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Comment by greid | Hacker News Reader

In terms of performance: I realize that this is a somewhat "toy" issue, and it's a sample size of 1, but for the currently ongoing "One Billion Row Challenge"[1] (an ongoing Java performance competition related to parsing and aggregating a 13 GB file), all of the current top-performers are using Unsafe. More specifically, the use of Unsafe appears to have been the change for a few entries that allowed getting below the 3-second barrier in the test.

1. https://github.com/gunnarmorling/1brc

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Submissions to this challenge are also hampered by the lack of prefetch intrinsic, vpshufb intrinsic, aesenc intrinsic, and graal's complete lack of vector intrinsics. As a total outsider to the Java ecosystem, it makes it seem like nobody in a place to make changes really knows or cares about enabling high-throughput code to run in the JVM.

What do you mean by that?

Also, graal can do some vectorization, and definitely has some libraries with vector intrinsics, e.g. truffle’s regex implementation uses similar algorithms to simdjson.

I think I was fairly specific? There's no way for a user to do vpshufb, aesenc, or a prefetch instruction. One would expect the former two things to be in jdk.incubator.vector, where they are not present. The last thing was explicitly removed, here's a link to one part of the process of it being removed: https://bugs.openjdk.org/browse/JDK-8068977

Code that uses jdk.incubator.vector does not actually get compiled to vector instructions under graal. This is why the top submission that uses jdk.incubator.vector is the only top solution that does not use graal.

I don't doubt that "similar algorithms to simdjson" may be used, but simdjson uses instruction sequences that are impossible to generate using the tools provided.

I mention these instructions because vpshufb comes up a lot in string parsing and formatting, because prefetch is useful for hiding latency when doing bulk accesses to a hash table, and because aesenc is useful for building cheap hash functions such as ahash.

Ah okay, it wasn’t clear that you were talking about explicit control over this through Vector API and similar.

For my solution (https://github.com/dzaima/1brc, uses jdk.incubator.vector significantly), switching all array reads/writes to identical Unsafe ones results in the solution running ~100x slower, so there's certainly truth to Unsafe messing with optimization - I suppose it means that the VM must consider all live objects to potentially have mutated. (as for the sibling comment, indeed, lack of vpshufb among other things can be felt significantly while attempting to use jdk.incubator.vector)

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