Jens Schmidt wrote: > could some one explain to me what the performance difference is > between a self complied kernel and a "standard" kernel. As Edward said, there isn't much normally. HOWEVER! Fedora 686 kernels are compiled with the 4G/4G patch. This means that the kernel can handle memory sizes of over 32 GB much more easily. It also means that switching between user space and kernel space is much slower. If you don't need more than 4 GB of memory (which seems likely), you can recompile the kernel without this and regain some performance. AMD64 kernels don't need any of this malarkey. > Is it possible > to express the difference some how in percentage. Well, figures of 10% have been bandied about, but it *really* depends on what you're doing. See http://www.redhat.com/archives/fedora-test-list/2004-April/msg01906.html and the surrounding thread for details. If you've got a machine with 4 MB pages (these days, that's most of them) you may find performance improvements if you compile in stuff you always use rather than load them as modules. This is because the MMU can cover the monolithic kernel with one of those 4 MB pages, but has to use 4 KB pages for modules. So when it accesses modules, it has to use TLB (translation look-aside buffer) entries for the module, which means it can't use them for user-space. Actually, this is not too far away from being a weaker version of the 4G/4G problem: 4G/4G kernels flush the TLB each time they switch between kernel and userspace, and it's the reloading of the TLB that slows them down. There may be other, much smaller, effects. If you've got an unusual CPU, then you may find that compiling while optimising for that gives a speedup in certain areas (IIRC, some C3s don't have all the 686 instructions, but can benefit from stuff like 3D-Now). Mostly, this is only really noticable in microbenchmarks. James. -- Notice of lack of confidentiality: This E-mail message is not intended for anyone in particular. If you are not the intended recipient, or you have not received this message, I don't much care. If you have received this message in error, please call your mother. Maybe she cares. -- "Gus Bjorklund, Wizard and VP of Stuff", Progress Software
