Hi,
I have a very simple routine which, on some JVMs/systems, which I have
not yet entirely narrowed down, suffers a 50x slowdown. The code is
included below.
In perf-java-flames, I see:
<clinit> -> readVarintTable (90%), of which:
readVarintTable -> readVarint (4%)
readVarintTable -> resolve_static_call -> libjvm.so (86%) <-- THE WAT?
So what is a perfectly trivial method doing spending 90% of it's time
inside resolve_static_call? What's going on?
My google searches turned up a note to do with loop unrolling and some
optimizations breaking for static methods, so I will try this with
non-static methods.
Slow on openjdk 1.8.0_191, Xeon E-2176M (Lenovo P1 laptop, 12-thread)
Fast on openjdk 1.8.0_191, Core i7-5600U (Lenovo T550 laptop, 4-thread)
I think Fast on Xeon E5620 (Supermicro rack, 8 thread).
I think Slow on AMD Epyc 7301 16-core, 64-thread. Will investigate more.
Knocking off the obvious:
* It's not doing meaningful amounts of allocation, and no GC.
* Total data size is 100M-1G.
* Both machines running same code, same dataset, ...
* This is single-threaded, and runs early in the JVM startup.
* It's doing I/O over JAR-resource -> BufferedInputStream ->
DataInputStream but it's not I/O contended, based on the calls in the
flamegraph.
But I feel that a 50x slowdown in an unexplained native call because of
... what, the number of cores ... bears some explanation. I can post the
flamegraphs if that helps.
And here is the code, which is as boring as anything, so what gives:
/** Reads a little-endian varint with no optimization for negative
numbers. */
private static int readVarint(@Nonnull DataInputStream in) throws
IOException {
int result = 0;
for (int shift = 0; shift < 32; shift += 7) {
int b = in.read();
if (b == -1)
throw new EOFException("Truncated varint in
stream.");
result |= (b & 0x7f) << shift;
if ((b & 0x80) == 0)
return result;
}
throw new IOException("Malformed varint in stream.");
}
@Nonnull
private static int[] readVarintTable(@Nonnull DataInputStream in,
@Nonnegative int sublength) throws IOException {
int[] out = new int[readVarint(in) * sublength];
for (int i = 0; i < out.length; i++)
out[i] = readVarint(in);
return out;
}
static {
try {
DataInputStream in = new DataInputStream(
new BufferedInputStream(
Parser.class.getResourceAsStream("Parser.dat")
)
);
table = readVarintTable(in);
} // etc
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