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Re: requested shared memory size overflows size_t

From: Tom Wilcox <hungrytom(at)gmail(dot)com>
To: Bob Lunney <bob_lunney(at)yahoo(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: requested shared memory size overflows size_t
Date: 2010-06-14 18:53:01
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
Hi Bob,

Thanks a lot. Here's my best attempt to answer your questions:

The VM is setup with a virtual disk image dynamically expanding to fill 
an allocation of 300GB on a fast, local hard drive (avg read speed = 
778MB/s ).
WAL files can have their own disk, but how significantly would this 
affect our performance?
The filesystem of the host OS is NTFS (Windows Server 2008 OS 64), the 
guest filesystem is Ext2 (Ubuntu 64).
The workload is OLAP (lots of large, complex queries on large tables run 
in sequence).

In addition, I have reconfigured my server to use more memory. Here's a 
detailed blow by blow of how I reconfigured my system to get better 
performance (for anyone who might be interested)...

In order to increase the shared memory on Ubuntu I edited the System V 
IPC values using sysctl:

sysctl -w kernel.shmmax=16106127360*
*sysctl -w kernel.shmall=2097152

I had some fun with permissions as I somehow managed to change the 
owner  of the postgresql.conf to root where it needed to be postgres, 
resulting in failure to start the service.. (Fixed with chown 
postgres:postgres ./data/postgresql.conf and chmod u=rwx ./data -R).

I changed the following params in my configuration file..


With this config, the following command took  6,400,000ms:

EXPLAIN ANALYZE UPDATE nlpg.match_data SET org = org;

With plan:
"Seq Scan on match_data  (cost=0.00..1392900.78 rows=32237278 width=232) 
(actual time=0.379..464270.682 rows=27777961 loops=1)"
"Total runtime: 6398238.890 ms"

With these changes to the previous config, the same command took  


Resulting plan:

"Seq Scan on match_data  (cost=0.00..2340147.72 rows=30888572 width=232) 
(actual time=0.094..452793.430 rows=27777961 loops=1)"
"Total runtime: 5614140.786 ms"

Then I performed these changes to the postgresql.conf file:


And ran this query (for a quick look - can't afford the time for the 
previous tests..):

EXPLAIN ANALYZE UPDATE nlpg.match_data SET org = org WHERE match_data_id 
< 100000;


"Index Scan using match_data_pkey1 on match_data  (cost=0.00..15662.17 
rows=4490 width=232) (actual time=27.055..1908.027 rows=99999 loops=1)"
"  Index Cond: (match_data_id < 100000)"
"Total runtime: 25909.372 ms"

I then ran EntrepriseDB's Tuner on my postgres install (for a dedicated 
machine) and got the following settings and results:

EXPLAIN ANALYZE UPDATE nlpg.match_data SET org = org WHERE match_data_id 
< 100000;

"Index Scan using match_data_pkey1 on match_data  (cost=0.00..13734.54 
rows=4495 width=232) (actual time=0.348..2928.844 rows=99999 loops=1)"
"  Index Cond: (match_data_id < 100000)"
"Total runtime: 1066580.293 ms"

For now, I will go with the config using 7000MB shared_buffers. Any 
suggestions on how I can further optimise this config for a single 
session, 64-bit install utilising ALL of 96GB RAM. I will spend the next 
week making the case for a native install of Linux, but first we need to 
be 100% sure that is the only way to get the most out of Postgres on 
this machine.

Thanks very much. I now feel I am at a position where I can really 
explore and find the optimal configuration for my system, but would 
still appreciate any suggestions.


On 11/06/2010 07:25, Bob Lunney wrote:
> Tom,
> First off, I wouldn't use a VM if I could help it, however, sometimes you have to make compromises.  With a 16 Gb machine running 64-bit Ubuntu and only PostgreSQL, I'd start by allocating 4 Gb to shared_buffers.  That should leave more than enough room for the OS and file system cache.  Then I'd begin testing by measuring response times of representative queries with significant amounts of data.
> Also, what is the disk setup for the box?  Filesystem?  Can WAL files have their own disk?  Is the workload OLTP or OLAP, or a mixture of both?  There is more that goes into tuning a PG server for good performance than simply installing the software, setting a couple of GUCs and running it.
> Bob
> --- On Thu, 6/10/10, Tom Wilcox <hungrytom(at)gmail(dot)com> wrote:
>> From: Tom Wilcox <hungrytom(at)gmail(dot)com>
>> Subject: Re: [PERFORM] requested shared memory size overflows size_t
>> To: "Bob Lunney" <bob_lunney(at)yahoo(dot)com>
>> Cc: "Robert Haas" <robertmhaas(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org
>> Date: Thursday, June 10, 2010, 10:45 AM
>> Thanks guys. I am currently
>> installing Pg64 onto a Ubuntu Server 64-bit installation
>> running as a VM in VirtualBox with 16GB of RAM accessible.
>> If what you say is true then what do you suggest I do to
>> configure my new setup to best use the available 16GB (96GB
>> and native install eventually if the test goes well) of RAM
>> on Linux.
>> I was considering starting by using Enterprise DBs tuner to
>> see if that optimises things to a better quality..
>> Tom
>> On 10/06/2010 15:41, Bob Lunney wrote:
>>> True, plus there are the other issues of increased
>> checkpoint times and I/O, bgwriter tuning, etc.  It may
>> be better to let the OS cache the files and size
>> shared_buffers to a smaller value.
>>> Bob Lunney
>>> --- On Wed, 6/9/10, Robert Haas<robertmhaas(at)gmail(dot)com> 
>> wrote:
>>>> From: Robert Haas<robertmhaas(at)gmail(dot)com>
>>>> Subject: Re: [PERFORM] requested shared memory
>> size overflows size_t
>>>> To: "Bob Lunney"<bob_lunney(at)yahoo(dot)com>
>>>> Cc: pgsql-performance(at)postgresql(dot)org,
>> "Tom Wilcox"<hungrytom(at)googlemail(dot)com>
>>>> Date: Wednesday, June 9, 2010, 9:49 PM
>>>> On Wed, Jun 2, 2010 at 9:26 PM, Bob
>>>> Lunney<bob_lunney(at)yahoo(dot)com>
>>>> wrote:
>>>>> Your other option, of course, is a nice 64-bit
>> linux
>>>> variant, which won't have this problem at all.
>>>> Although, even there, I think I've heard that
>> after 10GB
>>>> you don't get
>>>> much benefit from raising it further.  Not
>> sure if
>>>> that's accurate or
>>>> not...
>>>> -- Robert Haas
>>>> EnterpriseDB:
>>>> The Enterprise Postgres Company

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