Re: requested shared memory size overflows size_t

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..

default_statistics_target=10000
maintenance_work_mem=512MB
work_mem=512MB
shared_buffers=512MB
wal_buffers=128MB

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  5,610,000ms:

maintenance_work_mem=4GB
work_mem=4GB
shared_buffers=4GB
effective_cache_size=4GB
wal_buffers=1GB

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:

max_connections=3
effective_cache_size=15GB
maintenance_work_mem=5GB
shared_buffers=7000MB
work_mem=5GB

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;

Result:

"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.

Cheers,
Tom

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@gmail.com> wrote:

 

From: Tom Wilcox <hungrytom@gmail.com>
Subject: Re: [PERFORM] requested shared memory size overflows size_t
To: "Bob Lunney" <bob_lunney@yahoo.com>
Cc: "Robert Haas" <robertmhaas@gmail.com>, pgsql-performance@postgresql.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@gmail.com>      

wrote:
   

         

From: Robert Haas<robertmhaas@gmail.com>
Subject: Re: [PERFORM] requested shared memory
       

size overflows size_t
   

To: "Bob Lunney"<bob_lunney@yahoo.com>
Cc: pgsql-performance@postgresql.org,
       

"Tom Wilcox"<hungrytom@googlemail.com>
   

Date: Wednesday, June 9, 2010, 9:49 PM
On Wed, Jun 2, 2010 at 9:26 PM, Bob
Lunney<bob_lunney@yahoo.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: http://www.enterprisedb.com
The Enterprise Postgres Company

             

         

   

       

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