Columnarization in Rust
frankmcsherry.org
frankmcsherry.org
Well, that sums that up pretty succinctly. :)
> "... columnarization, a technique from the database community for laying out structured records in a format that is more convenient for serialization than the records themselves."
Column stores in comparison to row stores don't offer any serialization benefit per se. The main benefits are the following, I will be using a record (A,B,C,D,E) as example with all types u32 (4 bytes):
* If you only use some fields you have to load less data from memory/disk into the CPU cache and your working set is more probable to fit into cache. For example when filtering only the records where A=22 and B=45 you only have to actually load x(sizeof(A)+sizeof(B)) = x8 bytes instead of xrecord_size=x20. This can make a very significant difference. * When using compression to reduce the size of data, columns can often be compressed better because they only contain data of the same type and nature and thus probably share similarities. When using such a small record consisting only of integers it probably won't make a difference. But if e.g. some fields are country abbreviations, textual description or others are ids, one could easily imagine that there are gains.
Coming back to the point about serialization, using the same technique as described in the blog post, there won't[1] be a performance difference between column storage and row storage (e.g. using a struct). The method described in the blog post just lets the data array of the original vector be wrapped by a Vec<u8> without even moving the memory, so the method is independent of the data type that is stored in the vectors. Of course it will only work for data types that do not contains references, otherwise we could get illegal memory access after deserialization (which should be guaranteed by the rust type system because only Copy types are allowed).
The only thing this benchmark is testing is how fast a vector can be initialized.
[1] There can be an space improvement of keeping the data in a column layout compared to row layout when using normal structs. Normal structs normally align the total size to the size of the largest field in the struct. A struct containing i64 and i8 would contain 7 bytes of padding. In a column layout this overhead would be avoided. Still there would not be an improvement in this serialization scheme as it does not actually copy any data.
Unless I misunderstand your post, I think you may have missed important parts!
More is going on than "just wrapping the data of the original vector as a Vec<u8>". That is what happens when we are handed a Vec<uint>, but for other inner types (pairs, vectors, etc) there is more to do (and, actual code presented showing what that work is).
1. A Vec<(u8, u64)> definitely ends up as a (Vec<u8>, Vec<u64>), thereby avoiding padding you'd have if you just wrote out the elements as structs. You absolutely end up saving space, and part of doing this is definitely copying data. (responding to "as it does not actually copy any data").
2. The types themselves can be vectors, corresponding to a struct with owned pointers (whose elements can also own pointers, etc). This is not something that just casting the source array will deal with. It's important here that Rust hands you ownership, as otherwise it would be totally inappropriate for us to claim the underlying memory (which the code does). That part, recycling owned memory, is one of the big performance wins (about 2.5x faster for me than invoking the allocator each time I need to mint a new array).
3. "Copy" doesn't appear in the first post. The types don't have to be Copy, and indeed Vec<T> is not Copy, even if T is. Not sure where you got that from. But, no such requirement; yay!
Read part 2 for more about Copy, and how when your type is Copy you get a free implementation that does what you suggest (keeping each struct intact), except you can mix and match with Vecs and Options and stuff like that.
Hope this clears up some of the not-sense-making. Feel free to holler with other questions.
Cheers, Frank
1. In my comment I acknowledge that there are space savings with types that include padding. I don't understand why you imply in your answer that I didn't understand this point. Regarding my comment about not copying data: it was based on the benchmark[1] as linked in the blog post. The parts you measure do not copy any of the user data! It only converts your internal vectors of user types to a list of vectors of u8 type. So what you are doing is essentially moving the data. But when using move semantics the size of the user data does not matter any more, so there won't be a difference between your column layout and using the more complex types directly inside the vectors if measured the same way as in this benchmark. In my opinion your benchmark is flawed and does not support the argument you make in your blog post.
2. Ok yes that was bad wording on my side. If you provide adaptors for complex types with pointers like Vec than you can of course also serialize those.
3. I guess this is just about the same argument as point 2) and thus redundant.
I made the effort for you to write a serialization framework which does not do "columnarization" but which simulates row layout and also added an adaptor for vec and ran it with the same benchmark, and also reran the original columnar benchmark on my machine. Both benchmarks were compiled exactly the same way. You can find my code here[2]. Here are the results:
==============================
columnarization <uint> 12.1 GB/s, 742k values/s
columnarization <(uint, (uint, uint))> 5.3 GB/s, 107k values/s
columnarization <vec<uint>> 2.1 GB/s, 54k values/s
columnarization <Option<uint>> 1.58 GB/s, 163k values/s
==============================
row (simple_serialize) <uint> 12.1 GB/s, 730k values/s
row (simple_serialize) <(uint, (uint, uint))> 8.83 GB/s, 164 values/s
row (simple_serialize) <vec<uint>> 2.13 GB/s, 56 values/s
row (simple_serialize) <Option<uint>> 8.82 GB/s, 263k values/s
==============================
You see that columnization does not have a performance benefit in your benchmark and it even is significantly slower for Option<uint> type and pairs.
In your blog post you never mentioned that columnization will only has the potential to bring performance benefits to de-/serialization when using types with large padding overheads. I think this discussion would probably helped the blog post. It would be much better if it would either omit the currently wrong performance argument and just focus on the nicely typed API or if it would use a proper benchmark which would support your argument, which from my understanding would be only possible in a very limited set of use case scenarios.
Here is a list of other valid arguments you could have made instead:
* Format saves space at the cost of performance.
* Better than repr(packed) as it will also work on platforms that don't support unaligned access
[1] https://github.com/frankmcsherry/columnar/blob/master/exampl... [2] https://github.com/mkaufmann/simple_serialize
The reason the new numbers look better for Option with the Copy implementation is that you are now writing 16 bytes for each Option. They are a 50-50 mix of Some(0u) and None, which I wrote out in 5 bytes on average (always a 1 byte "present/not", and then 4 bytes of data on average). The Copy implementation is just writing 3x as much data and padding the throughput numbers, rather than reporting something more like goodput.
I sense that this isn't the right place to shake this out, so I'll stop posting. If there is a better way to follow up, let me know.
[edit: I've got you through github, thanks! I'll buy beers when I swing by TU next]
Using your original benchmark I than show that the row layout brings a large performance win in the benchmark. My numbers show this performance win not just in throughput (bytes per sec.) but als in "goodput" (values per sec.). Check my previous comment. I just noticed though that I sometimes forgot the k in the reported numbers, where it is missing you have ti multiply the number times 1000, I can't edit it anymore.
I guess we can agree to disagree and should continue the discussion in another form ;)
PS: I just updated my github information, I am now at the TU Munich
This code largely just copies pointers around.
And also what Steve said about Rust being rly rly smart, or at least move semantics opening the door for that.
Instead of (uint, int) we could have (int, sint) or (uint, sint).
There has been a huuuuuge amount of discussion about int/uint and naming, expect some clarification sometime soonish, as any changes (if any) would have to be made soon.