Counting Bytes Faster Than You'd Think Possible

Matt Stuchlik's recent exploration in high-performance computing focuses on counting the number of bytes with a value of 127 in a 250MB byte stream. His solution, which is approximately 550 times faster than a naive implementation, utilizes an optimized approach on an Intel Xeon E3-1271 v3 processor. The challenge involves reading the byte stream and counting occurrences of the target byte efficiently. Stuchlik's method employs SIMD (Single Instruction, Multiple Data) instructions to process data in chunks, leveraging AVX2 for performance gains.

A key innovation in his approach is the use of a memory read pattern that interleaves the processing of multiple 4K pages, which enhances data transfer rates by up to 30% in memory-bound scenarios. This technique takes advantage of the processor's hardware prefetchers, particularly the "Streamer," which can maintain multiple streams of data access. By interleaving access and unrolling the processing kernel, Stuchlik achieves significant performance improvements.

The implementation includes a series of assembly instructions that efficiently compare and count the target byte while managing potential overflow in accumulators. The final count is derived from a combination of narrow and wide accumulators, ensuring accuracy. Stuchlik concludes by noting the under-discussed nature of the page-interleaved read pattern and invites feedback on further memory optimization techniques.