Hi Remi,
On 19/06/2020 6:03 pm, Remi Forax wrote:
Hi Sergei,
the problem is that you are changing the semantics if there are several fields.
By example with the code below, you have the guarantee that the code will print
4 (if it prints something),
if you remove the assignment field = false, the code can print 0 or 4.
class A {
int i = 4;
volatile boolean field = false;
}
thread 1:
global = new A()
thread 2:
var a = global;
if (a != null) {
System.out.println(a.i);
}
Surely you intended to read field to get the volatile read to pair with
the volatile write of the initializer? Otherwise there is no
happens-before ordering.
David
regards,
Rémi
----- Mail original -----
De: "Сергей Цыпанов" <[email protected]>
À: "core-libs-dev" <[email protected]>
Envoyé: Vendredi 19 Juin 2020 06:57:25
Objet: [PATCH] remove redundant initialization of volatile fields with default
values
Hello,
while investigating an issue I've found out that assignment of default value to
volatile fields slows down object instantiation.
Consider the benchmark:
@State(Scope.Thread)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
@BenchmarkMode(value = Mode.AverageTime)
@Fork(jvmArgsAppend = {"-Xms2g", "-Xmx2g"})
public class VolatileFieldBenchmark {
@Benchmark
public Object explicitInit() {
return new ExplicitInit();
}
@Benchmark
public Object noInit() {
return new NoInit();
}
private static class ExplicitInit {
private volatile boolean field = false;
}
private static class NoInit {
private volatile boolean field;
}
}
This gives the following results as of my machine:
Benchmark Mode Cnt Score Error Units
VolatileFieldBenchmark.explicitInit avgt 40 11.087 ± 0.140 ns/op
VolatileFieldBenchmark.noInit avgt 40 3.367 ± 0.131 ns/op
I've looked into source code of java.base and found out several cases where the
default value is assigned to volatile field.
Getting rid of such assignements demonstates improvement as of object
instantiation, e.g. javax.security.auth.Subject:
Mode Cnt Score Error Units
before avgt 40 35.933 ± 2.647 ns/op
after avgt 40 30.817 ± 2.384 ns/op
As of testing tier1 and tier2 are both ok after the changes.
Best regards,
Sergey Tsypanov
