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Re: SHMMAX problem - how much is needed?

From: Laszlo Nagy <gandalf(at)shopzeus(dot)com>
To: Peter Koczan <pjkoczan(at)gmail(dot)com>
Cc: pgsql-admin(at)postgresql(dot)org
Subject: Re: SHMMAX problem - how much is needed?
Date: 2007-11-13 21:20:18
Message-ID: 473A1512.9060307@shopzeus.com (view raw or flat)
Thread:
Lists: pgsql-admin
Peter Koczan wrote:
>> Minimum shared mem:  140731400 bytes = 135.0 MB
>> Maximum shared mem:  268435456 bytes = 256.0 MB
>>
>> kern.ipc.shmmin="140731400"
>> kern.ipc.shmmax="268435456"
>>
>> Nov 13 20:46:19 neptunus postgres[669]: [1-2] DETAIL:  Failed system
>> call was shmget(key=5432001, size=21725184, 03600).
>>     
>
> The shared memory request, according to the error message, was for ~20
> MB, far less than the 135 MB minimum limit you put in the kernel.
>   
It means that I really did not know what it means. What are the 
disadvantages of reducing shmmin to one?

> You'll need to lower the limit or raise your request size, probably
> with regards to shared_buffers, as I think you're only allocating 2 MB
> to shared_buffers. You'll want to raise that on a production machine.
>   

I used shared_buffers=2048 which is 2048*8192 bytes = 16MB. The 
documentation tells that each buffer is 8K (more precisely, BLCKSZ) and 
it also tells that some thousand is good for a production environment.

shared_buffers (integer)

Sets the number of shared memory buffers used by the database server. 
The default is typically 1000, but may be less if your kernel settings 
will not support it (as determined during initdb). Each buffer is 8192 
bytes, unless a different value of BLCKSZ was chosen when building the 
server. This setting must be at least 16, as well as at least twice the 
value of max_connections 
<http://www.postgresql.org/docs/8.1/interactive/runtime-config-connection.html#GUC-MAX-CONNECTIONS>; 
however, settings significantly higher than the minimum are usually 
needed for good performance. Values of a few thousand are recommended 
for production installations. This option can only be set at server start.




Well, this is a test server and I'm using 4096 on a production server 
but now I'm confused. What should be a good value for this?

Thanks,

   Laszlo


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Subject: Re: SHMMAX problem - how much is needed?
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