Talk:FAQ Linux Memory Management
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[edit] Available: 503 or 52 MB?
"Notice that I have 512 MB of memory in my machine, but only 52 is listed as available by free. This is mainly because the kernel can't be swapped out, so the memory it occupies could never be freed. There may also be regions of memory reserved for/by the hardware for other purposes as well, depending on the system architecture."
This doesn't make sense since it essentially means the reserved regions take up 460 MB (512 - 52 MB) - seems quite insane to me. Someone probably misunderstood this and "corrected" it from 503 to 52 MB. A look back at previous page versions confirms this. Now maybe I'm missing something here since my experince with Linux is far from extensive and I wonder why no-one else has noticed this. So I'm not making any changes and hope someone knowledgeable can confirm and correct this.
[edit] 4GB Support
There was recently some talk at LKML and the changelogs that the 880MB limit is no more. Due that fact that almost everyone has ~1G they bumped the low limit up to 1024MB from 880. In that case setting up the 4GB support is not useful if you got =<1024MB..
[edit] Support for >4 GB RAM
What about this 64G RAM support (HIGHMEM64G=y in kernel config)? Is it essential, if I want to use 8G of physical RAM? Are they any CPU performance issues? Since which kernel version is it available? Also in 2.4.x?
[edit] Per-Process Limit
I was told, there is a limit of 3G RAM, which can be maximum allocated by a single process? Is it true? How to tune this up?
A single process cannot use more than 4G of RAM on a 32bit machine. If you want more, buy a 64 bit processor. 88.64.26.14 23:43, 5 January 2007 (UTC)
The per-process limit is 3GB of virtual memory (the VIRT column in top), not 3GB of RAM. The remaining 1GB is used for kernel.
[edit] Comments moved from article 2007/04
Very nice! Maybe a quick explanation of how to see if a process is being swapped? --Ron
Very good explanation about the difference between a buffer and a cache. --Fang
Just what I was looking for. I was worried after adding 256 meg RAM to an existing 256 meg RAM because "top" showed virtually all of the memory "used". How could that be? Your article explained it wonderfully! -- Lee
Thanks-- this is the first explanation I've found of just the simple statistics reported by 'free'. --Andrew
Very accurate and simple explanation of memory usage on Linux. The article I've been looking for for years. -- Water
Thanks! this is the most useful infomation i could find about understanding Linux memory reports. -- Gerryjun
I second that. I had a hard time finding an explanation for the difference between cache and buffer. --Mirko
