Thread: Increasing the shared memory

http://www.postgresql.org/docs/8.1/interactive/kernel-resources.html

On 4/2/07, Sorin N. Ciolofan <ciolofan@ics.forth.gr> wrote:
>
>
>
>
>
>
>  Hello!
>
>
>
>  I'd like to ask you if there is any Postgre configuration parameter (like
> the ones defined in postgresql.conf file) that could be used for increasing
> the shared memory for Postgre?
>
>
>
> Thank you very much
>
> With best regards,
>
> Sorin


--
marius@{bsd.com.ro, joy2share.com, vrajamarii.net, ispot.ro}

In response to "Sorin N. Ciolofan" <ciolofan@ics.forth.gr>:
>
>             I've a value of 1000 set for shared_buffers, does this means
> that I use 8kbX1000=8Mb of Shared Mem?
>
>
>
> The definition from the manual is quite confusing:
>
>
>
> shared_buffers (integer)
>
> Sets the amount of memory the database server uses for shared memory
> buffers. The default is typically 32 megabytes (32MB), but may be less if
> your kernel settings will not support it (as determined during initdb). This
> setting must be at least 128 kilobytes and at least 16 kilobytes times
> max_connections
> <http://www.postgresql.org/docs/current/static/runtime-config-connection.htm
> l#GUC-MAX-CONNECTIONS> .
>
>
>
> What does the integer number represent? Number of shared buffers? If yes,
> what size does each shared buffer have?
>
> "The default is typically 32 megabytes" suggests that this integer could
> also represent the number of megabytes?!?
>
> In the postgresql.conf file is an ambiguous comment that could induce the
> idea that each shared buffer has 8 kb.
>
> So, which is the meaning of this integer?

Older versions of PostgreSQL, the number was the _number_ of shared buffers.
Each buffer is typically 8K, but this can be altered at compile time (I
believe) and the PGDG has no way to know for sure if whatever packaged version
you installed might have done so.

However, on more recent versions of Postgres (although I don't know exactly
what version first included this) you can specify this value as "M" or "G"
to specify a number of megabytes or gigabytes.  Internally, this is still
converted to a number of 8K buffers, but it makes the config file easier to
read and understand.

--
Bill Moran
http://www.potentialtech.com

There is also a add on in contrib (pg_buffercache) that can be used to
give an indication of the number of buffers in use, this can be used to
help find a 'good' shared mem size for your configuration.

David.

In response to "Sorin N. Ciolofan" <ciolofan@ics.forth.gr>:

>                 I've tried first to increase the number of shared buffers, I
> doubled it, from 1000 to 2000 (16Mb)
>
>                 Unfortunately this had no effect.

The difference between 8M and and 16M of shared buffers is pretty minor.
Try bumping it up to 250M or so and see if that helps.

You could install the pg_buffercache addon and monitor your buffer usage
to see how much is actually being used.

However, if the problem is write performance (which I'm inferring from your
message that it is) then increasing shared_buffers isn't liable to make a
significant improvement, unless the inserts are doing a lot of querying as
well.  With inserts, the speed is going to (most likely) be limited by the
speed of your disks.  I may have missed this information in earlier posts,
did you provide details of you hardware configuration?  Have you done tests
to find out what speed your disks are running?  Have you monitored IO
during your inserts to see if the IO subsystem is maxed out?

Also, the original problem you were trying to solve has been trimmed from
this thread, which makes me wonder if any of my advice is relevant.

>
>                  Then I increased the number of max_locks_per_transaction
> from 64 to 128 (these shoul assure about 12 800 lock slots) considering
> max_connections=100 and max_prepared_transaction=5  (Quote from the manual -
> The shared lock table is created to track locks on max_locks_per_transaction
> * (max_connections
> <http://www.postgresql.org/docs/8.2/interactive/runtime-config-connection.ht
> ml#GUC-MAX-CONNECTIONS>  + max_prepared_transactions
> <http://www.postgresql.org/docs/8.2/interactive/runtime-config-resource.html
> #GUC-MAX-PREPARED-TRANSACTIONS> ) objects (e.g. tables);)
>
>                  I've also restarted
>
>                  This had also no effect. Because I can't see any difference
> between the maximum input accepted for our application with the old
> configuration and the maximum input accepted now, with the new
> configuration. It looks like nothing happened.
>
>
>
> Thanks
>
> Sorin
>
>   _____
>
> From: pgsql-admin-owner@postgresql.org
> [mailto:pgsql-admin-owner@postgresql.org] On Behalf Of Shoaib Mir
> Sent: Monday, April 02, 2007 6:02 PM
> To: Sorin N. Ciolofan
> Cc: pgsql-general@postgresql.org; pgsql-admin@postgresql.org
> Subject: Re: [ADMIN] Increasing the shared memory
>
>
>
> An extract from --> http://www.powerpostgresql.com/PerfList/ might help
> you....
>
> shared_buffers:
>
> As a reminder: This figure is NOT the total memory PostgreSQL has to work
> with. It is the block of dedicated memory PostgreSQL uses for active
> operations, and should be a minority of your total RAM on the machine, since
> PostgreSQL uses the OS disk cache as well. Unfortunately, the exact amount
> of shared buffers required is a complex calculation of total RAM, database
> size, number of connections, and query complexity. Thus it's better to go
> with some rules of thumb in allocating, and monitor the server (particuarly
> pg_statio views) to determine adjustments.
> On dedicated servers, useful values seem to be between between 8MB and 400MB
> (between 1000 and 50,000 for 8K page size). Factors which raise the desired
> shared buffers are larger active portions of the database, large complex
> queries, large numbers of simultaneous queries, long-running procedures or
> transactions, more available RAM, and faster/more CPUs. And, of course,
> other applications on the machine. Contrary to some expectations, allocating
> much too much shared_buffers can actually lower peformance, due time
> required for scanning. Here's some examples based on anecdotes and TPC tests
> on Linux machines:
>
>     * Laptop, Celeron processor, 384MB RAM, 25MB database: 12MB/1500
>     * Athlon server, 1GB RAM, 10GB decision-support database: 120MB/15000
>     * Quad PIII server, 4GB RAM, 40GB, 150-connection heavy transaction
> processing database: 240MB/30000
>     * Quad Xeon server, 8GB RAM, 200GB, 300-connection heavy transaction
> processing database: 400MB/50000
>
> Please note that increasing shared_buffers, and a few other memory
> parameters, will require you to modify your operating system's System V
> memory parameters. See the main PostgreSQL documentation for instructions on
> this.
>
> --
> Shoaib Mir
> EnterpriseDB (www.enterprisedb.com)
>
>


--
Bill Moran
http://www.potentialtech.com

"Sorin N. Ciolofan" <ciolofan@ics.forth.gr> writes:
>                  This had also no effect. Because I can't see any difference
> between the maximum input accepted for our application with the old
> configuration and the maximum input accepted now, with the new
> configuration. It looks like nothing happened.

This is the first you've mentioned about *why* you wanted to increase the
settings, and what it sounds like to me is that you are increasing the
wrong thing.  What's the actual problem?

            regards, tom lane

 I will simplify the things in order to describe when the error occurred:
The input of the application is some data which is read from files on disk,
processed and then inserted in the database in one transaction. This total
quantity of data represents an integer number of data files, n*q, where q is
a file which has always 60kb and n is the positive integer.
For n=23 and shared_buffers=1000 and max_locks_per_transaction=64 the
Postgres throws the following exception:

org.postgresql.util.PSQLException: ERROR: out of shared memory
    at
org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorI
mpl.java:1525)
    at
org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.ja
va:1309)
    at
org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:188)
    at
org.postgresql.jdbc2.AbstractJdbc2Statement.execute(AbstractJdbc2Statement.j
ava:452)
    at
org.postgresql.jdbc2.AbstractJdbc2Statement.executeWithFlags(AbstractJdbc2St
atement.java:340)
    at
org.postgresql.jdbc2.AbstractJdbc2Statement.executeUpdate(AbstractJdbc2State
ment.java:286)
    at
gr.forth.ics.rdfsuite.rssdb.repr.SSRepresentation.createClassTable(SSReprese
ntation.java:1936)
    at
gr.forth.ics.rdfsuite.rssdb.repr.SSRepresentation.store(SSRepresentation.jav
a:1783)
    at
gr.forth.ics.rdfsuite.swkm.model.db.impl.RDFDB_Model.storeSchema(RDFDB_Model
.java:814)
    at
gr.forth.ics.rdfsuite.swkm.model.db.impl.RDFDB_Model.store(RDFDB_Model.java:
525)
    at
gr.forth.ics.rdfsuite.services.impl.ImporterImpl.storeImpl(ImporterImpl.java
:79)
    ... 50 more

For n=23 I estimated that we create and manipulate about 8000 tables.
One of the suggestion received here was that maybe there are not sufficient
locks slots per transaction, that's why I've increased the
max_locks_per_transaction (to 128) in order to be able to manipulate about
12 800 tables.

So, I doubled both shared_buffers and  max_locks_per_transaction and for
n=23 I received the same error. I would expect to see a difference, even a
little one, for example from n=23 to n=24 but the maximum quantity of data
accepted was the same.

Thank you very much,
With best regards
Sorin

-----Original Message-----
From: Tom Lane [mailto:tgl@sss.pgh.pa.us]
Sent: Thursday, April 12, 2007 5:01 PM
To: Sorin N. Ciolofan
Cc: 'Shoaib Mir'; pgsql-general@postgresql.org; pgsql-admin@postgresql.org;
'Dimitris Kotzinos'
Subject: Re: [ADMIN] Increasing the shared memory

"Sorin N. Ciolofan" <ciolofan@ics.forth.gr> writes:
>                  This had also no effect. Because I can't see any
difference
> between the maximum input accepted for our application with the old
> configuration and the maximum input accepted now, with the new
> configuration. It looks like nothing happened.

This is the first you've mentioned about *why* you wanted to increase the
settings, and what it sounds like to me is that you are increasing the
wrong thing.  What's the actual problem?

            regards, tom lane

"Sorin N. Ciolofan" <ciolofan@ics.forth.gr> writes:
>  I will simplify the things in order to describe when the error occurred:
> The input of the application is some data which is read from files on disk,
> processed and then inserted in the database in one transaction. This total
> quantity of data represents an integer number of data files, n*q, where q is
> a file which has always 60kb and n is the positive integer.
> For n=23 and shared_buffers=1000 and max_locks_per_transaction=64 the
> Postgres throws the following exception:

> org.postgresql.util.PSQLException: ERROR: out of shared memory

> For n=23 I estimated that we create and manipulate about 8000 tables.

Okay, as far as I know the only user-causable way to get that message is
to run out of lock-table space, and a transaction does take a lock for
each table it touches, so I concur that raising
max_locks_per_transaction is an appropriate response.  If you didn't see
any change in the maximum N you could handle then I wonder whether you
actually did raise it --- does "show max_locks_per_transaction" reflect
the intended new value?

Another possibility is that there's something about your code that makes
the number of locks involved very nonlinear in N.  You could try
checking the number of rows in pg_locks immediately before commit at
some smaller values of N to confirm what the scaling is really like.

            regards, tom lane

Dear all,

Thanks for your advices. I'd like to ask you where can I download the
pg_buffercache add-on and also where can I find some documentation about how
can I install it?

Thank you
Sorin
-----Original Message-----
From: pgsql-general-owner@postgresql.org
[mailto:pgsql-general-owner@postgresql.org] On Behalf Of Bill Moran
Sent: Thursday, April 12, 2007 4:14 PM
To: Sorin N. Ciolofan
Cc: 'Shoaib Mir'; pgsql-general@postgresql.org; pgsql-admin@postgresql.org;
'Dimitris Kotzinos'
Subject: Re: [GENERAL] [ADMIN] Increasing the shared memory

In response to "Sorin N. Ciolofan" <ciolofan@ics.forth.gr>:

>                 I've tried first to increase the number of shared buffers,
I
> doubled it, from 1000 to 2000 (16Mb)
>
>                 Unfortunately this had no effect.

The difference between 8M and and 16M of shared buffers is pretty minor.
Try bumping it up to 250M or so and see if that helps.

You could install the pg_buffercache addon and monitor your buffer usage
to see how much is actually being used.

However, if the problem is write performance (which I'm inferring from your
message that it is) then increasing shared_buffers isn't liable to make a
significant improvement, unless the inserts are doing a lot of querying as
well.  With inserts, the speed is going to (most likely) be limited by the
speed of your disks.  I may have missed this information in earlier posts,
did you provide details of you hardware configuration?  Have you done tests
to find out what speed your disks are running?  Have you monitored IO
during your inserts to see if the IO subsystem is maxed out?

Also, the original problem you were trying to solve has been trimmed from
this thread, which makes me wonder if any of my advice is relevant.

>
>                  Then I increased the number of max_locks_per_transaction
> from 64 to 128 (these shoul assure about 12 800 lock slots) considering
> max_connections=100 and max_prepared_transaction=5  (Quote from the manual
-
> The shared lock table is created to track locks on
max_locks_per_transaction
> * (max_connections
>
<http://www.postgresql.org/docs/8.2/interactive/runtime-config-connection.ht
> ml#GUC-MAX-CONNECTIONS>  + max_prepared_transactions
>
<http://www.postgresql.org/docs/8.2/interactive/runtime-config-resource.html
> #GUC-MAX-PREPARED-TRANSACTIONS> ) objects (e.g. tables);)
>
>                  I've also restarted
>
>                  This had also no effect. Because I can't see any
difference
> between the maximum input accepted for our application with the old
> configuration and the maximum input accepted now, with the new
> configuration. It looks like nothing happened.
>

In response to "Sorin N. Ciolofan" <ciolofan@ics.forth.gr>:
>
> Dear all,
>
> Thanks for your advices. I'd like to ask you where can I download the
> pg_buffercache add-on and also where can I find some documentation about how
> can I install it?

It's part of the contrib directory that ships with the source tarball.
Depending on your OS and associated packaging system (which you don't
bother to mention) it's probably available via RPMs or DEBs or whatever.
For example, under FreeBSD it's in /usr/ports/databases/postgresql-contrib

--
Bill Moran
http://www.potentialtech.com

 Dear all,

About the pg_buffercache view:
I couldn't find the description for this view in the manual at
http://www.postgresql.org/docs/8.2/interactive/catalogs.html
However I found the readme file provided in the /contrib./pg_buffercache of
the source code for version 8.2.3
Here it's written the following description:

   Column         |  references          | Description

----------------+----------------------+------------------------------------
   bufferid       |                      | Id, 1..shared_buffers.
   relfilenode    | pg_class.relfilenode | Refilenode of the relation.
   reltablespace  | pg_tablespace.oid    | Tablespace oid of the relation.
   reldatabase    | pg_database.oid      | Database for the relation.
   relblocknumber |                      | Offset of the page in the
relation.
   isdirty        |                      | Is the page dirty?

I've 2 questions:
1)
I was not able to find the field "oid" from pg_database view. Could you
please tell me what is the actual name of the column for which reldatabase
is reffering to?
2)
In readme file is also written:
"Unused buffers are shown with all fields null except buffered".
A "used" buffer means that is used 100% or could it be filled only
partially?
Is there any way to know at a certain moment with precision how much shared
memory expressed in Mb is used?

With best regards,
Sorin

In response to "Sorin N. Ciolofan" <ciolofan@ics.forth.gr>:
>
>  Dear all,
>
> About the pg_buffercache view:
> I couldn't find the description for this view in the manual at
> http://www.postgresql.org/docs/8.2/interactive/catalogs.html
> However I found the readme file provided in the /contrib./pg_buffercache of
> the source code for version 8.2.3

Since pg_buffercache is contributed software, it's not documented in the
official PostgreSQL docs.

> Here it's written the following description:
>
>    Column         |  references          | Description
>
> ----------------+----------------------+------------------------------------
>    bufferid       |                      | Id, 1..shared_buffers.
>    relfilenode    | pg_class.relfilenode | Refilenode of the relation.
>    reltablespace  | pg_tablespace.oid    | Tablespace oid of the relation.
>    reldatabase    | pg_database.oid      | Database for the relation.
>    relblocknumber |                      | Offset of the page in the
> relation.
>    isdirty        |                      | Is the page dirty?
>
> I've 2 questions:
> 1)
> I was not able to find the field "oid" from pg_database view. Could you
> please tell me what is the actual name of the column for which reldatabase
> is reffering to?

At the end of the README is an example query that I think answers your
question:
SELECT c.relname, count(*) AS buffers
               FROM pg_class c, pg_buffercache b
               WHERE b.relfilenode = c.relfilenode
               GROUP BY c.relname
               ORDER BY 2 DESC LIMIT 10;


> 2)
> In readme file is also written:
> "Unused buffers are shown with all fields null except buffered".
> A "used" buffer means that is used 100% or could it be filled only
> partially?

Yes.  The buffer is either "used" or "not used", but pg_buffercache doesn't
know what percentage of it is used.  >0% is used.  0% is not used.

> Is there any way to know at a certain moment with precision how much shared
> memory expressed in Mb is used?

The precision is +/- 1 buffer.  I expect that trying to get more precision out
of the system will result in considerable performance degradation as the
data is collected and/or tracked.

--
Bill Moran
http://www.potentialtech.com

Hello!

 Do you know which could be the reasons that could conduce an application to
not release the shared buffers, even after the application was shut down?
 I noticed that only if a pg_ctl restart command is issued some of the
buffers are set free.

Thank you very much
With best regards,
Sorin

In response to "Sorin N. Ciolofan" <ciolofan@ics.forth.gr>:

>
> Hello!
>
>  Do you know which could be the reasons that could conduce an application to
> not release the shared buffers, even after the application was shut down?
>  I noticed that only if a pg_ctl restart command is issued some of the
> buffers are set free.

The reason would be "by design."

If the server flushes its cache every time the application restarts, the
cache isn't going to be very effective.

If PostgreSQL is using more shared buffers than you're comfortable with,
reduce the shared_buffers setting in the config.  That will allow the OS
to decide how to use the memory instead.

--
Bill Moran
http://www.potentialtech.com

I don't know the algorithm on which Postgre uses the shared buffers but I'd
like to find the principles behind it. Let's assume the following scenario:
I've set shared_buffers=3000
At the starting of Postgres there are 115 buffers used by database A
After the execution of some processing caused by a java methodA1()
invocation, 2850 buffers are used by A.
What happens next if these 2850 buffers remains used even if the methodA1()
finished its execution?
Suppose that now a methodA2() invocation occurs and this method works with
database A, too. Will be the 2850 buffers reused or will postgre throw an
"out of shared memory" exception?
What happens if a methodB() invocation occurs, assuming that this method
tries to work with database B?
How Postgre decides the allocation of shared_buffers?


Thanks
Sorin

-----Original Message-----
From: pgsql-general-owner@postgresql.org
[mailto:pgsql-general-owner@postgresql.org] On Behalf Of Bill Moran
Sent: Thursday, April 26, 2007 3:32 PM
To: Sorin N. Ciolofan
Cc: pgsql-general@postgresql.org; pgsql-admin@postgresql.org
Subject: Re: [ADMIN] [GENERAL] pg_buffercache view

In response to "Sorin N. Ciolofan" <ciolofan@ics.forth.gr>:

>
> Hello!
>
>  Do you know which could be the reasons that could conduce an application
to
> not release the shared buffers, even after the application was shut down?
>  I noticed that only if a pg_ctl restart command is issued some of the
> buffers are set free.

The reason would be "by design."

If the server flushes its cache every time the application restarts, the
cache isn't going to be very effective.

If PostgreSQL is using more shared buffers than you're comfortable with,
reduce the shared_buffers setting in the config.  That will allow the OS
to decide how to use the memory instead.

--
Bill Moran
http://www.potentialtech.com

---------------------------(end of broadcast)---------------------------
TIP 4: Have you searched our list archives?

               http://archives.postgresql.org/

In response to "Sorin N. Ciolofan" <ciolofan@ics.forth.gr>:
>
> I don't know the algorithm on which Postgre uses the shared buffers but I'd
> like to find the principles behind it. Let's assume the following scenario:
> I've set shared_buffers=3000
> At the starting of Postgres there are 115 buffers used by database A
> After the execution of some processing caused by a java methodA1()
> invocation, 2850 buffers are used by A.
> What happens next if these 2850 buffers remains used even if the methodA1()
> finished its execution?

They'll be reused the next time a query needs the same data, or discarded
when the buffer space is needed for something else.

> Suppose that now a methodA2() invocation occurs and this method works with
> database A, too. Will be the 2850 buffers reused

yes

> or will postgre throw an
> "out of shared memory" exception?

no

> What happens if a methodB() invocation occurs, assuming that this method
> tries to work with database B?

Buffers will be allocated/reallocate/reused as best fits the usage pattern
of the server.

> How Postgres decides the allocation of shared_buffers?

They're "buffers".

They follow the design of just about any other type of buffer:
http://foldoc.org/index.cgi?query=buffer&action=Search

"Buffers are used to decouple processes so that the reader and writer may
operate at different speeds or on different sized blocks of data."

> -----Original Message-----
> From: pgsql-general-owner@postgresql.org
> [mailto:pgsql-general-owner@postgresql.org] On Behalf Of Bill Moran
> Sent: Thursday, April 26, 2007 3:32 PM
> To: Sorin N. Ciolofan
> Cc: pgsql-general@postgresql.org; pgsql-admin@postgresql.org
> Subject: Re: [ADMIN] [GENERAL] pg_buffercache view
>
> In response to "Sorin N. Ciolofan" <ciolofan@ics.forth.gr>:
>
> >
> > Hello!
> >
> >  Do you know which could be the reasons that could conduce an application
> to
> > not release the shared buffers, even after the application was shut down?
> >  I noticed that only if a pg_ctl restart command is issued some of the
> > buffers are set free.
>
> The reason would be "by design."
>
> If the server flushes its cache every time the application restarts, the
> cache isn't going to be very effective.
>
> If PostgreSQL is using more shared buffers than you're comfortable with,
> reduce the shared_buffers setting in the config.  That will allow the OS
> to decide how to use the memory instead.
>
> --
> Bill Moran
> http://www.potentialtech.com
>
> ---------------------------(end of broadcast)---------------------------
> TIP 4: Have you searched our list archives?
>
>                http://archives.postgresql.org/
>
>


--
Bill Moran
http://www.potentialtech.com

Hello!

I'd like to ask you what line should be added in pg_hba.conf file in order
to grant access to a user with ip
139.100.99.98 to a db named "myDB" with user "scott" with password
"mikepwd"?

After modifying this file is enough to issue
pg_ctl reload
or should I restart postgres?

Thank you
With best regards,

Sorin N. Ciolofan написа:
> Hello!
>
> I'd like to ask you what line should be added in pg_hba.conf file in order
> to grant access to a user with ip
> 139.100.99.98 to a db named "myDB" with user "scott" with password
> "mikepwd"?
>
> After modifying this file is enough to issue
> pg_ctl reload
> or should I restart postgres?
>

http://www.postgresql.org/docs/current/static/auth-pg-hba-conf.html

(Hint: look for "The pg_hba.conf file is read on...")



--
Milen A. Radev