From: | Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> |
---|---|
To: | pgsql-hackers(at)postgresql(dot)org |
Subject: | Incorrect formula for SysV IPC parameters |
Date: | 2016-02-03 03:51:19 |
Message-ID: | 20160203.125119.66820697.horiguchi.kyotaro@lab.ntt.co.jp |
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Hello, I found that the formulas to calculate SEMMNI and SEMMNS
are incorrect in 9.2 and later.
http://www.postgresql.org/docs/9.5/static/kernel-resources.html
All of them say that the same thing as following,
| SEMMNI Maximum number of semaphore identifiers (i.e., sets)
|
| at least ceil((max_connections + autovacuum_max_workers + 4) / 16)
|
| SEMMNS Maximum number of semaphores system-wide
|
| ceil((max_connections + autovacuum_max_workers + 4) / 16) * 17
| plus room for other applications
But actually the number of semaphores PostgreSQL needs is
calculated as following in 9.4 and later.
numSemas = MaxConnections + NUM_AUXILIARY_PROCS(=4)
MaxConnections = max_connections + autovacuum_max_workers + 1 +
max_worker_processes
So, the formula for SEMMNI should be
ceil((max_connections + autovacuum_max_workers + max_worker_processes + 5) / 16)
and SEMMNS should have the same fix.
In 9.3 and 9.2, the documentation says the same thing but
actually it is calculated as following,
numSemas = MaxConnections + NUM_AUXILIARY_PROCS(=4)
MaxConnections = max_connections + autovacuum_max_workers + 1 +
GetNumShmemAttachedBgworkers()
Omitting GetNumShmemAttachedBgworkers, the actual formula is
ceil((max_connections + autovacuum_max_workers + 5) / 16)
In 9.1, NUM_AUXILIARY_PROCS is 3 so the documentations is correct.
I attached two patches for 9.2-9.3 and 9.4-9.6dev
respectively. patch command complains a bit on applying it on
9.2.
On the platforforms that doesn't have tas operation needs
additional 1024 + 64 semaphores but I understand it as out of
scope of the documentation.
One concern is that 'at least' and 'plus room for other
applications' are mixed in the table 17-1, especially for SEMMNI
and SEMMNS. It seems to me that they should be in the same
wording, but it is not an actual problem.
regards,
--
Kyotaro Horiguchi
NTT Open Source Software Center
Attachment | Content-Type | Size |
---|---|---|
fix_doc_sysvipc_92-93.patch | text/x-patch | 1.3 KB |
fix_doc_sysvipc_94-96.patch | text/x-patch | 1.4 KB |
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