Linux – What are the differences between the “generic” and “server” kernel images provided by Ubuntu

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In particular, I'm wondering if there are any patches or config adjustments made to the disk cache size in the server edition. I'm running on a small system (256M RAM), and would like to experiment with keeping the disk cache size smaller so that there's more memory available for applications.

I've found this page at Ubuntu's website, which neither answers my questions nor is about the 9.04 release.

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I'm guessing the difference is mostly in the fact that the server kernel is not preemptive, like the desktop kernel is. This is a bit complicated:

Preempting means kicking process A from the CPU in favor of process B, in simple terms. End-users expect a responsive system. Therefore, a desktop will run a preemptive kernel, which can favor user interaction above running programs. This means that the kernel can 'kick' a background program, favoring a user program by granting it runtime on the CPU, even before the background program's timeslice is over and before the background program yields the CPU.

A server on the other hand, is built to run a couple of programs in the background (like Apache) and not to interact with a user. Therefore, a server will run a non preemptive kernel, and so be optimized for running programs in the background.

You can read a bit about Ubuntu kernel configuration here. The article is a couple of years old, but I'm pretty sure the preemption bit is still one of the biggest differences between Ubuntu's desktop and server kernels. Another difference is the choice of I/O scheduler: the server uses the deadlines scheduler, the desktop the CFQ. Admittedly, I took that from the linked article.

As for disk cache, Linux caches. Period. Linux will happily use all of your RAM as disk cache. It doesn't matter if you have 512MB of RAM or 1GB of RAM: Linux will use it if it sees fit to. There is not tweaking of this that I am aware of.

Also know that there is no downside to this: as your programs require more RAM, less RAM will be used for cache. The fact that your VM has 256MB only and that some of that is used as cache, will not hamper performance of your programs: if the programs request RAM, they will get it instantly, at the cost of having less cache. That is a Good Thing ^TM. Again: memory used as cache is still available to your applications. It will be reclaimed instantly and giving to a memory hungry application if the kernel thinks that appropriate. Running a different kernel will not change this.

Related Solutions

Linux Apache2 – How to Disable All Disk Caching

I don't think you can disable all disk caching in Linux.

As a hack, you could keep running "sync; echo 3 > /proc/sys/vm/drop_caches" to flush almost anything that is cached in memory. From the console

watch -n 1 `sync; echo 3 > /proc/sys/vm/drop_caches`

would do the trick. In the above example nothing will remain cached by the kernel for more than a second, though it will have no effect on data held in memory by Apache or other processes. It may also not flush stuff from any memory-mapped files that are still open with portions locked.

If you only want nothing cached at the start of a test run, and don't care if it caches stuff during the tests, then you could just add a single call to "sync" and "echo 3 > /proc/sys/vm/drop_caches" at the start of your test run.

If your test involves scripts that access a database you will need to be able to tell your database back-end to not cache stuff in RAM between tests also.

Ubuntu – MySQL: How to get the “Maximum possible memory usage” down

You have a server with 256M, but you can't use all of that -- remember there's some OS overhead. Add to that with the fact you're over committing as other folks have mentioned and you'll definitely thrash here. 256M is only enough for a small DB, 20 connections is a lot with what you've got configured.

1) reduce your max connections to 4 (you're using 3 out of 20)

2) optimize your query cache better; 8M is really large, and 64M total is a lot based on your hits/prunes; try a 4/32 combo and see how it goes. Really I think a 2/24 combo would work for you.

3) you have no sorts requiring temp tables, why is that max_heap_table_size verb in there? Comment that out, use the defaults

4) do you actually have 128 tables? Try cutting that table_cache in half to 64 or 48

5) reduce thread_cache_size to 4

6) optimize those tables to reduce fragmenting

Those are some things to start with. It looks like you threw a bunch of numbers in a config without any actual profiling to know what you needed and have created a mess; if all else fails go back to the defaults and get rid of your custom settings and start over using some performance tuning guides you can find on Google. Get the output of SHOW VARIABLES and SHOW STATUS, find any one of a bajillion tuning guides and plug in your actual, real numbers into their equations and that'll tell you the exact-ish numbers you need to put in your config file.

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Related Solutions

Linux Apache2 – How to Disable All Disk Caching

Ubuntu – MySQL: How to get the “Maximum possible memory usage” down

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