doc/src/sgml/config.sgml | 20 +++++++++++++++++--
src/backend/utils/misc/guc.c | 2 +-
src/backend/utils/misc/postgresql.conf.sample | 2 +-
3 files changed, 20 insertions(+), 4 deletions(-)
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 061697b54cb..4e8c982dd59 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -1603,7 +1603,7 @@ include_dir 'conf.d'
</term>
<listitem>
<para>
- Specifies the maximum amount of disk space that a session can use
+ Specifies the maximum amount of disk space that a process can use
for temporary files, such as sort and hash temporary files, or the
storage file for a held cursor. A transaction attempting to exceed
this limit will be canceled.
@@ -1613,7 +1613,7 @@ include_dir 'conf.d'
</para>
<para>
This setting constrains the total space used at any instant by all
- temporary files used by a given <productname>PostgreSQL</> session.
+ temporary files used by a given <productname>PostgreSQL</> process.
It should be noted that disk space used for explicit temporary
tables, as opposed to temporary files used behind-the-scenes in query
execution, does <emphasis>not</emphasis> count against this limit.
@@ -2011,6 +2011,22 @@ include_dir 'conf.d'
be inefficient. The default value is 2. Setting this value to 0
disables parallel query execution.
</para>
+
+ <para>
+ Note that parallel queries may consume very substantially more
+ resources than non-parallel queries, because each worker process is
+ a completely separate process which has roughly the same impact on the
+ system as an additional user session. This should be taken into
+ account when choosing a value for this setting, as well as when
+ configuring other settings that control resource utilization, such
+ as <xref linkend="guc-work-mem">. Resource limits such as
+ <varname>work_mem</> are applied individually to each worker,
+ which means the total utilization may be much higher across all
+ processes than it would normally be for any single process.
+ For example, a parallel query using 4 workers may use up to 5 times
+ as much CPU time, memory, I/O bandwidth, and so forth as a query which
+ uses no workers at all.
+ </para>
</listitem>
</varlistentry>
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 60148b871b3..6ac5184b374 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -1947,7 +1947,7 @@ static struct config_int ConfigureNamesInt[] =
{
{"temp_file_limit", PGC_SUSET, RESOURCES_DISK,
- gettext_noop("Limits the total size of all temporary files used by each session."),
+ gettext_noop("Limits the total size of all temporary files used by each process."),
gettext_noop("-1 means no limit."),
GUC_UNIT_KB
},
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index a0abc2c2e00..5faf22a37a5 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -138,7 +138,7 @@
# - Disk -
-#temp_file_limit = -1 # limits per-session temp file space
+#temp_file_limit = -1 # limits per-process temp file space
# in kB, or -1 for no limit
# - Kernel Resource Usage -
--
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