Red Hat

If you really want to free up some memory space, by clearing out free page caches, directory entries and inode caches, try the below:

Code:

for i in 1 2 3; do sync; sleep 1; done; sysctl -w vm.drop_caches=3

Do not expect to see huge free memory instantly though.

Reboot should not be the first troubleshooting step, especially for Un*x.

If your server's memory is being consumed almost 100% constantly, check free -m and concentrate on used column for -/+ buffers/cache:. This tells you how much memory is used by the applications apart from the cache and file system buffers.

If that amount is too high, run sar -B and concentrate on majflt/s field. This tells you how many major page faults (when kernel does not find a page in RAM and incurs reading pages from disk swap area) are occurring per second.

Compare that data with the data gathered when your system had reasonable free memory. If you see differences are high, check which processes are eating up memory, which can be found by issuing top and sorting the RES column.

Or it means you have a poorly coded application that has a memory leak.

If you need to find which process have the most memory, consider looking at the output from:-

Code:

ps -el | sort -n -k10 | tail

This shows the heaviest memory users. The allocated memory size is in column 10 with the PID & PPID in columns 4 & 5 respectively.

I used this to prove that a new Cobol routine in development (same server as production) was hammering the memory and the code was amended before being released to production.


You don't really need to worry about real memory being 100% full, that's quite normal even if you can't see what's seeming using it. It saves the server from performing real I-O to get frequently used files. You need to think about swap more than anything else from a performance view. Memory being swapped to/from disk is costly in processing terms.

You can list swap in use with vmstat A bit in the manual page states that under the memory section the heading swpd is "the amount of virtual memory used."

I hope that this is helpful.


Robin
Liverpool/Blackburn
UK

No need to reboot for cache issue. find exact problem.
I do reboot when there is some upgrade or some specific issue that really addressed well for a reboot.
Some of my servers are running up for 2 yrs :-) Sorry I didnot patch the OS for some reasons

There are two kinds of "cache" that can and will consume all your free memory - and it's a good thing.

The first is I/O buffers. This is a cache for moving data around. If you're moving 100 GB from one physical disk to another, that data is going through this I/O buffer space. It allows for more efficient multitasking. The read disk fills the buffer, and the write disk gets its data from the buffer. It's an efficient way of doing things.

The second is file cache. This is a filesystem read cache. Frequently accessed files are stored in this read cache to make accessing them quicker.

Both are tuned and managed automatically by the kernel. There is no user intervention required. There is no need to reboot to "clear" them. If you start up a new program that requires a lot of memory, the kernel will prune the I/O buffer and the file cache automatically to make room for your new program.

You can completely ignore I/O buffer and file cache for memory usage purposes. Count only the system + user space. Take your "used" total, and subtract file cache and buffer space, and the resulting number should = system + user.

You are having the Linux Newbie Memory Freakout.

Cache memory is as good as free memory.

If your system has 99% cache, it has 99% free.

Take a deep breath and relax. Everything is fine.

Memory is under VM mnagement, so it is no longer much of an issue. Sometimes excess paging is, or lack of swap space. Mostly, memory obsessions are left over from the trauma of primitive mainframe days. So many damaged souls!

Now on a 64 bit CPU you can mmap() a 100 TB file into your memory space and walk around in it like it was memory, which of course it is as you touch pages. In fact, you can seek up in an empty file 100 TB and write something to a page, and have a 1 page file. As you write the lower pages, the file allocation goes up. Hello sparse array! The rest of the space is generated null characters, if you read where you have not written.