
How much shared memory do you need?

options         SHMMNI=96               # number of shared memory identifiers default is 32
options         SHMMAXPGS=20480         # number of pages a shgment can have and the total number of pages? default 1024
                                        # max seg size is 25meg
options         SHMSEG=64               # number of segments per process
options         SEMMNI=40               # number of semaphore identifiers
options         SEMMNS=240              # number of semaphores in the system
options         MSGSEG=4096             # max number of message segments is this

With Postgres set for 512 backends, I can't start it.
I can run with 128, but at 10240 have been running out during large transactions.
I am hoping I can get it at 20480.



Wed, 29 Sep 1999 15:26:44 -0700
Jason Venner <jason@idiom.com>
shared memory 651, freebsd 2.2.7
Jason Venner <jason@idiom.com> writes:
> How much shared memory do you need?

What's probably getting you is the SEMMNS & SEMMNI limits --- we need a
semaphore per backend, and an identifier for every 16 semas.

I thought there was a discussion of kernel resource settings
somewhere in the documentation, but I'm not sure where offhand.

			regards, tom lane

Wed, 29 Sep 1999 19:48:14 -0400
Tom Lane <tgl@sss.pgh.pa.us>
Re: [HACKERS] shared memory 651, freebsd 2.2.7 
>
>
> How much shared memory do you need?
>
> options         SHMMNI=96               # number of shared memory identifiers default is 32
> options         SHMMAXPGS=20480         # number of pages a shgment can have and the total number of pages? default 1024
>                                         # max seg size is 25meg
> options         SHMSEG=64               # number of segments per process
> options         SEMMNI=40               # number of semaphore identifiers
> options         SEMMNS=240              # number of semaphores in the system
> options         MSGSEG=4096             # max number of message segments is this
>
> With Postgres set for 512 backends, I can't start it.
> I can run with 128, but at 10240 have been running out during large transactions.
> I am hoping I can get it at 20480.

    What  do  you  need 512 backends for? Such a high concurrency
    doesn't make things better (locking, spin locks etc.).

    Would eventually some kind of a middle tear application  with
    a  limited  number  of  work  processes  which connect to the
    database help?


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#========================================= wieck@debis.com (Jan Wieck) #



Thu, 30 Sep 1999 03:16:22 +0200 (MET DST)
wieck@debis.com (Jan Wieck)
Re: [HACKERS] shared memory 651, freebsd 2.2.7
At 03:16 AM 9/30/99 +0200, Jan Wieck wrote:

>    Would eventually some kind of a middle tear application  with
>    a  limited  number  of  work  processes  which connect to the
>    database help?

This is exactly how some web servers, i.e. AOLServer (which
I use) help to throttle traffic.  This server lets you
limit the number of pooled backends, when the pool's exhausted,
the web server blocks new threads until there's a free back
end.

This means you can configure your web server to service the
number of concurrent back-ends you think it can deal with,
based on its hardware configuration and the load placed on
it due to your specific queries and database contents, and
web load. 

AOLServer's been doing this for at least five years.  For
web work, it's seems to be the right place to do it, because
there are other things that impact the load the server can
deal with.  The db access throttle is just one of various
throttles one can imagine when trying to tune the entire
site.

Of course, this is web specific in detail.  Not necessarily
in concept, though...



- Don Baccus, Portland OR <dhogaza@pacifier.com>
  Nature photos, on-line guides, Pacific Northwest
  Rare Bird Alert Service and other goodies at
  http://donb.photo.net.

Wed, 29 Sep 1999 20:52:23 -0700
Don Baccus <dhogaza@pacifier.com>
Re: [HACKERS] shared memory 651, freebsd 2.2.7