Re: requested shared memory size overflows size_t

With that clarification, I stand squarely behind what others are saying ...
if performance is important to you, then you should always run databases on
dedicated hardware, with the OS running on bare metal with no
virtualization. VirtualBox has even more I/O losses than Hyper-V. It's
simply not designed for this, and you're giving away a ton of performance.

If nothing else, my confusion should indicate to you how unconventional and
poorly performing this virtualizaed setup is ... I simply assumed that the
only plausible reason you were piggybacking on virtualization on Windows was
a mandated lack of alternative options.

Reload the hardware with an OS which PGSQL supports well, and get rid of the
VirtualBox and Windows layers. If you have hardware that only Windows
supports well, then you may need to make some hardware changes.

I haven't said anything about which Unix-like OS .... you may find people
arguing passionately for BSD vs. Linux .... however, the difference between
these is negligible compared to "virtualized vs. real system", and at this
point considerations like support base, ease of use and familiarity also
come into play.

IMHO Ubuntu would be a fine choice, and PGSQL is a "first-class" supported
package from the distributor ... however, at customer sites, I've typically
used Red Hat AS because they have a corporate preference for it, even though
it is less convenient to install and manage.

On Mon, Jun 14, 2010 at 7:41 PM, Tom Wilcox <hungrytom(at)gmail(dot)com> wrote:

> Hi Dave,
>
> I am definitely able to switch OS if it will get the most out of Postgres.
> So it is definitely a case of choosing the OS on the needs if the app
> providing it is well justified.
>
> Currently, we are running Ubuntu Server 64-bit in a VirtualBox VM.
>
> Cheers,
> Tom
>
>
> Dave Crooke wrote:
>
>> Tom
>>
>> I always prefer to choose apps based on business needs, then the OS based
>> on the needs for the app.
>>
>> Cynically, I often feel that the best answer to "we have a policy that
>> says we're only allowed to use operating system x" is to ignore the policy
>> .... the kind of people ignorant enough to be that blinkered are usually not
>> tech-savvy enough to notice when it gets flouted :-)
>>
>> More seriously, is the policy "Windows only on the metal" or could you run
>> e.g. VMware ESX server? I/O is the area that takes the biggest hit in
>> virtualization, and ESX server has far less overhead loss than either
>> Hyper-V (which I presume you are using) or VMWare Workstation for NT
>> (kernels).
>>
>> If it's a Windows-only policy, then perhaps you can run those traps in
>> reverse, and switch to a Windows database, i.e. Microsoft SQL Server.
>>
>> Cheers
>> Dave
>>
>> On Mon, Jun 14, 2010 at 1:53 PM, Tom Wilcox <hungrytom(at)gmail(dot)com <mailto:
>> hungrytom(at)gmail(dot)com>> wrote:
>>
>>
>> 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(at)gmail(dot)com
>> <mailto:hungrytom(at)gmail(dot)com>> wrote:
>>
>>
>> From: Tom Wilcox <hungrytom(at)gmail(dot)com
>> <mailto:hungrytom(at)gmail(dot)com>>
>>
>> Subject: Re: [PERFORM] requested shared memory size
>> overflows size_t
>> To: "Bob Lunney" <bob_lunney(at)yahoo(dot)com
>> <mailto:bob_lunney(at)yahoo(dot)com>>
>>
>> Cc: "Robert Haas" <robertmhaas(at)gmail(dot)com
>> <mailto:robertmhaas(at)gmail(dot)com>>,
>> pgsql-performance(at)postgresql(dot)org
>> <mailto: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
>> <mailto:robertmhaas(at)gmail(dot)com>>
>>
>> wrote:
>>
>>
>> From: Robert Haas<robertmhaas(at)gmail(dot)com
>> <mailto:robertmhaas(at)gmail(dot)com>>
>>
>> Subject: Re: [PERFORM] requested shared memory
>>
>> size overflows size_t
>>
>> To: "Bob Lunney"<bob_lunney(at)yahoo(dot)com
>> <mailto:bob_lunney(at)yahoo(dot)com>>
>>
>> Cc: pgsql-performance(at)postgresql(dot)org
>> <mailto:pgsql-performance(at)postgresql(dot)org>,
>>
>>
>> "Tom Wilcox"<hungrytom(at)googlemail(dot)com
>> <mailto: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
>> <mailto: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: http://www.enterprisedb.com
>> The Enterprise Postgres Company
>>
>>
>>
>>
>>
>>
>>
>> -- Sent via pgsql-performance mailing list
>> (pgsql-performance(at)postgresql(dot)org
>> <mailto:pgsql-performance(at)postgresql(dot)org>)
>>
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>>
>>
>