Introduce JK to Disable Memory Reallocation Optimization (#325)

Summary:

When implementing the stream chunker, we anticipated that stream buffers after chunking will end up growing to the size that previously triggered chunking. As a tradeoff between minimizing reallocations (for performance) and actually releasing memory (to relieve memory pressure), we heuristically determine the new buffer capacity for each stream to be larger that required. The issue with this optimization is that it conflicts with the rest of our memory tracking logic since we now have retained memory in the memory pool that is not accounted for. We now know through local testing that disabling this optimization leads to better memory pressure relief. In this diff, we introduce a JK, [dwio/nimble_chunking:disable_memory_reallocation_optimization](https://www.internalfb.com/intern/justknobs/?name=dwio%2Fnimble_chunking#disable_memory_reallocation_optimization), that will be enabled just for DISCO experiments. This will help us understand the full impact of this optimization and whether it should be retained.

Differential Revision: D87494427

Author: macvincent

CMakeLists.txt

@@ -20,6 +20,7 @@ set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED True)
 
 add_compile_definitions(DISABLE_META_INTERNAL_COMPRESSOR=1)
+add_compile_definitions(DISABLE_META_INTERNAL_MEMORY_REALLOCATION_OPTIMIZATION=1)
 
 # Sets new behavior for CMP0135, which controls how timestamps are extracted
 # when using ExternalProject_Add():

dwio/nimble/velox/StreamChunker.h

@@ -17,6 +17,9 @@
 #pragma once
 
 #include "dwio/nimble/velox/StreamData.h"
+#ifndef DISABLE_META_INTERNAL_MEMORY_REALLOCATION_OPTIMIZATION
+#include "justknobs/JustKnobProxy.h"
+#endif
 
 namespace facebook::nimble {
 namespace detail {
@@ -32,6 +35,14 @@ uint64_t getNewBufferCapacity(
     uint64_t maxChunkSize,
     uint64_t currentCapacityCount,
     uint64_t requiredCapacityCount) {
+#ifndef DISABLE_META_INTERNAL_MEMORY_REALLOCATION_OPTIMIZATION
+  static facebook::jk::BooleanKnob disableMemoryReallocationOptimization(
+      "dwio/nimble_chunking:disable_memory_reallocation_optimization");
+  if (disableMemoryReallocationOptimization()) {
+    return requiredCapacityCount;
+  }
+#endif
+
   const auto maxChunkElementCount = maxChunkSize / sizeof(T);
   if (currentCapacityCount < maxChunkElementCount) {
     return std::max<uint64_t>(