[GitHub] [flink] zentol commented on a change in pull request #10179: [FLINK-14734][runtime] Add a ResourceSpec in SlotSharingGroup to describe its overall resources

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zentol commented on a change in pull request #10179: [FLINK-14734][runtime] Add a ResourceSpec in SlotSharingGroup to describe its overall resources
URL: https://github.com/apache/flink/pull/10179#discussion_r346803103
 
 

 ##########
 File path: flink-core/src/main/java/org/apache/flink/api/common/operators/ResourceSpec.java
 ##########
 @@ -149,6 +150,39 @@ public ResourceSpec merge(final ResourceSpec other) {
 		return target;
 	}
 
+	/**
+	 * Subtracts another resource spec from this one.
+	 *
+	 * @param other The other resource spec to subtract.
+	 * @return The subtracted resource spec.
+	 */
+	public ResourceSpec subtract(final ResourceSpec other) {
+		checkNotNull(other, "Cannot subtract null resources");
+
+		if (this.equals(UNKNOWN) || other.equals(UNKNOWN)) {
+			return UNKNOWN;
+		}
+
+		checkArgument(other.lessThanOrEqual(this), "Cannot subtract a larger ResourceSpec from this one.");
+
+		final ResourceSpec target = new ResourceSpec(
+			this.cpuCores.subtract(other.cpuCores),
+			this.taskHeapMemory.subtract(other.taskHeapMemory),
+			this.taskOffHeapMemory.subtract(other.taskOffHeapMemory),
+			this.onHeapManagedMemory.subtract(other.onHeapManagedMemory),
+			this.offHeapManagedMemory.subtract(other.offHeapManagedMemory));
+
+		target.extendedResources.putAll(extendedResources);
+
+		for (Resource resource : other.extendedResources.values()) {
+			target.extendedResources.merge(resource.getName(), resource, (v1, v2) -> {
+				final Resource subtracted = v1.subtract(v2);
+				return subtracted.getValue().compareTo(BigDecimal.ZERO) == 0 ? null : subtracted;
 
 Review comment:
   why are we using null instead of 0? `Resource` imposes no restriction on the value, hence 0 being a possible one. It is odd that , given 2 resources with a value of 0, the result of a subtraction is suddenly null.

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