Is there a bandwidth improvement from installing a 32-bit operating system on a 64-bit machine?

Question

Knuth recently objected to 64-bit systems, saying that for programs which fit in 4 gigs of memory, "they effectively throw away half of the cache" because the pointers are twice as big as on a 32-bit system.

My question is: can this problem be avoided by installing a 32-bit operating system on a 64-bit machine? And are there any bandwidth-intensive benchmarks which demonstrate the advantage in this case?

Answer 1

Bandwidth is not really the correct term here. What Knuth was really talking about was data density, as it relates to cache footprint. Imagine that you have a 16KB L1 data cache: If you're purely storing pointers, you can store 2^14/2^2 = 2^12 = 4096 32-bit pointers, but only 2048 64-bit pointers. If the performance of your application depends on being able to keep track of over 2K different buffers, you may see a real performance benefit from a 32-bit address space. However, most real code is not this way, and real performance benefits from a caching system often come from being able to cache common integer and floating-point data structures, not huge quantities of pointers. If your working set is not pointer-heavy, the downside of 64-bit becomes negligible, and the upside becomes much more obvious if you're performing a lot of 64-bit integer arithmetic.