Unsafe vs MemorySegments / Bounds checking...

[Maurizio Cimadamore]

On 27/10/2024 14:19, Johannes Lichtenberger wrote:
> Hello,
>
> I've watched the Devoxx talk[1] from Thomas and Roy about the 1 
> billion rows challenge. I assume Unsafe is going to be deprecated and 
> removed at some point, but it seems for max performance it's still a 
> good fit (of course if you know what to do). Wouldn't it be also 
> possible to remove bounds checks from MemorySegments if you really 
> want for instance via some JVM flag or something? Of course I know the 
> spacial and temporal bounds checks are a feature and very nice (in 
> most cases) :-)
Hi,

the process for Deprecating Unsafe has already started:

https://openjdk.org/jeps/471

Re. 1brc, that was a fun challenge to watch. My general sense is that 
results are a bit skewed towards the "extreme" - e.g. I don't think many 
Java developers would really like to read (or write!) code that looks 
like those in the top 10. For instance, in most instances I've seen, the 
top 10 examples use some way to memory map a file, but they never do the 
unmap (this is done by using a global arena) which I'm not sure will be 
considered "good practice". If you write code like that, and peak 
performance is your only concern, then yes, you can get into a place 
where bound checks matter (a lot) in terms of CPU cycles. But I'm not 
sure how well that translates to us mortals :-)

That said, note that there are at least a couple of Unsafe-less 
solutions that get pretty close to the top spot:

https://github.com/gunnarmorling/1brc/blob/main/src/main/java/dev/morling/onebrc/CalculateAverage_gonix.java

https://github.com/gunnarmorling/1brc/blob/main/src/main/java/dev/morling/onebrc/CalculateAverage_merykitty.java

One is based on ByteBuffer, the other on MemorySegment. They do their 
job in 3s - the top spot does it in 1.5. So... not too bad!

Maurizio