Home » What are high memory and low memory on Linux?

What are high memory and low memory on Linux?

Solutons:


On a 32-bit architecture, the address space range for addressing RAM is:

0x00000000 - 0xffffffff

or 4'294'967'295 (4 GB).

The linux kernel splits that up 3/1 (could also be 2/2, or 1/3 1) into user space (high memory) and kernel space (low memory) respectively.

The user space range:

0x00000000 - 0xbfffffff

Every newly spawned user process gets an address (range) inside this area. User processes are generally untrusted and therefore are forbidden to access the kernel space. Further, they are considered non-urgent, as a general rule, the kernel tries to defer the allocation of memory to those processes.

The kernel space range:

0xc0000000 - 0xffffffff

A kernel processes gets its address (range) here. The kernel can directly access this 1 GB of addresses (well, not the full 1 GB, there are 128 MB reserved for high memory access).

Processes spawned in kernel space are trusted, urgent and assumed error-free, the memory request gets processed instantaneously.

Every kernel process can also access the user space range if it wishes to. And to achieve this, the kernel maps an address from the user space (the high memory) to its kernel space (the low memory), the 128 MB mentioned above are especially reserved for this.


1 Whether the split is 3/1, 2/2, or 1/3 is controlled by the CONFIG_VMSPLIT_... option; you can probably check under /boot/config* to see which option was selected for your kernel.

The first reference to turn to is Linux Device Drivers (available both online and in book form), particularly chapter 15 which has a section on the topic.

In an ideal world, every system component would be able to map all the memory it ever needs to access. And this is the case for processes on Linux and most operating systems: a 32-bit process can only access a little less than 2^32 bytes of virtual memory (in fact about 3GB on a typical Linux 32-bit architecture). It gets difficult for the kernel, which needs to be able to map the full memory of the process whose system call it’s executing, plus the whole physical memory, plus any other memory-mapped hardware device.

So when a 32-bit kernel needs to map more than 4GB of memory, it must be compiled with high memory support. High memory is memory which is not permanently mapped in the kernel’s address space. (Low memory is the opposite: it is always mapped, so you can access it in the kernel simply by dereferencing a pointer.)

When you access high memory from kernel code, you need to call kmap first, to obtain a pointer from a page data structure (struct page). Calling kmap works whether the page is in high or low memory. There is also kmap_atomic which has added constraints but is more efficient on multiprocessor machines because it uses finer-grained locking. The pointer obtained through kmap is a resource: it uses up address space. Once you’ve finished with it, you must call kunmap (or kunmap_atomic) to free that resource; then the pointer is no longer valid, and the contents of the page can’t be accessed until you call kmap again.

This is relevant to the Linux kernel; I’m not sure how any Unix kernel handles this.

The High Memory is the segment of memory that user-space programs can address. It cannot touch Low Memory.

Low Memory is the segment of memory that the Linux kernel can address directly. If the kernel must access High Memory, it has to map it into its own address space first.

There was a patch introduced recently that lets you control where the segment is. The tradeoff is that you can take addressable memory away from user space so that the kernel can have more memory that it does not have to map before using.

Additional resources:

  • http://tldp.org/HOWTO/KernelAnalysis-HOWTO-7.html
  • http://linux-mm.org/HighMemory

Related Solutions

Joining bash arguments into single string with spaces

[*] I believe that this does what you want. It will put all the arguments in one string, separated by spaces, with single quotes around all: str="'$*'" $* produces all the scripts arguments separated by the first character of $IFS which, by default, is a space....

AddTransient, AddScoped and AddSingleton Services Differences

TL;DR Transient objects are always different; a new instance is provided to every controller and every service. Scoped objects are the same within a request, but different across different requests. Singleton objects are the same for every object and every...

How to download package not install it with apt-get command?

Use --download-only: sudo apt-get install --download-only pppoe This will download pppoe and any dependencies you need, and place them in /var/cache/apt/archives. That way a subsequent apt-get install pppoe will be able to complete without any extra downloads....

What defines the maximum size for a command single argument?

Answers Definitely not a bug. The parameter which defines the maximum size for one argument is MAX_ARG_STRLEN. There is no documentation for this parameter other than the comments in binfmts.h: /* * These are the maximum length and maximum number of strings...

Bulk rename, change prefix

I'd say the simplest it to just use the rename command which is common on many Linux distributions. There are two common versions of this command so check its man page to find which one you have: ## rename from Perl (common in Debian systems -- Ubuntu, Mint,...

Output from ls has newlines but displays on a single line. Why?

When you pipe the output, ls acts differently. This fact is hidden away in the info documentation: If standard output is a terminal, the output is in columns (sorted vertically) and control characters are output as question marks; otherwise, the output is...

mv: Move file only if destination does not exist

mv -vn file1 file2. This command will do what you want. You can skip -v if you want. -v makes it verbose - mv will tell you that it moved file if it moves it(useful, since there is possibility that file will not be moved) -n moves only if file2 does not exist....

Is it possible to store and query JSON in SQLite?

SQLite 3.9 introduced a new extension (JSON1) that allows you to easily work with JSON data . Also, it introduced support for indexes on expressions, which (in my understanding) should allow you to define indexes on your JSON data as well. PostgreSQL has some...

Combining tail && journalctl

You could use: journalctl -u service-name -f -f, --follow Show only the most recent journal entries, and continuously print new entries as they are appended to the journal. Here I've added "service-name" to distinguish this answer from others; you substitute...

how can shellshock be exploited over SSH?

One example where this can be exploited is on servers with an authorized_keys forced command. When adding an entry to ~/.ssh/authorized_keys, you can prefix the line with command="foo" to force foo to be run any time that ssh public key is used. With this...

Why doesn’t the tilde (~) expand inside double quotes?

The reason, because inside double quotes, tilde ~ has no special meaning, it's treated as literal. POSIX defines Double-Quotes as: Enclosing characters in double-quotes ( "" ) shall preserve the literal value of all characters within the double-quotes, with the...

What is GNU Info for?

GNU Info was designed to offer documentation that was comprehensive, hyperlinked, and possible to output to multiple formats. Man pages were available, and they were great at providing printed output. However, they were designed such that each man page had a...

Set systemd service to execute after fstab mount

a CIFS network location is mounted via /etc/fstab to /mnt/ on boot-up. No, it is not. Get this right, and the rest falls into place naturally. The mount is handled by a (generated) systemd mount unit that will be named something like mnt-wibble.mount. You can...

Merge two video clips into one, placing them next to each other

To be honest, using the accepted answer resulted in a lot of dropped frames for me. However, using the hstack filter_complex produced perfectly fluid output: ffmpeg -i left.mp4 -i right.mp4 -filter_complex hstack output.mp4 ffmpeg -i input1.mp4 -i input2.mp4...

How portable are /dev/stdin, /dev/stdout and /dev/stderr?

It's been available on Linux back into its prehistory. It is not POSIX, although many actual shells (including AT&T ksh and bash) will simulate it if it's not present in the OS; note that this simulation only works at the shell level (i.e. redirection or...

How can I increase the number of inodes in an ext4 filesystem?

It seems that you have a lot more files than normal expectation. I don't know whether there is a solution to change the inode table size dynamically. I'm afraid that you need to back-up your data, and create new filesystem, and restore your data. To create new...

Why doesn’t cp have a progress bar like wget?

The tradition in unix tools is to display messages only if something goes wrong. I think this is both for design and practical reasons. The design is intended to make it obvious when something goes wrong: you get an error message, and it's not drowned in...