Re: requested shared memory size overflows size_t - Mailing list pgsql-performance
| From | Dave Crooke |
|---|---|
| Subject | Re: requested shared memory size overflows size_t |
| Date | |
| Msg-id | AANLkTilht6MjUhOm0Dv3yQ2lrYoIw59oVb0PeBL7iAbv@mail.gmail.com Whole thread Raw |
| In response to | Re: requested shared memory size overflows size_t (Tom Wilcox <hungrytom@gmail.com>) |
| Responses |
Re: requested shared memory size overflows size_t
|
| List | pgsql-performance |
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.
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@gmail.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<mailto:hungrytom@gmail.com>>On Mon, Jun 14, 2010 at 1:53 PM, Tom Wilcox <hungrytom@gmail.com <mailto:hungrytom@gmail.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@gmail.com<mailto:bob_lunney@yahoo.com>> <mailto:robertmhaas@gmail.com>>,
Subject: Re: [PERFORM] requested shared memory size
overflows size_t
To: "Bob Lunney" <bob_lunney@yahoo.com
pgsql-performance@postgresql.org
<mailto:pgsql-performance@postgresql.org><mailto:robertmhaas@gmail.com>> <mailto:robertmhaas@gmail.com>>
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<mailto:bob_lunney@yahoo.com>> <mailto:pgsql-performance@postgresql.org>, <mailto:hungrytom@googlemail.com>>
Subject: Re: [PERFORM] requested shared memory
size overflows size_t
To: "Bob Lunney"<bob_lunney@yahoo.com<mailto:bob_lunney@yahoo.com>>
Date: Wednesday, June 9, 2010, 9:49 PM
On Wed, Jun 2, 2010 at 9:26 PM, Bob
Lunney<bob_lunney@yahoo.com<mailto:pgsql-performance@postgresql.org>)
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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