Red Hat

Is there any kernel tune parameters available to limit RAM usages at certain level .

Code:

EG .
RAM: 4 GB
Swap: 2 GB

I Need if my RAM usages reached 3 GB Kernel will start swaping new pages . ..


--Shirish Shukla

Why do you need this?

I suppose you could use the badram module, and tell it that vast swaths of memory are bad...

I can't see the use of this except for testing purposes, to see what happens when you start swapping...

Linux: Tuning Swappiness | KernelTrap might shed some light on it.

Yes Corona...
We have a test lab RHEL6 with 4GB RAM and have about 3GB of Swap .. that will be soon go for development phase .. before we want to test the functionality by creating high load and swap usages and test the same with monitoring tools and tuned parameters .

Thanks for your valuable inputs ..

--Shirish Shukla

Shirish,

For test purposes, the easiest way to effectively reduce the amount of physical RAM available for use by processes is to crank up the kernel parameter vm.min_free_kbytes. This sets the low water mark for the amount of memory held as free for new processes and is normally set to something like 50MB. Turning this up to 3.5GB in your example will mean the system will run out of physical memory and start actively paging very quickly. See the man pages for sysctl and sysctl.conf.

This parameter is also very useful for effectively eliminating cache and buffer space when testing I/O throughput.

Of course, the whole point of designing a system configuration is to ensure that the system would only ever actively page as an absolute last resort, one small step better than crashing. Systems should NEVER actively page in normal operation. Disk I/O (milliseconds) is literally a million times slower than RAM operations (nanoseconds), so system performance will immediately become completely unacceptable as soon as the system starts to actively page.

Active paging (shown by "sar -B" or "vmstat") is completely different from swap space usage (shown by "sar -S" or "free"). It's OK for a system to statically park pages of memory for long inactive processes on the swap partition. But, if the system is actively paging, it's reading and writing thousand of blocks to and from swap space per second.

If you even suspect that this system will start actively paging under heavy load, BUY MORE RAM.

Remember to reset vm.min_free_kbytes to the original value when your tests are complete...

Thanks for clearing this . can you pls let me confirm ..

Code:

 
RAM: 1GB
Swap: 2GB
Kernel: 2.6.3
 
AND 
# cat /proc/sys/vm/min_free_kbytes
3794

So what basis this calculated and am missing logic behind same ..some docx state that it's number of pages of physical RAM available .

What logic used to calculate this value .. ??