[feature] io-uring support

[jonahlund]

I'm wondering if adding support for io-uring could be feasible without requiring any major rewrites, and if it's feasible, would there be enough benefits in doing so for async programs, instead of simply using spawn_blocking?

I could take a look at implementing something like this if it's deemed relevant enough.

[marvin-j97]

First and foremost, it would have to implemented in lsm-tree (and value-log), fjall is just a wrapper around that. I'm not sure how feasible it is to rewrite everything into an async core, and if it really provides great performance improvements. And to benchmark, you have to actually migrate everything, which would be a lot of work. There are some benchmarks that don't look great: facebook/rocksdb#11017

But, there are certain isolated operations that may be optimizable using io-uring without having to migrate the entire code base, specifically:

• Compaction/Merging: because during a compaction we just want to push data into a new file as fast as possible, so not waiting may improve performance - see https://openinx.github.io/ppt/io-uring.pdf Case 3, they measured 200% faster compactions
• Multi Get: https://rocksdb.org/blog/2022/10/07/asynchronous-io-in-rocksdb.html

instead of simply using spawn_blocking

There are situations where you don't need to wrap everything in spawn_blocking either. A rough guide is to not do work between two await points for more than 10-100µs.

• Reading a block from SSD takes ~1.5-70µs (large block loading will become a bit faster in the future)
• Writing small items without fsync takes about 1-2µs
• Even my enterprise SSD fsyncs at around ~30µs, so I can even do durable writes without blocking that long

But, when you write with fsync (consumer SSD: ~1ms, HDD: ~15ms) or read from an HDD (~15ms), you definitely want spawn_blocking, because you will wait for so long, you could perform other meaningful work while waiting.

[jonahlund]

I see, thank you for the insightful response!

So I guess my initial thought was if async io-uring would be added to fjall (lsm-tree), it would probably be best kept as a separate optional feature rather than a core rework, kind of like how sled provides a complementary flush_async method for async io, providing better efficiency for async programs. But as you mentioned, the question is how beneficial is it to add true async io as opposed to use spawn_blocking.

[marvin-j97]

kind of like how sled provides a complementary flush_async method for async io

Note that that function pretty much just moves the flush into a thread pool and awaits that:

https://github.com/spacejam/sled/blob/005c023ca94d424d8e630125e4c21320ed160031/src/tree.rs#L945

it would probably be best kept as a separate optional feature rather than a core rework

The problem is you would have to basically double the entire library to have the sync and async implementations separately.
As of now I think async IO is best use sparingly in the situations I mentioned - I would love to see if it turns out that it increases performance there - but I find the io_uring API to not be too convenient to work with, so I haven't actually really put any time into it... fjall-rs/lsm-tree#68