I am somewhat surprised to see the bucket memory layout which is: [k1/v1],[k2,v2],[k3/v3] etc. where k1,k2,k3 are keys and v1,v2,v3 are values. The CPU cache will not contain more than one [k,v] pair - because the CPU cache line is about 64 bytes and the size of [k,v] pair was about 56 bytes.
So iterating through the bucket looking for a key will require each iteration to fetch the next [k,v] pair from RAM.
Compare this with the following layout: k1,k2,k3,… followed by v1,v2,v3. Looking up the first key in the bucket will end up loading at least one more key in the CPU cache-line. And this should make iterations faster.
The downside of this approach is if the lookup almost all the time results in the first key in the bucket. Then [k1,v1],[k2,v2],k3,v3] packing is better-because the value is also in the CPU cache line .
I am wondering if people on this forum knowvmore about this trade-off. Thanks!!
I am more interested to learn about Swiss tables than bug fix :)
What are the best places to learn modern implementations of traditional data structures. Many of these utilise SIMD for last mile usage of modern hardware
I am somewhat surprised to see the bucket memory layout which is: [k1/v1],[k2,v2],[k3/v3] etc. where k1,k2,k3 are keys and v1,v2,v3 are values. The CPU cache will not contain more than one [k,v] pair - because the CPU cache line is about 64 bytes and the size of [k,v] pair was about 56 bytes.
So iterating through the bucket looking for a key will require each iteration to fetch the next [k,v] pair from RAM.
Compare this with the following layout: k1,k2,k3,… followed by v1,v2,v3. Looking up the first key in the bucket will end up loading at least one more key in the CPU cache-line. And this should make iterations faster.
The downside of this approach is if the lookup almost all the time results in the first key in the bucket. Then [k1,v1],[k2,v2],k3,v3] packing is better-because the value is also in the CPU cache line .
I am wondering if people on this forum knowvmore about this trade-off. Thanks!!
The trade off is discussed here: https://github.com/golang/go/issues/70835
I am more interested to learn about Swiss tables than bug fix :)
What are the best places to learn modern implementations of traditional data structures. Many of these utilise SIMD for last mile usage of modern hardware
OP here, I wrote another blog post that explains how Swiss Tables work, see https://news.ycombinator.com/item?id=44597562
In addition to this comment's siblings resources, I also suggest this really good Cppcon presentation on Swisstable https://www.youtube.com/watch?v=ncHmEUmJZf4
I'd recommend reading the Swiss table design notes[1] in the Abseil documentation. You might also like F14 maps[2] from Folly.
[1]: https://abseil.io/about/design/swisstables
[2]: https://engineering.fb.com/2019/04/25/developer-tools/f14/
could read one of the implementations. there’s the original abseil implementation and rust’s in the hashbrown crate. probably many more.
Great write up. It almost made me miss my old DevOps job.