Chips and Cheese investigated Zen 5's AVX-512 behavior and found that while AVX-512 is enabled and functional, using these instructions significantly reduces clock speeds. Their testing shows a consistent frequency drop across various AVX-512 workloads, with performance ultimately worse than using AVX2 despite the higher theoretical throughput of AVX-512. This suggests that AMD likely enabled AVX-512 for compatibility rather than performance, and users shouldn't expect a performance uplift from applications leveraging these instructions on Zen 5. The power consumption also significantly increases with AVX-512 workloads, exceeding even AMD's own TDP specifications.
The blog post details the creation of an extremely fast phrase search algorithm leveraging the AVX-512 instruction set, specifically the VPCONFLICTM instruction. This instruction, designed to detect hash collisions, is repurposed to efficiently find exact occurrences of phrases within a larger text. By cleverly encoding both the search phrase and the text into a format suitable for VPCONFLICTM, the algorithm can rapidly compare multiple sections of the text against the phrase simultaneously. This approach bypasses the character-by-character comparisons typical in other string search methods, resulting in significant performance gains, particularly for short phrases. The author showcases impressive benchmarks demonstrating substantial speed improvements compared to existing techniques.
Several Hacker News commenters express skepticism about the practicality of the described AVX-512 phrase search algorithm. Concerns center around the limited availability of AVX-512 hardware, the potential for future deprecation of the instruction set, and the complexity of the code making it difficult to maintain and debug. Some question the benchmark methodology and the real-world performance gains compared to simpler SIMD approaches or existing optimized libraries. Others discuss the trade-offs between speed and portability, suggesting that the niche benefits might not outweigh the costs for most use cases. There's also a discussion of alternative approaches and the potential for GPUs to outperform CPUs in this task. Finally, some commenters express fascination with the cleverness of the algorithm despite its practical limitations.
Summary of Comments ( 45 )
https://news.ycombinator.com/item?id=43215781
Hacker News users discussed the potential implications of the observed AVX-512 frequency behavior on Zen 5. Some questioned the benchmarks, suggesting they might not represent real-world workloads and pointed out the importance of considering power consumption alongside frequency. Others discussed the potential benefits of AVX-512 despite the frequency drop, especially for specific workloads. A few comments highlighted the complexity of modern CPU design and the trade-offs involved in balancing performance, power efficiency, and heat management. The practicality of disabling AVX-512 for higher clock speeds was also debated, with users considering the potential performance hit from switching instruction sets. Several users expressed interest in further benchmarks and a more in-depth understanding of the underlying architectural reasons for the observed behavior.
The Hacker News post titled "Zen 5's AVX-512 Frequency Behavior," linking to a Chips and Cheese article, has generated a moderate number of comments, primarily discussing the technical details and implications of the article's findings.
Several commenters focus on the performance trade-offs observed with AVX-512 on Zen 5. Some highlight the significant frequency drops when using AVX-512 instructions, questioning the practical benefit given the reduced clock speeds. One commenter points out the potential for increased power consumption despite the lower frequency due to the higher voltage required for AVX-512. Others discuss the impact on overall system performance, noting that even if AVX-512 provides theoretical advantages, the frequency reduction could negate these gains in real-world applications.
The discussion also touches on the complexities of power management in modern CPUs. Commenters explain how different instruction sets place varying demands on the power delivery system, leading to dynamic frequency adjustments. One comment suggests that the observed behavior might be due to power limits being reached, rather than an inherent limitation of the Zen 5 architecture. Another commenter speculates about the potential for future optimizations, suggesting that BIOS updates or software tweaks could mitigate the frequency drops.
A few comments delve into the technical details of AVX-512 implementation, discussing topics like vector units and instruction throughput. One commenter questions the efficiency of using AVX-512 for certain workloads, given the observed performance characteristics. Another commenter mentions the challenges of software utilizing AVX-512 effectively and the importance of compiler optimization.
Some comments compare Zen 5's AVX-512 behavior to other architectures, including Intel's offerings. One commenter suggests that while Zen 5 may face frequency reductions, it still offers competitive performance in AVX-512 workloads compared to some Intel CPUs.
Overall, the comments section provides valuable insights into the technical nuances and practical implications of AVX-512 on Zen 5. The discussion highlights the complex interplay between instruction sets, frequency scaling, and power management in modern CPUs. While some comments express concerns about the observed performance trade-offs, others offer potential explanations and suggest avenues for future optimization. The discussion remains focused on the technical aspects raised by the linked article, without delving into broader market analysis or speculation.