On Fri, Mar 27, 2009 at 08:39:57AM +0100, Thomas H?berle wrote: > A client is currently considering to switch Linux as operating system on his > devices. > In a recent discussion the theme "virtual memory & MMU usage" came up. > All used CPUs have a MMU, but it is not used due to performance reasons as > the developers state. > Someone threw a number into the discussion and said enabling the MMU would > cause performance losses of about 30-40 percent, thus if we switch to Linux > at all it would have to be ucLinux.
Wow that's a claim I wouldn't believe without some serious detail explaining why on that particular cpu for that particular software setup it had that kind of impact. > Unfortunatley I don't have enough experience in low-level programming, so I > don't know if this is true, but I would like to know! > As far as I understand the scope of your project are systems that simply > don't have a MMU due to hardware design reasons. > Is this correct? > Of course I COULD run it on systems with a disabled MMU, but why should I if > my processor has the luxury of a MMU and I already payed for it? Is the > trade-off for hardware supported virtual memory (=enabling the MMU) really > that significant as mentioned above? What are your experiences? Virtual memory is just one of the nice features of an MMU. You also get shared libraries (well you can fake it on some uclinux types even without an MMU but with very serious limitations). You also get fork(). That's a big deal to a lot of code. Fork requires being able to copy an address space (preferably efficiently), which an MMU can help you do. It might be possible to do without an MMU, but I am not sure, since I have never seen anyone do so. You get the ability to memory map files. Handy for some tasks. You get the ability to do memory protection, so one process can't accidentally write to another processes memory. Very good for containing programming errors to a single process, and helps for debuging since a stray pointer will be much more likely to go outside the virtual memory space of the process than go outside physical memory. I am personally rather annoyed at dealing with a coldfire without an MMU, given there exists coldfires with MMUs that could run proper software. They cost a lot more unfortunately. -- Len Sorensen _______________________________________________ uClinux-dev mailing list [email protected] http://mailman.uclinux.org/mailman/listinfo/uclinux-dev This message was resent by [email protected] To unsubscribe see: http://mailman.uclinux.org/mailman/options/uclinux-dev
