Re: Shared Buffer Size

Hi Preetika,

a few months ago, when I installed my first PostgreSQL, I have had the same
problem. I've try to get any information about optimal memory config, and
working, but there wasn't any "optimal memory setting calculator" on the
internet, just some guide in the posgre documentation (
http://www.postgresql.org/docs/9.0/interactive/kernel-resources.html#SYSVIPC).
I got FreeBSD 8.2 AMD64, with 8 GB of memory (this server is just for
PostgreSQL and a little PHP app with 2 user), and I have theese setting in
postgresql.conf (which are not the default):

listen_addresses = '192.168.1.1' # what IP address(es) to listen on;
port = 5432 # (change requires restart)
max_connections = 200 # (There are 20 user, with Microsoft Access client and
ODBC connections... (min 6 connection / user))

shared_buffers = 1900MB # min 128kB
temp_buffers = 64MB # min 800kB
work_mem = 64MB # min 64kB
maintenance_work_mem = 1024MB # min 1MB
max_stack_depth = 64MB # min 100kB

shared_preload_libraries = '$libdir/plpgsql.so' # (change requires restart)

checkpoint_segments = 32 # in logfile segments, min 1, 16MB each
checkpoint_timeout = 15min # range 30s-1h
checkpoint_completion_target = 0.5 # checkpoint target duration, 0.0 - 1.0

effective_cache_size = 4096MB

track_activities = on
track_counts = on
#track_functions = none # none, pl, all
#track_activity_query_size = 1024 # (change requires restart)

update_process_title = off
deadlock_timeout = 1s
max_locks_per_transaction = 256 # min 10

And the sysctl.conf from BSD, which are relevant for theese postgre
settings:
kern.ipc.shmall=524288
kern.ipc.shmmax=2147483648
kern.ipc.semmap=512
kern.ipc.shm_use_phys=1

And the last one is the loader.conf from BSD, which are relevant for theese
postgre settings:
kern.ipc.semmni=512
kern.ipc.semmns=1024
kern.ipc.semmnu=512

Theese settings based on my experience, with lot of reboot and restart and
reload config - I hope this can help you, and I accept any comment, if I
need to set everything else :-)

Thanks,
Carl
2011/5/27 preetika tyagi <preetikatyagi(at)gmail(dot)com>

> Hi Derrick,
>
> Thank you for your response.
> I saw this document and trying to understand "Interaction with the
> Operating System Cache" which is mentioned in this document.
>
> I have the following question-
> Hows does the shared buffer in Postgres rely on the Operating System
> cache?
> Suppose my RAM is 8 GB and shared_buffer is 24 MB in postgres. And there
> are some dirty pages in shared_buffer and I need to write a dirty page back
> to the disk to bring in a new page. What happens in this case? The dirty
> page will be written to the disk considering the shared_buffer size as 24
> MB? or it will not be written and will stay in RAM which is 8 GB?
>
> Thanks,
> Preetika
>
>
> On Fri, May 27, 2011 at 2:11 PM, Derrick Rice <derrick(dot)rice(at)gmail(dot)com>wrote:
>
>> Check out the "Inside the PostgreSQL Buffer Cache" link here:
>>
>> http://projects.2ndquadrant.com/talks
>>
>> Thanks to Greg Smith (active here).
>>
>> Derrick
>>
>>
>> On Fri, May 27, 2011 at 3:36 PM, preetika tyagi <preetikatyagi(at)gmail(dot)com>wrote:
>>
>>> Hi All,
>>>
>>> I am little confused about the internal working of PostgreSQL. There is a
>>> parameter shared_buffer in postgres.conf and I am assuming that it is used
>>> for buffer management in PostgreSQL. If there is a need to bring in a new
>>> page in the buffer and size exceeds the shared_buffer limit, a victim dirty
>>> page will be written back to the disk.
>>>
>>> However, I have read on many links that PostgreSQL depends on the OS for
>>> caching. (http://www.varlena.com/GeneralBits/Tidbits/perf.html#shbuf)
>>>
>>> So my question is, the actual limit of the shared buffer will be defined
>>> by OS or the shared_buffer parameter in the postgres.conf to figure whether
>>> a victim dirty page needs to be selected for disk write or not?
>>>
>>> Thanks!
>>>
>>
>>
>