pseudo swap and performance

[Suvarna Galgali]

Hi

We have following configuration
server 1 ( memory utlization > 70 % )
alloc used
device 4.0GB 0MB 1
device 3.9Gb 0MB 0
pseudo-swap 12.9GB 2.0 GB na

Server 2 ( memory utilization < 40 %)

device 2.GB 0MB 1
device 31.2GB 0MB 1
pseudo-swap 13.0GB 1.2GB na

both machine has physical memory 16 GB.

questions
1) Do we have allocate more swap on server1 to bring down the the memory utilization ?
2) Do we really need 31 GB device space on server 2 when we don't hv any performance issues
3) How much we should allocate to both servers

[A. Clay Stephenson]

All the device swap that you really need in order to fully utilize your address space is 25% of RAM (and that is even exagerated because of kernel data structures like buffer cahce); yours is a perfect of example of the application of pseudo-swap; you have a machine with lots of memory and you really don't want to assign disk that will never be used. If you are concerned about > 70% memory usage, why? What you should really monitor is the amount of DEVICE swap that is being used. If it is not (or very rarely) used then you have enough.

The old rules about 2-3X memory size for swap really don't apply --- and remember 2-3X grows 4-6X when mirrored; that's a lot of disk that does nothing. One rule to remember is that every device swap region should be mirrored.

I also am a firm believer that swap spaces have absolutely nothing to do with dump spaces and you definitely do not want to mirror dump spaces.

[Michael Schulte zur Sur]

Hi ACS,

not mirror dump space? What happens, if the disk goes and the OS decides for once to claim it. No crash?

thanks,

Michael

[A. Clay Stephenson]

By definition, when a system needs dump space it is already crashing so the loss of a valid dump area will not crash a system. There are two reasons not to mirror dump space: 1) Very wasteful of disk that will very rarely (hopefully never) be used. Do you now want to be twice as wasteful? 2) When mirrored, the dump space is literally a crapshoot; the system is going to dump to 1 of the mirrors. The last thing I want to do is figure out which of the two physical disks actually houses the dump

[Steve Steel]

Hi


You have already had good advice but for memory usage background see

Memory Usage - â What is using all of the memoryâ ?

by:eric.herberholz@hp.com

Last modified: July 26, 2004

Latest version available at external ftp site:

ftp://eh:spear9@hprc.external.hp.com/memory.htm



It gives you some really good tools to see what is happening


Steve

[Ted Buis]

If you don't have any page outs then you don't need to do anything. If you do have page outs, the solution is often to purchase more RAM, but first check that your buffer cache is not set too high (rarely any need to go over 500MB). If you have page outs, and you can't add RAM, then pseudo swap might hurt you. This is because with pseudo swap turned on some RAM pages cannot be paged out to disk. This can force those page that can be paged out to be paged out more frequently. It is similar locking a process in RAM, except that the pages chosen to be locked are effectively selected by chance.

To answer your specific questions:
1) Only consider adding more swap if page outs are occuring or if process can't run do to lack of virtual memory.
2) It depends on the applicaions.
3) it depends on the applicatons.