From 05c7dc582a48d738bed82bcb24d3a7619fec8bc9 Mon Sep 17 00:00:00 2001
From: Antonio Andelic <antonio@clickhouse.com>
Date: Thu, 18 Jul 2024 13:40:03 +0200
Subject: [PATCH] Add some comments

---
 src/Common/Jemalloc.h     | 3 +++
 src/Common/MemoryWorker.h | 6 ++++++
 2 files changed, 9 insertions(+)

diff --git a/src/Common/Jemalloc.h b/src/Common/Jemalloc.h
index 629d039b483..dfa265c5e59 100644
--- a/src/Common/Jemalloc.h
+++ b/src/Common/Jemalloc.h
@@ -41,6 +41,9 @@ T getJemallocValue(const char * name)
     return value;
 }
 
+/// Each mallctl call consists of string name lookup which can be expensive.
+/// This can be avoided by translating name to "Management Information Base" (MIB)
+/// and using it in mallctlbymib calls
 template <typename T>
 struct JemallocMibCache
 {
diff --git a/src/Common/MemoryWorker.h b/src/Common/MemoryWorker.h
index 5f02fd0b1d0..6c0a578aa61 100644
--- a/src/Common/MemoryWorker.h
+++ b/src/Common/MemoryWorker.h
@@ -8,6 +8,12 @@ namespace DB
 {
 
 #if USE_JEMALLOC
+/// Correct MemoryTracker based on stats.resident read from jemalloc.
+/// This requires jemalloc built with --enable-stats which we use.
+/// The worker spawns a background thread which moves the jemalloc epoch (updates internal stats),
+/// and fetches the current stats.resident whose value is sent to global MemoryTracker.
+/// Additionally, if the current memory usage is higher than global hard limit,
+/// jemalloc's dirty pages are forcefully purged.
 class MemoryWorker
 {
 public: