{{tag>Brouillon Hardware CPU}}

= Notes IPC shared memory

Voir :
* https://www.youtube.com/watch?v=9OZofG500Js  Processus et Sémaphores Chaker El Amrani
* https://gerardnico.com/linux/shared_memory
* http://www.thegeekstuff.com/2010/08/ipcs-command-examples/
* http://www.postgresql.org/docs/current/static/kernel-resources.html
* http://www.postgresql.org/docs/9.1/static/kernel-resources.html#SYSVIPC
* http://sysnet-adventures.blogspot.com/2014/02/omreport-fails-object-not-found.html
* ''ipcrm'' / ''ipcmk''

Voir aussi :
* https://fr.wikipedia.org/wiki/Situation_de_comp%C3%A9tition

<code bash>
#Total de la RAM
total=$(cat /proc/meminfo |grep MemTotal|awk '{print $2}')

# 90% de la mémoire physique max
ratio=0.9

# On applique le ratio et on converti en pages de 4096
shmall=$(echo $total/4096*$ratio+1 |bc)
shmmax=$(echo $total*$ratio |bc)

# On arrondi la valeur
shmall=$(echo $shmall |sed -e 's/\..*$//')
shmmax=$(echo $shmmax |sed -e 's/\..*$//')

echo $shmall > /proc/sys/kernel/shmall
echo $shmmax > /proc/sys/kernel/shmmax

# ipcs -lm
#################


#################
# Overcommit memory ?
# sysctl -w vm.overcommit_memory=2

# Page size ?
# getconf PAGE_SIZE
# cat /proc/sys/kernel/shmmni
#################

#### SEMAPHORES
# http://www.postgresql.org/docs/9.1/static/runtime-config-connection.html#GUC-MAX-CONNECTIONS
# SEMMNS is the second parameter in output of cat /proc/sys/kernel/sem (default value 32000) 
# SEMMNI is the last parameter there, default value 128 - this is the limiting factor - SEMMNI must be at least at least ceil(max_connections / 16)

# http://www.puschitz.com/TuningLinuxForOracle.shtml#TheSEMMNSParameter
# http://www.postgresql.org.es/node/229
####
</code>

Connaître la PAGE_SIZE \\
Voir https://en.wikipedia.org/wiki/Page_%28computer_memory%29
<code bash>
getconf PAGE_SIZE
cat /proc/sys/kernel/shmmni
</code>

<code ->
$ ipcs

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages    

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status      
0x0052e2c1 32768      postgres   600        56         20                      
0x00000000 17924097   wouter     600        33554432   2          dest         
0x00000000 49446914   wouter     600        524288     2          dest         
[...]

$ ipcs -i 17924097 -m -p

Shared memory Segment shmid=17924097
uid=1000    gid=1000    cuid=1000   cgid=1000
mode=01600  access_perms=0600
bytes=33554432  lpid=3808   cpid=1457   nattch=2
att_time=Fri Jul  3 10:43:28 2015  
det_time=Fri Jul  3 10:43:28 2015  
change_time=Fri Jul  3 10:03:00 2015 
</code>
== Debug

Source : http://www.makelinux.net/alp/035

The ipcs command provides information on interprocess communication facilities, including shared segments. Use the -m flag to obtain information about shared memory. For example, this code illustrates that one shared memory segment, numbered 1627649, is in use:

 
<code ->
% ipcs -m 

 

------ Shared Memory Segments -------

key       shmid     owner     perms     bytes     nattch    status 

0x00000000 1627649   user    640       25600     0 
</code>


If this memory segment was erroneously left behind by a program, you can use the ipcrm command to remove it.

<code ->
% ipcrm shm 1627649 
</code>

Periodically remove allocated semaphores which belong to the user running the plugin. This can be achieved like this (example for the user "nrpe")\\
Source : http://folk.uio.no/trondham/software/check_openmanage.html
<code bash>
ipcrm $(ipcs -s | awk '/nrpe/ {print "-s ",$2}')
</code>