Re: requested shared memory size overflows size_t

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> 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> 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: http://www.enterprisedb.com
>>>>> The Enterprise Postgres Company
>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>
>>>
>>
>>
>
>
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