3 ms·

> Why would you avoid something as trivial L3 cache is orders of magnitude faster than using RAM. You're talking a maximum of 50GB/s for DDR5, versus 1500GB/s

by zymhan 4y ago

> Why would you avoid something as trivial L3 cache is orders of magnitude faster than using RAM. You're talking a maximum of 50GB/s for DDR5, versus 1500GB/s for L3 cache https://en.wikipedia.org/wiki/List_of_interface_bit_rates#Dynamic_random-access_memory https://en.wikipedia.org/wiki/List_of_interface_bit_rates#Dy... https://meterpreter.org/amd-ryzen-9-7900x-benchmark-zen-4-improves-l3-cache-bandwidth-by-50-compared-to-zen-3/ https://meterpreter.org/amd-ryzen-9-7900x-benchmark-zen-4-im... It's a paradigm-shifting increase in processing speed when you don't need to hit RAM.

zX41ZdbW 4y ago
+ totally agree with that. There is a use case when you can improve performance by keeping compressed (LZ4) data in RAM and decompressing by small blocks that fit in cache. This is demonstrated by ClickHouse[1][2] - the whole data processing after decompression fits in cache, and compression saves the RAM bandwidth. [1] https://presentations.clickhouse.com/meetup53/optimizations/ https://presentations.clickhouse.com/meetup53/optimizations/ [2] https://github.com/ClickHouse/ClickHouse https://github.com/ClickHouse/ClickHouse
wmf 4y ago
The economic point stands that you aren't going to get a processor with only cache and no RAM because virtually no workloads want such an unbalanced system.
- lmm 4y ago
  As SSDs get faster and L3 caches get larger, will conventional RAM get squeezed out? I know Optane failed a few years back, but that kind of convergence seems inevitable in the long term.
  - prng2021 4y ago
    Isn’t it inevitable that conventional RAM will continue to get larger and faster as well?
    - lmm 4y ago
      IDK, I got the impression that while RAM was getting larger and higher-throughput, that was coming at the cost of higher latency.
xenadu02 4y ago
You're correct but that is still a niche segment because markets that need 128MB of super-fast memory are almost always happy to pay a little bit more to get 4GB+ of "L4" (aka DRAM).

by zymhan 4y ago

zX41ZdbW 4y ago

+ totally agree with that. There is a use case when you can improve performance by keeping compressed (LZ4) data in RAM and decompressing by small blocks that fit in cache. This is demonstrated by ClickHouse[1][2] - the whole data processing after decompression fits in cache, and compression saves the RAM bandwidth. [1] https://presentations.clickhouse.com/meetup53/optimizations/ https://presentations.clickhouse.com/meetup53/optimizations/ [2] https://github.com/ClickHouse/ClickHouse https://github.com/ClickHouse/ClickHouse

wmf 4y ago

The economic point stands that you aren't going to get a processor with only cache and no RAM because virtually no workloads want such an unbalanced system.

lmm 4y ago
As SSDs get faster and L3 caches get larger, will conventional RAM get squeezed out? I know Optane failed a few years back, but that kind of convergence seems inevitable in the long term.
- prng2021 4y ago
  Isn’t it inevitable that conventional RAM will continue to get larger and faster as well?
  - lmm 4y ago
    IDK, I got the impression that while RAM was getting larger and higher-throughput, that was coming at the cost of higher latency.

xenadu02 4y ago

You're correct but that is still a niche segment because markets that need 128MB of super-fast memory are almost always happy to pay a little bit more to get 4GB+ of "L4" (aka DRAM).