Meaning of SHMALL kernel variable

JMCraig · 12-19-2007, 08:17 PM

Hi Folks,

I'm trying to set up a DB server so it has plenty of RAM (for Postgresql 8.2). The Postgres instructions suggest modifying the values for these kernel variables:

SHMMAX
SHMALL

OK. No problem, I can do that (I know where to set them dynamically and at boot time).

The examples and explanation are as follows:

Quote:

For example, to allow 128 MB, and explicitly set the maximum total shared memory size to 2097152 pages (the default):

$ sysctl -w kernel.shmmax=134217728
$ sysctl -w kernel.shmall=2097152

OK. I get SHMMAX (maximum total shared memory--I actually want more like 384MB).

Someone please tell me what kernel.shmall=2097152 pages means. What does page mean in this context and how does this relate to SHMMAX?

(Frankly, I don't want to set this value to the default, I want it to be big--the server has lots of RAM and I want to use it.)

Thanks for any insight you can give me.

John

gilead · 12-19-2007, 08:25 PM

There's some info on Werner Puschitz's Oracle site at http://www.puschitz.com/TuningLinuxF...ngSharedMemory that might help:

Code:

Setting SHMALL Parameter

This parameter sets the total amount of shared memory pages that can 
be used system wide. Hence, SHMALL should always be at least 
ceil(shmmax/PAGE_SIZE).

The default size for SHMALL in RHEL 3/4 and 2.1 is 2097152 which is also 
Oracle's recommended minimum setting for 9i and 10g on x86 and x86-64 
platforms. In most cases this setting should be sufficient since it means 
that the total amount of shared memory available on the system is 
2097152*4096 bytes (shmall*PAGE_SIZE) which is 8 GB. PAGE_SIZE is usually 
4096 bytes unless you use Big Pages or Huge Pages which supports the 
configuration of larger memory pages.

JMCraig · 12-20-2007, 08:35 AM

Thanks for the info. Most helpful to see it so clearly laid out. Is PAGE_SIZE a characteristic of the Linux install, the file system, or what?

gilead · 12-20-2007, 11:54 AM

It's memory pages they're referring to in this case. I don't know much about the nuts and bolts of it, but there's some info about optimising memory usage at http://www.puschitz.com/TuningLinuxF...ryOptimization

JMCraig · 12-20-2007, 03:16 PM

I found one reference that said that w/ 32-bit Linux, you're dealing with a 4K page. So, I tried that number ( SHMMAX / 4096 ) and that seemed to work.

alpapan · 04-28-2010, 06:07 PM

getconf PAGE_SIZE