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Commit38da053

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Try to avoid semaphore-related test failures on NetBSD/OpenBSD.
These two platforms have a remarkably tight default limit on thenumber of SysV semaphores in the system: SEMMNS is only 60out-of-the-box. Unless manual action is taken to raise that,we'll only be able to allocate 3 sets of 16 usable semaphoreseach, leading to initdb setting max_connections to just 20.That's problematic because the core regression tests expectto be able to launch 20 concurrent sessions, leaving us withno headroom. This seems to be the cause of intermittentbuildfarm failures on some machines.While there's no getting around the fact that you'd better raiseSEMMNS for production use on these platforms, it does seem desirablefor "make check" to pass reliably without that. We can make thathappen, at least for awhile longer, with two small changes:* Change sysv_sema.c's SEMAS_PER_SET to 19, so that we can eat upall of the available semas not just most of them.* Change initdb to make the smallest max_connections value it willconsider be 25 not 20.As of HEAD this will leave us with four free semaphores (using thedefault values for other relevant parameters such as max_wal_senders).So we won't need to consider this again until we've invented fivemore background processes. Maybe by then we can switch both theseplatforms to some other semaphore API.For the moment, do this only in master; there've not been fieldcomplaints that might justify a back-patch.Discussion:https://postgr.es/m/db2773a2-aca0-43d0-99c1-060efcd9954e@gmail.com
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+16
-10
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3 files changed

+16
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‎doc/src/sgml/runtime.sgml

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Original file line numberDiff line numberDiff line change
@@ -782,19 +782,19 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
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<row>
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<entry><varname>SEMMNI</varname></entry>
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<entry>Maximum number of semaphore identifiers (i.e., sets)</entry>
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<entry>at least <literal>ceil(num_os_semaphores /16)</literal> plus room for other applications</entry>
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<entry>at least <literal>ceil(num_os_semaphores /19)</literal> plus room for other applications</entry>
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</row>
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<row>
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<entry><varname>SEMMNS</varname></entry>
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<entry>Maximum number of semaphores system-wide</entry>
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<entry><literal>ceil(num_os_semaphores /16) *17</literal> plus room for other applications</entry>
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<entry><literal>ceil(num_os_semaphores /19) *20</literal> plus room for other applications</entry>
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</row>
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<row>
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<entry><varname>SEMMSL</varname></entry>
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<entry>Maximum number of semaphores per set</entry>
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<entry>at least17</entry>
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<entry>at least20</entry>
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</row>
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<row>
@@ -841,7 +841,7 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
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(<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
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(<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
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(<xref linkend="guc-max-wal-senders"/>), allowed background
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process (<xref linkend="guc-max-worker-processes"/>), etc., in sets of16.
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process (<xref linkend="guc-max-worker-processes"/>), etc., in sets of19.
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The runtime-computed parameter <xref linkend="guc-num-os-semaphores"/>
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reports the number of semaphores required. This parameter can be viewed
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before starting the server with a <command>postgres</command> command like:
@@ -851,17 +851,17 @@ $ <userinput>postgres -D $PGDATA -C num_os_semaphores</userinput>
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</para>
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<para>
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Each set of16 semaphores will
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also contain a17th semaphore which contains a <quote>magic
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Each set of19 semaphores will
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also contain a20th semaphore which contains a <quote>magic
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number</quote>, to detect collision with semaphore sets used by
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other applications. The maximum number of semaphores in the system
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is set by <varname>SEMMNS</varname>, which consequently must be at least
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as high as <literal>num_os_semaphores</literal> plus one extra for
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each set of16 required semaphores (see the formula in <xref
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each set of19 required semaphores (see the formula in <xref
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linkend="sysvipc-parameters"/>). The parameter <varname>SEMMNI</varname>
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determines the limit on the number of semaphore sets that can
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exist on the system at one time. Hence this parameter must be at
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least <literal>ceil(num_os_semaphores /16)</literal>.
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least <literal>ceil(num_os_semaphores /19)</literal>.
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Lowering the number
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of allowed connections is a temporary workaround for failures,
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which are usually confusingly worded <quote>No space

‎src/backend/port/sysv_sema.c

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Original file line numberDiff line numberDiff line change
@@ -50,8 +50,14 @@ typedef int IpcSemaphoreId;/* semaphore ID returned by semget(2) */
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* we allocate. It must be *less than* your kernel's SEMMSL (max semaphores
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* per set) parameter, which is often around 25. (Less than, because we
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* allocate one extra sema in each set for identification purposes.)
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*
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* The present value of 19 is chosen with one eye on NetBSD/OpenBSD's default
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* SEMMNS setting of 60. Remembering the extra sema per set, this lets us
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* allocate three sets with 57 useful semaphores before exceeding that, which
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* is enough to run our core regression tests. Users of those systems will
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* still want to raise SEMMNS for any sort of production work, though.
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*/
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#defineSEMAS_PER_SET16
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#defineSEMAS_PER_SET19
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#defineIPCProtection(0600)/* access/modify by user only */
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‎src/bin/initdb/initdb.c

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1119,7 +1119,7 @@ test_config_settings(void)
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#defineMIN_BUFS_FOR_CONNS(nconns)((nconns) * 10)
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staticconstinttrial_conns[]= {
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100,50,40,30,20
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100,50,40,30,25
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};
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staticconstinttrial_bufs[]= {
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16384,8192,4096,3584,3072,2560,2048,1536,

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