Compute pool metrics

[vasilmkd]

I've been putting off doing this for so long.

[armanbilge]

Don't have experience with MBeans but this seems straightforward enough 👍

[armanbilge]

Pedagogical: why keep this behind a forwarder?

[vasilmkd]

Because of this BS:

class Stupid {
  val blah = 5
}

compiles to the following bytecode:

public class cats.effect.Stupid {
  private final int blah;

  private volatile boolean bitmap$init$0;

  public int blah();
    Code:
       0: aload_0
       1: getfield      #15                 // Field bitmap$init$0:Z
       4: ifeq          14
       7: aload_0
       8: getfield      #17                 // Field blah:I
      11: goto          24
      14: new           #19                 // class scala/UninitializedFieldError
      17: dup
      18: ldc           #21                 // String Uninitialized field: /Users/vasil/Code/cats-effect/core/shared/src/main/scala/cats/effect/Stupid.scala: 20
      20: invokespecial #25                 // Method scala/UninitializedFieldError."<init>":(Ljava/lang/String;)V
      23: athrow
      24: pop
      25: aload_0
      26: getfield      #17                 // Field blah:I
      29: ireturn

  public cats.effect.Stupid();
    Code:
       0: aload_0
       1: invokespecial #30                 // Method java/lang/Object."<init>":()V
       4: aload_0
       5: iconst_5
       6: putfield      #17                 // Field blah:I
       9: aload_0
      10: iconst_1
      11: putfield      #15                 // Field bitmap$init$0:Z
      14: return
}

[vasilmkd]

The blah() getter is synchronized on the bitmap field, which is volatile, incurring unnecessary synchronization points.

[armanbilge]

Huh, I thought this happened even for private[this] fields which is why you moved everything to the constructor?

[vasilmkd]

It does in the constructor. But for private[this] fields, the getter is not automatically generated by the compiler. A "forwarder" method accesses the field raw.

[vasilmkd]

class Forwarder {
  private[this] val blah: Int = 5

  def blahForwarder: Int = blah
}

compiles to

public class cats.effect.Forwarder {
  private final int blah;

  private volatile boolean bitmap$init$0;

  public int blahForwarder();
    Code:
       0: aload_0
       1: getfield      #16                 // Field blah:I
       4: ireturn

  public cats.effect.Forwarder();
    Code:
       0: aload_0
       1: invokespecial #22                 // Method java/lang/Object."<init>":()V
       4: aload_0
       5: iconst_5
       6: putfield      #16                 // Field blah:I
       9: aload_0
      10: iconst_1
      11: putfield      #24                 // Field bitmap$init$0:Z
      14: return
}

[vasilmkd]

Forwarder is a Cats Effect nomenclature. I think I heard Daniel say it once, or use it, and I continued doing so. It doesn't have any meaning. It's the same as getter, the point is to write it ourselves and circumvent the synchronization that the Scala compiler would do otherwise. 😄

[armanbilge]

Thanks 🤓 I'm very glad there's a workaround! But I wish I understood why the Scala compiler does this.