On Fri, Jul 15, 2005 at 06:21:44PM -0400, David Chau wrote: > I'm working on a driver for the Broadcom 1250, and I am using reserved > memory for some data buffers. The board comes with 256 MB of RAM, so I > boot Linux with "mem=253M" to reserve some RAM at the top of memory, and > then mmap() /dev/mem starting at 253 MB. > > The problem is that accessing this memory is ridiculously slow. A simple > benchmark revealed that it takes about 200 cycles to read a 64-bit > number. mmap will create uncached mappings for anything above the highest RAM address. > If I mmap() /dev/zero instead, a read takes under 3 cycles. Because you have a cache hits. No RAM is that fast. Above 200 cycles really is how horribly slow RAM is compared to a moderatly clocked system. > For those of you who knows how the Linux VM works, could you tell me why > the memory access is so slow? It look like it might be invoking the > page-fault handler on every read. How can I make memory access faster? Ralf
