On Sun, Apr 24, 2005 at 10:59:40PM +0200, Wolfgang Denk wrote: > Dear Marcelo,
Hi Wolfgang! > thanks for starting this discussion, and for providing a patch for 8xx. Thanks so much for spending the time to do this, some very interesting results inside.. > However, I think we should not only look at the TLB handling problems > on the 8xx processors. This is probably just a part of the problem. > In general the 2.6 performance on (small) embedded systems is much, > much worse than what we see with a 2.4 kernel. > > I put some results (2.4.25 vs. 2.6.11.7 on a MPC860 and on a MPC8240) > at http://www.denx.de/twiki/bin/view/Know/Linux24vs26 > > Here is the summary: > > Using the 2.6 kernel on embedded systems implicates the following > disadvantages: > * Slow to build: 2.6 takes 30...40% longer to compile > * Big memory footprint in flash: the 2.6 compressed kernel image is > 30...40% bigger > > * Big memory footprint in RAM: the 2.6 kernel needs 30...40% more > RAM; the available RAM size for applications is 700kB smaller I've shrank the v2.6 kernel build to a size significantly smaller than our v2.4 build, and performance did not increase at all. >From that, I could figure that the performance problem, in this case, was not related to decreased available free memory. From then on I started going the TLB direction. But yes, in general, v2.6 image is bigger and memory consumption is higher than v2.4. One important project in this area is linux-tiny, which allows one to disable unwanted features. > * Slow to boot: 2.6 takes 5...15% longer to boot into multi-user mode Others have mentioned, and I agree, that sysfs is likely to be the major cause for boot-time slowdown. Have you tried disabling sysfs? > * Slow to run: context switches up to 96% slower, local communication > latencies up to 80% slower, file system latencies up to 76% slower, > local communication bandwidth less than 50% in some cases. I've noticed the v2.6 scheduler context switching _more_ than v2.4... Question: Such huge regressions are seen on MPC8xx only, MPC82xx slowdown is not so bad, correct? > It's a disappointing result, indeed. Yes we are in bad shape :(
