JAVA-5788 Improve ByteBufferBsonOutput::writeCharacters

[franz1981]

Fixes JAVA-5788

While running few benchmarks vs https://github.com/quarkusio/quarkus-super-heroes/blob/main/README.md using native image (graal CE) we found

writeCharacters heap-based code path to be bloated by:

• many not inlined calls: which Graal CE should be blamed since it has a very simple and naive inliner
• an excessive usage of logic to handle the current buffer to be used to write chars into
• zero loop unrolling due to the complexity of the main loop; since native image cannot profile branches cannot "shape" the assembly to grant latin to be a fast path

This patch is manually inlining the fast paths for both latin chars and heap buffers, assuming them to have enough available space.
This should help immensely JVM mode as well (i.e. JDK Hotspot) - so if there's any JMH bench I can re-use or run to verify it would be great.

NOTE: i could change the assumption which make the optimization to be triggered to be more "elastic" and use the heap buffers (byte[]) remaining capacity and perform checkpoints to refill it, too, but it would make the logic way more complex - still more elastic.
Let me know what you prefer 🙏

[franz1981]

@jyemin I see I have to open an issue there but I need a bit of help to find if there is already a JMH benchmark which already cover this; if there is not I can provides a new one myself

[franz1981]

This is a flamegraph coming from the same test running in JVM mode (JDK 21)

which reports a similar story.

[franz1981]

@jyemin I have just verified that since we are using the Netty driver as well, the core path there will uses (direct) Netty buffers too, so I think I will expand this optimization to include them, checking which optimization path is more appropriate there e g.

• branchless ASCII check with 8 chars (using 2 longs storing 4 UTF-16 chars each)
• branchless zero check leveraging ASCII bytes
• batch buffer writes

[franz1981]

I have added an extra commit to improve the case for the Netty buffers as well.
I can't modify the original JIRA, to explain and include such, how I can do it?

[franz1981]

I have performed some JMH benchmarks and the Netty approach, despite is way better than the original code, is still much slower than the plain array case, byte per byte; I will experiment a bit more by using some Netty internalNioBuffer's API - although, since we create a Netty swapped wrapper (not needed actually) it could not work

[franz1981]

I've added ff360c0 to show what means using the internal NIO buffer from Netty
I'll post some JMH bench results soon - although native image CE won't benefit a lot from this since NIO ByteBuffer(s) have some "wide" enough OOP hierarchy which can still make single byte access to be too costly - which is something the "batchy" version instead was saving.

[franz1981]

The new approach is pretty much on par with the array based (slightly slower than it actually) - but miles better than the batchy unrolled one.
I am waiting for some feedback before proceeding in this (including benchmarks)

[franz1981]

@jyemin any chance I can help moving this forward? Anything I can help with? 🙏
This has a very large performance impact to both netty and non-netty users...

[jyemin]

Hi @franz1981 we're actually working on a larger performance epic and one of the areas we are investigating is the writeCharacters method. @vbabanin will follow up soon.

[franz1981]

thanks @jyemin and I'm happy (or any in my team) to help - since i'm a Netty commiter, whatever I can help with re my knowledge there I'll do it 🙏

[vbabanin]

Hi @franz1981, apologies for the late response. Thank you for the PR - i like the ideas! We’ve been working on similar improvements as part of our performance optimization investigations.

We added additional tests to guard against regressions and included enhancements for multi-byte encoding to reduce overhead from method calls and bounds checks. I’ll be proceeding with that implementation in #1651 with ByteBuf abstraction.

That said, your Netty-specific optimizations go beyond what we’ve implemented so far, and optimizing Netty integration in general is something we could explore.

We don’t yet have a Netty baseline to evaluate their impact, but I’ve added them to our driver-benchmark suite, currently under review in #1647. Once that’s merged and we can measure those changes, I’ll revisit the Netty-specific optimizations in this PR and follow up here.

One concern I have is that internalNioBuffer is documented as

Internal use only: Exposes the internal NIO buffer.

We aim to avoid using non-public APIs, as there’s no guarantee they’ll remain stable, and we should preserve compatibility for our users.

Regarding JMH: we don’t have any JMH benchmarks set up for this area. Instead, we use a custom benchmarking framework to measure performance. You could consider using BenchmarkSuite from there and running it locally to verify the performance of the Netty improvements. To enable Netty, configure NettyTransportSettings via MongoClientSettings on MongoClient created in Benchmark class.

[franz1981]

@vbabanin

It's fair enough related the internal NIO api, although (I am a Netty committer) I know for sure this won't go away for 4.2 as well (which means other N years before Netty 5 which have a much different Buffer API regardless).
I still suggest to use the ByteBuf::get(index) API since it will cut further the cost of moving the reader/writer offsets and further improving performance

Said that, I have sent some time ago #1630 which improve another "hot spot" found in production, so please check if it fits