Add some comments

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
 {

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: