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chore: optimize lazy val with power of two. #22428

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chore: optimize lazy val with power of two. #22428

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11 changes: 6 additions & 5 deletions library/src/scala/runtime/LazyVals.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -27,17 +27,18 @@ object LazyVals {

private val base: Int = {
val processors = java.lang.Runtime.getRuntime.availableProcessors()
8 * processors * processors
val rawSize = 8 * processors * processors
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@He-Pin He-Pin Jan 21, 2025 •
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not sure why it's 8 here, Some CPU has 384 cores now, maybe 2 * 2 is better than 8.

The current implementation may use a little more memory than it was, but as many server just has 64 cores, which is the same.

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@szymon-rd szymon-rd Apr 26, 2025 •
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I see from blame that it is here for some time. But availableProcessors gives the number of logical cores, so no relation to CPU count, just the actual cores. And I don't think this code is actually used in the new lazy vals impl?

//find the next power of 2
1 << (32 - Integer.numberOfLeadingZeros(rawSize - 1))
}

private val mask: Int = base - 1

private val monitors: Array[Object] =
Array.tabulate(base)(_ => new Object)

private def getMonitor(obj: Object, fieldId: Int = 0) = {
var id = (java.lang.System.identityHashCode(obj) + fieldId) % base

if (id < 0) id += base
monitors(id)
monitors((java.lang.System.identityHashCode(obj) + fieldId) & mask)
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Use & instead of %

}

private final val LAZY_VAL_MASK = 3L
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