Incorrect formula for SysV IPC parameters

Started by Kyotaro HORIGUCHIalmost 10 years ago8 messages

Kyotaro HORIGUCHI

horiguchi.kyotaro@lab.ntt.co.jp

almost 10 years ago

2 attachment(s)

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

Attachments:

fix_doc_sysvipc_92-93.patchtext/x-patch; charset=us-asciiDownload

>From 6da5ad413dff4724fee75f1ba09013b6033f76ca Mon Sep 17 00:00:00 2001
From: Kyotaro Horiguchi <horiguchi.kyotaro@lab.ntt.co.jp>
Date: Wed, 3 Feb 2016 11:35:43 +0900
Subject: [PATCH] Fix the formula to calculate SEMMNI and SEMMNS in
 documentation

---
 doc/src/sgml/runtime.sgml | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index 0db3807..45579da 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -645,13 +645,13 @@ psql: could not connect to server: No such file or directory
        <row>
         <entry><varname>SEMMNI</></>
         <entry>Maximum number of semaphore identifiers (i.e., sets)</>
-        <entry>at least <literal>ceil((max_connections + autovacuum_max_workers + 4) / 16)</literal></>
+        <entry>at least <literal>ceil((max_connections + autovacuum_max_workers + 5) / 16)</literal></>
        </row>
 
        <row>
         <entry><varname>SEMMNS</></>
         <entry>Maximum number of semaphores system-wide</>
-        <entry><literal>ceil((max_connections + autovacuum_max_workers + 4) / 16) * 17</literal> plus room for other applications</>
+        <entry><literal>ceil((max_connections + autovacuum_max_workers + 5) / 16) * 17</literal> plus room for other applications</>
        </row>
 
        <row>
-- 
1.8.3.1

fix_doc_sysvipc_94-96.patchtext/x-patch; charset=us-asciiDownload

>From aaf51a50e942edfa35af20a8a1d8b8719155664b Mon Sep 17 00:00:00 2001
From: Kyotaro Horiguchi <horiguchi.kyotaro@lab.ntt.co.jp>
Date: Wed, 3 Feb 2016 11:19:58 +0900
Subject: [PATCH] Fix the formula to calculate SEMMNI and SEMMNS in
 documentation

---
 doc/src/sgml/runtime.sgml | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index 650d455..4b2e403 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -645,13 +645,13 @@ psql: could not connect to server: No such file or directory
        <row>
         <entry><varname>SEMMNI</></>
         <entry>Maximum number of semaphore identifiers (i.e., sets)</>
-        <entry>at least <literal>ceil((max_connections + autovacuum_max_workers + 4) / 16)</literal></>
+        <entry>at least <literal>ceil((max_connections + autovacuum_max_workers max_worker_processes + 5) / 16)</literal></>
        </row>
 
        <row>
         <entry><varname>SEMMNS</></>
         <entry>Maximum number of semaphores system-wide</>
-        <entry><literal>ceil((max_connections + autovacuum_max_workers + 4) / 16) * 17</literal> plus room for other applications</>
+        <entry><literal>ceil((max_connections + autovacuum_max_workers + max_worker_processes + 5) / 16) * 17</literal> plus room for other applications</>
        </row>
 
        <row>
-- 
1.8.3.1

Fujii Masao

masao.fujii@gmail.com

almost 10 years ago

In reply to: Kyotaro HORIGUCHI (#1)

Re: Incorrect formula for SysV IPC parameters

On Wed, Feb 3, 2016 at 12:51 PM, Kyotaro HORIGUCHI
<horiguchi.kyotaro@lab.ntt.co.jp> wrote:

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.

Good catch!

ISTM that you also need to change the descriptions about SEMMNI and SEMMNS
under the Table 17-1.

Regards,

--
Fujii Masao

--
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers

Kyotaro HORIGUCHI

horiguchi.kyotaro@lab.ntt.co.jp

almost 10 years ago

In reply to: Fujii Masao (#2)

2 attachment(s)

Re: Incorrect formula for SysV IPC parameters

At Thu, 4 Feb 2016 21:43:04 +0900, Fujii Masao <masao.fujii@gmail.com> wrote in <CAHGQGwHgBsat29_ZqK3aXg4a5Lsa0JUv579VkGWX3R_g0KOncw@mail.gmail.com>

On Wed, Feb 3, 2016 at 12:51 PM, Kyotaro HORIGUCHI
<horiguchi.kyotaro@lab.ntt.co.jp> wrote:

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

But actually the number of semaphores PostgreSQL needs is
calculated as following in 9.4 and later.

...

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

...

ceil((max_connections + autovacuum_max_workers + 5) / 16)

In 9.1, NUM_AUXILIARY_PROCS is 3 so the documentations is correct.

Good catch!

Thanks.

ISTM that you also need to change the descriptions about SEMMNI and SEMMNS
under the Table 17-1.

Oops! Thank you for pointing it out.

The original description doesn't mention the magic-number ('5' in
the above formulas, which previously was '4') so I haven't added
anything about it.

Process of which the number is limited by max_worker_processes is
called 'background process' (not 'backend worker') in the
documentation so I used the name to mention it in the additional
description.

The difference in the body text for 9.2, 9.3 is only a literal
'4' to '5' in the formula.

regards,

--
Kyotaro Horiguchi
NTT Open Source Software Center

Attachments:

fix_doc_sysvipc_92-93.patchtext/x-patch; charset=us-asciiDownload

>From 6ed2f296cc5899f75a2e817f470e5da07bcb0d2c Mon Sep 17 00:00:00 2001
From: Kyotaro Horiguchi <horiguchi.kyotaro@lab.ntt.co.jp>
Date: Wed, 3 Feb 2016 11:35:43 +0900
Subject: [PATCH] Fix the formula to calculate SEMMNI and SEMMNS in
 documentation

---
 doc/src/sgml/runtime.sgml | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index 0db3807..2bc0a91 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -645,13 +645,13 @@ psql: could not connect to server: No such file or directory
        <row>
         <entry><varname>SEMMNI</></>
         <entry>Maximum number of semaphore identifiers (i.e., sets)</>
-        <entry>at least <literal>ceil((max_connections + autovacuum_max_workers + 4) / 16)</literal></>
+        <entry>at least <literal>ceil((max_connections + autovacuum_max_workers + 5) / 16)</literal></>
        </row>
 
        <row>
         <entry><varname>SEMMNS</></>
         <entry>Maximum number of semaphores system-wide</>
-        <entry><literal>ceil((max_connections + autovacuum_max_workers + 4) / 16) * 17</literal> plus room for other applications</>
+        <entry><literal>ceil((max_connections + autovacuum_max_workers + 5) / 16) * 17</literal> plus room for other applications</>
        </row>
 
        <row>
@@ -712,7 +712,7 @@ psql: could not connect to server: No such file or directory
     linkend="sysvipc-parameters">).  The parameter <varname>SEMMNI</>
     determines the limit on the number of semaphore sets that can
     exist on the system at one time.  Hence this parameter must be at
-    least <literal>ceil((max_connections + autovacuum_max_workers + 4) / 16)</>.
+    least <literal>ceil((max_connections + autovacuum_max_workers + 5) / 16)</>.
     Lowering the number
     of allowed connections is a temporary workaround for failures,
     which are usually confusingly worded <quote>No space
-- 
1.8.3.1

fix_doc_sysvipc_94-96.patchtext/x-patch; charset=us-asciiDownload

>From c196c82665e4081a6f90938ac86bb63e4e60e45c Mon Sep 17 00:00:00 2001
From: Kyotaro Horiguchi <horiguchi.kyotaro@lab.ntt.co.jp>
Date: Wed, 3 Feb 2016 11:07:12 +0900
Subject: [PATCH] Fix the formula to calculate SEMMNI and SEMMNS in
 documentation

---
 doc/src/sgml/runtime.sgml | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index cda05f5..dd42dd0 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -645,13 +645,13 @@ psql: could not connect to server: No such file or directory
        <row>
         <entry><varname>SEMMNI</></>
         <entry>Maximum number of semaphore identifiers (i.e., sets)</>
-        <entry>at least <literal>ceil((max_connections + autovacuum_max_workers + 4) / 16)</literal></>
+        <entry>at least <literal>ceil((max_connections + autovacuum_max_workers + max_worker_processes + 5) / 16)</literal></>
        </row>
 
        <row>
         <entry><varname>SEMMNS</></>
         <entry>Maximum number of semaphores system-wide</>
-        <entry><literal>ceil((max_connections + autovacuum_max_workers + 4) / 16) * 17</literal> plus room for other applications</>
+        <entry><literal>ceil((max_connections + autovacuum_max_workers + max_worker_processes + 5) / 16) * 17</literal> plus room for other applications</>
        </row>
 
        <row>
@@ -699,20 +699,22 @@ psql: could not connect to server: No such file or directory
 
    <para>
     <productname>PostgreSQL</> uses one semaphore per allowed connection
-    (<xref linkend="guc-max-connections">) and allowed autovacuum worker
-    process (<xref linkend="guc-autovacuum-max-workers">), in sets of 16.
+    (<xref linkend="guc-max-connections">), allowed autovacuum worker process
+    (<xref linkend="guc-autovacuum-max-workers">) and allowed background
+    process (<xref linkend="guc-max-worker-processes">), in sets of 16.
     Each such set will
     also contain a 17th semaphore which contains a <quote>magic
     number</quote>, to detect collision with semaphore sets used by
     other applications. The maximum number of semaphores in the system
     is set by <varname>SEMMNS</>, which consequently must be at least
     as high as <varname>max_connections</> plus
-    <varname>autovacuum_max_workers</>, plus one extra for each 16
+    <varname>autovacuum_max_workers</> and <varname>max_worker_processes</>,
+    plus one extra for each 16
     allowed connections plus workers (see the formula in <xref
     linkend="sysvipc-parameters">).  The parameter <varname>SEMMNI</>
     determines the limit on the number of semaphore sets that can
     exist on the system at one time.  Hence this parameter must be at
-    least <literal>ceil((max_connections + autovacuum_max_workers + 4) / 16)</>.
+    least <literal>ceil((max_connections + autovacuum_max_workers + max_worker_processes + 5) / 16)</>.
     Lowering the number
     of allowed connections is a temporary workaround for failures,
     which are usually confusingly worded <quote>No space
-- 
1.8.3.1

Fujii Masao

masao.fujii@gmail.com

almost 10 years ago

In reply to: Kyotaro HORIGUCHI (#3)

1 attachment(s)

Re: Incorrect formula for SysV IPC parameters

On Fri, Feb 5, 2016 at 2:17 PM, Kyotaro HORIGUCHI
<horiguchi.kyotaro@lab.ntt.co.jp> wrote:

At Thu, 4 Feb 2016 21:43:04 +0900, Fujii Masao <masao.fujii@gmail.com> wrote in <CAHGQGwHgBsat29_ZqK3aXg4a5Lsa0JUv579VkGWX3R_g0KOncw@mail.gmail.com>

On Wed, Feb 3, 2016 at 12:51 PM, Kyotaro HORIGUCHI
<horiguchi.kyotaro@lab.ntt.co.jp> wrote:

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

But actually the number of semaphores PostgreSQL needs is
calculated as following in 9.4 and later.

...

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

...

ceil((max_connections + autovacuum_max_workers + 5) / 16)

In 9.1, NUM_AUXILIARY_PROCS is 3 so the documentations is correct.

Good catch!

Thanks.

ISTM that you also need to change the descriptions about SEMMNI and SEMMNS
under the Table 17-1.

Oops! Thank you for pointing it out.

The original description doesn't mention the magic-number ('5' in
the above formulas, which previously was '4') so I haven't added
anything about it.

Process of which the number is limited by max_worker_processes is
called 'background process' (not 'backend worker') in the
documentation so I used the name to mention it in the additional
description.

The difference in the body text for 9.2, 9.3 is only a literal
'4' to '5' in the formula.

Thanks for updating the patches!

They look good to me except that the formulas don't include the number of
background processes requesting shared memory access, i.e.,
GetNumShmemAttachedBgworkers(), in 9.3. Isn't it better to use the following
formula in 9.3?

ceil((max_connections + autovacuum_max_workers + number of
background proceses + 5) / 16)

Attached patch uses the above formula for 9.3. I'm thinking to push your
patches to 9.2, 9.4, 9.5, master, also push the attached one to 9.3.
Comments?

Regards,

--
Fujii Masao

Attachments:

fix_doc_sysvipc_93.patchapplication/octet-stream; name=fix_doc_sysvipc_93.patchDownload

diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index 0db3807..5990905 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -645,13 +645,13 @@ psql: could not connect to server: No such file or directory
        <row>
         <entry><varname>SEMMNI</></>
         <entry>Maximum number of semaphore identifiers (i.e., sets)</>
-        <entry>at least <literal>ceil((max_connections + autovacuum_max_workers + 4) / 16)</literal></>
+        <entry>at least <literal>ceil((max_connections + autovacuum_max_workers + number of background processes + 5) / 16)</literal></>
        </row>
 
        <row>
         <entry><varname>SEMMNS</></>
         <entry>Maximum number of semaphores system-wide</>
-        <entry><literal>ceil((max_connections + autovacuum_max_workers + 4) / 16) * 17</literal> plus room for other applications</>
+        <entry><literal>ceil((max_connections + autovacuum_max_workers + number of background processes + 5) / 16) * 17</literal> plus room for other applications</>
        </row>
 
        <row>
@@ -699,20 +699,23 @@ psql: could not connect to server: No such file or directory
 
    <para>
     <productname>PostgreSQL</> uses one semaphore per allowed connection
-    (<xref linkend="guc-max-connections">) and allowed autovacuum worker
-    process (<xref linkend="guc-autovacuum-max-workers">), in sets of 16.
+    (<xref linkend="guc-max-connections">), allowed autovacuum worker
+    process (<xref linkend="guc-autovacuum-max-workers">) and allowed
+    background processes requesting shared memory access, in sets of 16.
     Each such set will
     also contain a 17th semaphore which contains a <quote>magic
     number</quote>, to detect collision with semaphore sets used by
     other applications. The maximum number of semaphores in the system
     is set by <varname>SEMMNS</>, which consequently must be at least
     as high as <varname>max_connections</> plus
-    <varname>autovacuum_max_workers</>, plus one extra for each 16
+    <varname>autovacuum_max_workers</> plus number of background
+    processes requesting shared memory access, plus one extra for each 16
     allowed connections plus workers (see the formula in <xref
     linkend="sysvipc-parameters">).  The parameter <varname>SEMMNI</>
     determines the limit on the number of semaphore sets that can
     exist on the system at one time.  Hence this parameter must be at
-    least <literal>ceil((max_connections + autovacuum_max_workers + 4) / 16)</>.
+    least <literal>ceil((max_connections + autovacuum_max_workers +
+    number of background processes + 5) / 16)</>.
     Lowering the number
     of allowed connections is a temporary workaround for failures,
     which are usually confusingly worded <quote>No space

Kyotaro HORIGUCHI

horiguchi.kyotaro@lab.ntt.co.jp

almost 10 years ago

In reply to: Fujii Masao (#4)

Re: Incorrect formula for SysV IPC parameters

Thanks for looking at this.

At Fri, 12 Feb 2016 23:19:45 +0900, Fujii Masao <masao.fujii@gmail.com> wrote in <CAHGQGwHEnT+-S+axWKQPBYSg6z852OfgS6gDXi0Ycpq5QW=iww@mail.gmail.com>

ISTM that you also need to change the descriptions about SEMMNI and SEMMNS
under the Table 17-1.

Oops! Thank you for pointing it out.

The original description doesn't mention the magic-number ('5' in
the above formulas, which previously was '4') so I haven't added
anything about it.

Process of which the number is limited by max_worker_processes is
called 'background process' (not 'backend worker') in the
documentation so I used the name to mention it in the additional
description.

The difference in the body text for 9.2, 9.3 is only a literal
'4' to '5' in the formula.

Thanks for updating the patches!

They look good to me except that the formulas don't include the number of
background processes requesting shared memory access, i.e.,
GetNumShmemAttachedBgworkers(), in 9.3. Isn't it better to use the following
formula in 9.3?

ceil((max_connections + autovacuum_max_workers + number of
background proceses + 5) / 16)

Attached patch uses the above formula for 9.3. I'm thinking to push your
patches to 9.2, 9.4, 9.5, master, also push the attached one to 9.3.
Comments?

One concern is that users don't have any simple way to know how
many bg-workers a server runs in their current configuration.
The formula donsn't make sense without it.

regards,

--
Kyotaro Horiguchi
NTT Open Source Software Center

--
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers

Fujii Masao

masao.fujii@gmail.com

almost 10 years ago

In reply to: Kyotaro HORIGUCHI (#5)

Re: Incorrect formula for SysV IPC parameters

On Mon, Feb 15, 2016 at 2:31 PM, Kyotaro HORIGUCHI
<horiguchi.kyotaro@lab.ntt.co.jp> wrote:

Thanks for looking at this.

At Fri, 12 Feb 2016 23:19:45 +0900, Fujii Masao <masao.fujii@gmail.com> wrote in <CAHGQGwHEnT+-S+axWKQPBYSg6z852OfgS6gDXi0Ycpq5QW=iww@mail.gmail.com>

ISTM that you also need to change the descriptions about SEMMNI and SEMMNS
under the Table 17-1.

Oops! Thank you for pointing it out.

The original description doesn't mention the magic-number ('5' in
the above formulas, which previously was '4') so I haven't added
anything about it.

Process of which the number is limited by max_worker_processes is
called 'background process' (not 'backend worker') in the
documentation so I used the name to mention it in the additional
description.

The difference in the body text for 9.2, 9.3 is only a literal
'4' to '5' in the formula.

Thanks for updating the patches!

They look good to me except that the formulas don't include the number of
background processes requesting shared memory access, i.e.,
GetNumShmemAttachedBgworkers(), in 9.3. Isn't it better to use the following
formula in 9.3?

ceil((max_connections + autovacuum_max_workers + number of
background proceses + 5) / 16)

Attached patch uses the above formula for 9.3. I'm thinking to push your
patches to 9.2, 9.4, 9.5, master, also push the attached one to 9.3.
Comments?

One concern is that users don't have any simple way to know how
many bg-workers a server runs in their current configuration.

Users need to read the document of the extensions they want to load,
to see the number of background worker processes which will be running.

The formula donsn't make sense without it.

IMO, documenting "incorrect" formula can cause more troubles.

Regards,

--
Fujii Masao

--
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers

Fujii Masao

masao.fujii@gmail.com

almost 10 years ago

In reply to: Fujii Masao (#4)

Re: Incorrect formula for SysV IPC parameters

On Fri, Feb 12, 2016 at 11:19 PM, Fujii Masao <masao.fujii@gmail.com> wrote:

On Fri, Feb 5, 2016 at 2:17 PM, Kyotaro HORIGUCHI
<horiguchi.kyotaro@lab.ntt.co.jp> wrote:

At Thu, 4 Feb 2016 21:43:04 +0900, Fujii Masao <masao.fujii@gmail.com> wrote in <CAHGQGwHgBsat29_ZqK3aXg4a5Lsa0JUv579VkGWX3R_g0KOncw@mail.gmail.com>

On Wed, Feb 3, 2016 at 12:51 PM, Kyotaro HORIGUCHI
<horiguchi.kyotaro@lab.ntt.co.jp> wrote:

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

But actually the number of semaphores PostgreSQL needs is
calculated as following in 9.4 and later.

...

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

...

ceil((max_connections + autovacuum_max_workers + 5) / 16)

In 9.1, NUM_AUXILIARY_PROCS is 3 so the documentations is correct.

Good catch!

Thanks.

ISTM that you also need to change the descriptions about SEMMNI and SEMMNS
under the Table 17-1.

Oops! Thank you for pointing it out.

The original description doesn't mention the magic-number ('5' in
the above formulas, which previously was '4') so I haven't added
anything about it.

Process of which the number is limited by max_worker_processes is
called 'background process' (not 'backend worker') in the
documentation so I used the name to mention it in the additional
description.

The difference in the body text for 9.2, 9.3 is only a literal
'4' to '5' in the formula.

Thanks for updating the patches!

They look good to me except that the formulas don't include the number of
background processes requesting shared memory access, i.e.,
GetNumShmemAttachedBgworkers(), in 9.3. Isn't it better to use the following
formula in 9.3?

ceil((max_connections + autovacuum_max_workers + number of
background proceses + 5) / 16)

Attached patch uses the above formula for 9.3. I'm thinking to push your
patches to 9.2, 9.4, 9.5, master, also push the attached one to 9.3.
Comments?

Pushed. Thanks for the report and patches!

Regards,

--
Fujii Masao

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Kyotaro HORIGUCHI

horiguchi.kyotaro@lab.ntt.co.jp

almost 10 years ago

In reply to: Fujii Masao (#7)

Re: Incorrect formula for SysV IPC parameters

Thank you for pushing this.

At Tue, 16 Feb 2016 15:07:32 +0900, Fujii Masao <masao.fujii@gmail.com> wrote in <CAHGQGwGWhkDECnSB3GySt0PXigd8CqY7oj=sfUDz3ur4CmdZOw@mail.gmail.com>

Attached patch uses the above formula for 9.3. I'm thinking to push your
patches to 9.2, 9.4, 9.5, master, also push the attached one to 9.3.
Comments?

Pushed. Thanks for the report and patches!

regards,

--
Kyotaro Horiguchi
NTT Open Source Software Center

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