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Linux 2.4 VM Documentation
popoutman writes "Mel Gorman has announced the availability of a guide to the 2.4 kernel VM
including a walkthrough of the VM code. Anyone interested in obtaining a solid understanding of the Linux 2.4 VM will certainly want to take a look at this documentation. Mel says that the effort is at least several weeks from being finished, but that he's releasing it now with the hopes of getting feedback to be sure he's on the right track.
He also notes that the 2.5 VM is still too much of a moving target for him to document it just yet." See also a Kerneltrap story.
We design aned document things first, work out the bugs in the high level design and then code.
Well maybe not all the time and with serveral itterations, but I only manage your credit raiting, not you kernel VM.
thank God the internet isn't a human right.
...that virtual memory works because of small, magical faeries and gnomes.
/syle
Define your function "Olog", please. Surely Mr. "Wagner LLC Consulting Co. - Getting it right the first time" couldn't have meant O(log n)... :)
People who don't know what it stands for would not want to read the document anyway, don't you think?
This is a good question -- there are several features the VM system allows for in addition to overcommitting physical memory. If your system has enough memory to run with no swap/paging space set up, the VM system still provides these features without ever paging or swapping out memory.
The first is demand paging. That is, only those pages of an executable that are needed are brought into memory, and on-demand. Yes, this saves memory, but more importantly, it makes program startup much faster. Without demand paging, the whole program would need to be brought into memory at startup. Nowadays, when disks are much slower relative to CPUs, than they used to be, this makes a big difference, especially in the Unix shell-pipeline style of programming, where you run many different short-lived programs.
Secondly, the VM system allows for shared memory segments between processes. This allows for shared memory, threads and shared libraries.
Finally, the VM system implements caching of the file system, which we all know and love as a good use for all the memory we stuff into our machines now.
Good point -- this is such a given on Unix systems that I didn't word it very well. What I should have said is that it enforces memory separation and protection between most memory segments and most processes, and allows for sharing of segments when explicitly setup. This is perhaps the most important thing the VM system does.
yeah, right. let's also integrate to the kernel Perl, Python, Tcl, Lua, Emacs (for Elisp), Guile, Hugs, OCAML, Bash, Apache (for PHP) and Gecko (I want my Mozilla to work faster too!). I wonder, why is X server still not there? And don't forget about at least two CORBA brokers: Gnorba (everyone would love faster Gnome) and OmniOrb (just for a case). Hey, let's put everything into the kernel! Ooops... It's not kernel anymore and it doesn't want to run either. What was the mistake?
Less is more !
I suspect it would be a waste of your time. Lets look at a bit of history.
There used to be a kernel-space HTTP server. It was integrated into the kernel for a specific reason: zero-copy access to the network interface memory. It was fast and relatively feature-poor. If it crashed (fortunately, a rare occurance), you got a kernel panic.
Along came a user-space, zero-copy HTTP server. It was faster and had a few more features to boot. Being a user-space program, if it crashed, you got a core-dump. It could also be run in a chroot jail, a gigantic step more secure than running in ring-0.
Two lessons can be read from this:
1. Don't integrate something with the kernel unless there is a specific advantage you hope to gain from it. Will making a JVM part of the kernel really speed it up? Are you sure?
2. Don't under-estimate the speed of a properly designed user-space Linux program. The kernel developers have done a magnificant job tuning the kernel and providing APIs for performance-critical apps.
Because it's important. :-)
Some people think that virtual memory means paging to secondary memory, such as disk. It doesn't. Virtual memory systems can support this, but many OSes (e.g. QNX) support virtual memory with no disk paging. (OK, QNX does support disk paging, but only as an afterthought, so that QNX can be self-compiled. GCC takes a lot of memory.)
Virtual memory provides a virtual address space for each process. The benefits include:
This is just off the top of my head.
sub f{($f)=@_;print"$f(q{$f});";}f(q{sub f{($f)=@_;print"$f(q{$f});";}f});