[Lugstuff] shared memory kernel

[Theodore Knab]

Thanks Thomas,

I saw that article before, but I re-read it again.

It seems paging hell seems to be the result of setting too much shared
memory.

But, what is paging hell ?


On 16/09/10 09:31 -0400, Thomas Gallen wrote:
> I believe this will answer most of your questions:
> 
> http://www.pythian.com/news/245/the-mysterious-world-of-shmmax-and-shmall/
> 
> Thomas
> 
> On Sep 16, 2010, at 8:43 AM, Theodore Knab wrote:
> 
> > Hi I was wondering if anyone knows how to tune the three shared memory parameters
> > found in the /etc/sysctl.conf.  For those of you not familiar with
> > sysctl, it holds tunable kernel configurations. 
> > 
> > http://en.wikipedia.org/wiki/Sysctl
> > 
> > Normally, you have to increase the default settings for shared
> > memory use when messing with database applications and other applications that lock a lot
> > of memory.
> > 
> > The three I am focusing on are:
> > kernel.shmax
> > kernel.shmall
> > kernel.shmni
> > 
> > More specifically, I am wondering what happens if the tuning numbers for
> > shmax are set too high ? Does the kernel think it has more RAM that it
> > does ? The reverse seems to be true. For example, if the shared memory
> > number is too low applications requiring more shared memory will crash
> > or just not start.
> > 
> > For example, my home system runs Zone Minder for keeping track of
> > camera data. The default settings for shmax of 32Mb had to be 
> > increased for this application to work.
> > 
> > In contrast, I found a Informix machine that was tuned with a
> > shmmax that I thing was outrageously high.
> > 
> > For example, I found a system with 24GB had this parameter:
> > kernel.shmmax = 4398046511104 
> > 
> > According to the Google calculation this number translates to:
> > 4 398 046 511 104 bytes = 4096 gigabytes
> > 
> > This is the default setting that IBM Informix has on their website for
> > their Database application. So, I can understand how this may have been 
> > set. However, it seems not many people would have 4096GB or more on their machine.
> > 
> > http://publib.boulder.ibm.com/infocenter/idshelp/v115/index.jsp?topic=/com.ibm.relnotes.doc/ids_1150xc5/mach/ids_machine_notes_11.50.linppc64.html
> > 
> > I have since reset the setting to 75% of the total RAM in bytes, but I am still
> > wondering what happens to a Linux system. The machine in question locked
> > up twice due to all the memory being used. This is not something that
> > created a log entry. Also, the machine seemed to become unstable when it started swapping. 
> > It was just as if all the applications on the machine were trying to consuming more RAM 
> > that the system had.  For example, recently while running a database upgrade script on the
> > machine, I could not run simple shell commands. When I tried to log in,
> > I could type a few characters but only after a few minutes of waiting.
> > I tried to type 'ctrl-alt-del' but it appeared the program the shutdown
> > program could not be run in the time I had available to wait.  
> > It was not a hard crash where errors appear. 
> > 
> > Could this be a possible symptom of the kernel.shmmax being too high ?
> > 
> > Here are the current setting I am using for the 24GB machine.
> > kernel.shmmax = 18956485632 
> > kernel.shmall = 4194304 
> > kernel.shmmni = 4096
> > 
> > I used this script to calculate the max shared memory:
> > #!/bin/sh
> > #limits the kernel's shared memory to 1/2 actually physical memory
> > mem_bytes=`awk '/MemTotal:/ { printf "%0.f",$2 * 1024}' /proc/meminfo`
> > mem_max=`expr $mem_bytes / 2` #devide by 2
> > mem_max=`expr $mem_bytes / 4` #devide by 3
> > 
> > echo "mem max is $mem_max"
> > 
> > mem_max=`expr $mem_max + $mem_max + $mem_max` #gets a 75%
> > page_size=`getconf PAGE_SIZE`
> > shmall=`expr $mem_bytes / $page_size`
> > echo \# Maximum shared segment size in bytes
> > echo kernel.shmmax = $mem_max
> > echo \# Maximum number of shared memory segments in pages
> > echo kernel.shmall = $shmall
> > 
> > 
> > -- 
> > Ted Knab
> > 
> > _______________________________________________
> > Lugstuff mailing list
> > Lugstuff at annapolislinux.org
> > http://list.annapolislinux.org/cgi-bin/mailman/listinfo/lugstuff
> 

-- 
Ted Knab
Stevensville, MD USA