On Thu, Apr 17, 2003 at 10:53:57AM -0700, Dennis Castleman wrote: > Anybody know the performance differences I can expect using a MIPS 5K core > @250 Mhz in 64bit mode versus 32bit mode? As a rule of thumb - less performance. 64-bit code is typically larger resulting in lower cache hit rate. And since performance optimization these days is essentially equivalent to maximising the cache hit rate going 64-bit usually means a performance drop due to the drastically larger size of code and data. On the positive side for 64-bit stuff there's the possibility to do 64-bit computations with just one instruction, move data with less instructions, use twice as many double precission fp registers that are offered by 64-bit ABIs and more calling sequences. The first two paragraphs were sort of a generic statement regarding 32-bit vs. 64-bit software on MIPS processors and affect both kernel and userspace. There's a few additional issues with the Linux kernel. The 32-bit kernels requires all memory to be addressable through KSEG0 which limits it to at most 512MB; typically the limit is more like 256MB. Memory above 512MB physical address can only be used as highmem. That's fairly inefficient and requires alot of special care when writing new kernel software. For processors that suffer from virtual aliases in their data cache highmem currently is frighteningly inefficient - and high memory pressure on lowmem doesn't help either. So from a certain point on that's simply making a 64-bit kernel is simply the better idea - even for running 32-bit software. That in particular applies to very I/O intensive stuff. In short - the right choice is a tradeoff between the hardware platform and the application's requirements. Choose wrong and you'll curse loudly :-) Ralf
