performance: arrayElementVarHandle / calculated index / aligned vs unaligned

Thu Dec 19 13:41:37 UTC 2024

Thanks a lot for rewriting/reproducing!

I've in the meantime tried to take some more complexity out:
* replaced arrayElementVarHandle (which uses MemoryLayout#scale with exact
math) with a "plain" version (`byteSize() * index`, at the risk of silent
overflows).
* I also eliminated the "VarHandle#collectCoordinates(h,1, scale)" in favor
of a plain varHandle.get(segment, i * byteSize()), after verifying they
have identical performance.

So we're down to a plain "VarHandle.get(segment, i * byteSize)"  in four
combinations: ({aligned, unaligned} x {i, i+1}), and the original
observation still holds:
the combo "aligned read" with "i + 1" somehow trips C2:
Alignment.aligned         avgt       0.151          ns/op
Alignment.alignedPlus1    avgt       0.298          ns/op
Alignment.unaligned       avgt       0.153          ns/op
Alignment.unalignedPlus1  avgt       0.153          ns/op

This leaves us with the assumption that the alignment check for the +1 case
somehow evades C2 optimization.
And indeed, we can remove all VarHandle business and only benchmark the
alignment check, and see that it fails to recognize that it is
loop-invariant:

    // simplified copy of AbstractMemorySegmentImpl#isAlignedForElement
    private static boolean isAligned(MemorySegment segment, long offset,
long byteAlignment) {
        return (((segment.address() + offset)) & (byteAlignment - 1)) == 0;
    }

    @Benchmark
    public void isAligned() {
        for (long i = 1; i < COUNT; ++i) {
            if (!isAligned(segment, i * 8, 8)) throw new
IllegalArgumentException();
        }
    }

    @Benchmark
    public void isAlignedPlus1() {
        for (long i = 0; i < COUNT - 1; ++i) {
            if (!isAligned(segment, (i + 1) * 8, 8)) throw new
IllegalArgumentException();
        }
    }

=>

Alignment.isAligned       thrpt       35160425.491          ops/ns
Alignment.isAlignedPlus1  thrpt              7.242          ops/ns

So this seems to be the culprit. Is it an issue? Idk. Using a plain offset
multiplication instead of the overflow-protecting MemoryLayout#scale
actually seems to have a bigger impact on performance than this.

> hsdis

I actually looked at hsdis and tried to use Jorn's precompiled dylib, but
it doesn't seem to load for me. Installing the whole toolchain and building
my own is probably beyond what I'm trying to do here (esp since I'm not
even sure I could read the assembly...)

Matthias

On Wed, Dec 18, 2024 at 3:13 PM Per-Ake Minborg <per-ake.minborg at oracle.com>
wrote:

> Hi Matthias!
>
> I've rewritten the benchmark slightly (just to make them "normalized" the
> way we use to write them) even though your benchmarks work equally well.
> See attachment. By using the commands
>
> jvmArgsAppend = {
>
>         "-XX:+PrintCompilation",
>         "-XX:+UnlockDiagnosticVMOptions",
>         "-XX:+PrintInlining" }
>
> in a @Fork annotation, and observing the output, it appears all the
> methods are inlined properly. So, even though some methods are more
> complex, it appears they are treated in the same way when it comes to
> inlining.
>
>  By looking at the actual assembly generated for the benchmarks using
> these commands (for an M1 in my case):
>
> @Fork(value = 1, jvmArgsAppend = {
>         "-Xbatch",
>         "-XX:-TieredCompilation",
>         "-XX:CompileCommand=dontinline,org.openjdk.bench.java.lang.foreign.Alignment::findAligned*",
>         "-XX:CompileCommand=PrintAssembly,org.openjdk.bench.java.lang.foreign.Alignment::findAligned*"
> })
>
> I can see that the C2 compiler is able to unroll the segment access in the
> findAligned method but not in the findAlignedNext method. This is one
> reason findAligned is faster. In order to see real assembly, the
> "hsdis-aarch64.dylib" must be present and it is recommended to use a
> "fast-debug" version of the JDK. Read more on Jorn Vernee's blog here:
> https://jornvernee.github.io/hsdis/2022/04/30/hsdis.html
>
>
> The question then becomes why that is the case. This drives us into
> another field of expertise where I am not the right person to provide an
> answer. Generally, there is no guarantee as to how the C2 compiler works
> and we are improving it continuously. Maybe someone else can provide
> additional information.
>
> Best, Per Minborg
>
>
>
>
> ------------------------------
> *From:* panama-dev <panama-dev-retn at openjdk.org> on behalf of Matthias
> Ernst <matthias at mernst.org>
> *Sent:* Wednesday, December 18, 2024 9:26 AM
> *To:* panama-dev at openjdk.org <panama-dev at openjdk.org>
> *Subject:* performance: arrayElementVarHandle / calculated index /
> aligned vs unaligned
>
> Hi,
>
> I'm trying to use the foreign memory api to interpret some variable-length
> encoded data, where an offset vector encodes the start offset of each
> stride. Accessing element `i` in this case involves reading `offset[i+1]`
> in addition to `offset[i]`. The offset vector is modeled as a
> `JAVA_LONG.arrayElementVarHandle()`.
>
> Just out of curiosity about bounds and alignment checks I switched the
> layout to JAVA_LONG_UNALIGNED for reading (data is still aligned) and I saw
> a large difference in performance where I didn't expect one, and it seems
> to boil down to the computed index `endOffset[i+1]` access, not for the
> `[i]` case. My expectation would have been that all variants exhibit the
> same performance, since alignment checks would be moved out of the loop.
>
> A micro-benchmark (attached) to demonstrate:
> long-aligned memory segment, looping over the same elements in 6 different
> ways:
> {aligned, unaligned} x {segment[i] , segment[i+1],  segment[i+1] (w/ base
> offset) } gives very different results for aligned[i+1] (but not for
> aligned[i]):
>
> Benchmark                         Mode  Cnt    Score   Error  Units
> Alignment.findAligned            thrpt       217.050          ops/s
> Alignment.findAlignedPlusOne     thrpt       110.366          ops/s. <=
> #####
> Alignment.findAlignedNext    thrpt       110.377          ops/s. <= #####
> Alignment.findUnaligned          thrpt       216.591          ops/s
> Alignment.findUnalignedPlusOne   thrpt       215.843          ops/s
> Alignment.findUnalignedNext  thrpt       216.483          ops/s
>
> openjdk version "23.0.1" 2024-10-15
> OpenJDK Runtime Environment (build 23.0.1+11-39)
> OpenJDK 64-Bit Server VM (build 23.0.1+11-39, mixed mode, sharing)
> Macbook Air M3
>
> Needless to say that the difference was smaller with more app code in
> play, but large enough to give me pause. Likely it wouldn't matter at all
> but I want to have a better idea which design choices to pay attention to.
> With the foreign memory api, I find it a bit difficult to distinguish
> convenience from performance-relevant options (e.g. using path expressions
> vs computed offsets vs using a base offset. Besides "make layouts and
> varhandles static final" what would be other rules of thumb?)
>
> Thx
> Matthias
>
>
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