[igodard]

There are dededicated Well-Known-Regions for code, stack, globals and TLS that catch the great majority of memory references. The PLB is only consulted when we miss in the WKRs. What's not in the WKRs? MAP_SHARED mmap()s. How many of those are in your program? How often do you access one for the first time or after long enough that the entry has been evicted from the PLB?

Like any cache, the optimal PLB size is determined by the working set. In the typical code we are seeing, the program has a couple of open files, half a dozen mmaps where the heap grew itself, and portal blocks for assorted libraries. The working sets are much smaller than a conventional TLB, and with SAS we have several cycles available in parallel with the caches.

The upshot is that a PLB can be large, cool, and slow. As for the range compares, the PLB permits the same sore of address sub-setting as is done in mixed size TLBs. Think about how many bits in the typical address range differ between lower and upper bounds.