Before Solaris could have large page support for program text and data, we needed a business case. PRISM stands for Process Relocation in Intimate Shared Memory, and was my first big innovation whilst in PAE. The idea is simple: stop the process, copy a region of small pages somewhere, unmap the source region, remap the source region with large pages, copy the data back, and then allow the process to continue. At the time ISM was the only source of large pages.

My first solution used the LD_PRELOAD shared library interposition technique, but quickly moved on to LD_AUDIT interposition because this provides more fine-grained control. Operating at process startup (with the inclusion of an optional dummy malloc() and free() to preallocate the heap before the relocation took place), PRISM generated plenty of useful data to fuel the MPSS and Large Pages OOB projects. It also highlighted the usefulness of local copies of readonly text and data for large scale NUMA machines.

The PRISM library helped some of our published CPU benchmark numbers, and so had to be shipped with some versions of our compilers. This triggered the patent filing process, with my patent finally being awarded a year or so later.

About five years later, with MPSS and Large Pages OOB in place, I revisited the PRISM idea with Shatter, a tool to break up large pages into smaller ones. This contributed part of Nicolai Kosce’s dataspace profiling initiative (DProfile), which was trying to understand the effect of page colouring on performance.