Message-ID: <4C0014AB.4060707@2ndquadrant.com>
Date: Fri, 28 May 2010 15:08:27 -0400
From: Greg Smith <greg@2ndquadrant.com>
User-Agent: Thunderbird 2.0.0.24 (X11/20100317)
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To: Balkrishna Sharma <b_ki@hotmail.com>
CC: pgsql-admin@postgresql.org
Subject: Re: Relation between RAM / shmmax / shmall / shared_buffers
References: <BAY149-w5102A9288345674F2C689AF0E80@phx.gbl>
In-Reply-To: <BAY149-w5102A9288345674F2C689AF0E80@phx.gbl>
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Balkrishna Sharma wrote:
> 1. Keeping the kernel parameter kernel.shmmax at 75% of RAM (i.e. at 
> 3GB )kernel.shmmax = 3221225472
> 2. Keeping the kernel parameter shmall at the same value. Because 
> shmall is measured in number of pages and each page on my linux is 
> 4096 bytes, having kernel.shmall = 786432     (786432 * 4096 = 
> 3221225472, same as shmmax)

There's little reason to put shmmax at over 50% of RAM, because 
shared_buffers is going to be significantly lower than that even.  I use 
the attached script for this job now; I got sick of doing the math 
manually all the time.  If you're on a system that supports returning 
memory info using getconf, it outputs the lines you need to put into the 
kernel configuration.

-- 
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com   www.2ndQuadrant.us


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#!/bin/bash

# Output lines suitable for sysctl configuration based
# on total amount of RAM on the system.  The output
# will allow up to 50% of physical memory to be allocated
# into shared memory.

# On Linux, you can use it as follows (as root):
# 
# ./shmsetup >> /etc/sysctl.conf
# sysctl -p

# Early FreeBSD versions do not support the sysconf interface
# used here.  The exact version where this works hasn't
# been confirmed yet.

page_size=`getconf PAGE_SIZE`
phys_pages=`getconf _PHYS_PAGES`

if [ -z "$page_size" ]; then
  echo Error:  cannot determine page size
  exit 1
fi

if [ -z "$phys_pages" ]; then
  echo Error:  cannot determine number of memory pages
  exit 2
fi

shmall=`expr $phys_pages / 2`
shmmax=`expr $shmall \* $page_size` 

echo \# Maximum shared segment size in bytes
echo kernel.shmmax = $shmmax
echo \# Maximum number of shared memory segments in pages
echo kernel.shmall = $shmall

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