MDEV-30889: max_global_memory_used status variable

Implement a new status variable max_global_memory_used, which gives the peak
value of global_memory_used.

When mallinfo2() shows a lot of memory being free in the malloc
implementation, this can be used to understand if this is because that
memory was at one point allocated to the server during a peak load situation
and later freed; or if it is caused by internal fragmentation in the malloc
implementation.
Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>

sql/mysqld.cc

@@ -3784,6 +3784,8 @@ static void my_malloc_size_cb_func(long long size, my_bool is_thread_specific)
     DBUG_PRINT("info", ("global thd memory_used: %lld  size: %lld",
                         (longlong) thd->status_var.global_memory_used, size));
     thd->status_var.global_memory_used+= size;
+    set_if_bigger(thd->status_var.max_global_memory_used,
+                  thd->status_var.global_memory_used);
   }
   else
     update_global_memory_status(size);
@@ -3852,6 +3854,7 @@ static int init_early_variables()
   set_current_thd(0);
   set_malloc_size_cb(my_malloc_size_cb_func);
   global_status_var.global_memory_used= 0;
+  global_status_var.max_global_memory_used= 0;
   init_alloc_root(PSI_NOT_INSTRUMENTED, &startup_root, 1024, 0, MYF(0));
   init_alloc_root(PSI_NOT_INSTRUMENTED, &read_only_root, 1024, 0,
 		  MYF(MY_ROOT_USE_MPROTECT));

sql/sql_class.cc

@@ -795,6 +795,7 @@ THD::THD(my_thread_id id, bool is_wsrep_applier)
   status_var.local_memory_used= sizeof(THD);
   status_var.max_local_memory_used= status_var.local_memory_used;
   status_var.global_memory_used= 0;
+  status_var.max_global_memory_used= 0;
   variables.pseudo_thread_id= thread_id;
   variables.max_mem_used= global_system_variables.max_mem_used;
   main_da.init();
@@ -1921,7 +1922,10 @@ void add_to_status(STATUS_VAR *to_var, STATUS_VAR *from_var)
     update_global_memory_status(from_var->global_memory_used);
   }
   else
+  {
    to_var->global_memory_used+= from_var->global_memory_used;
+   set_if_bigger(to_var->max_global_memory_used, to_var->global_memory_used);
+  }
 }
 
 /*

sql/sql_class.h

@@ -1061,6 +1061,7 @@ typedef struct system_status_var
   volatile int64 local_memory_used;
   /* Memory allocated for global usage */
   volatile int64 global_memory_used;
+  volatile int64 max_global_memory_used;
 } STATUS_VAR;
 
 /*
@@ -1113,6 +1114,9 @@ static inline void update_global_memory_status(int64 size)
   // workaround for gcc 4.2.4-1ubuntu4 -fPIE (from DEB_BUILD_HARDENING=1)
   int64 volatile * volatile ptr= &global_status_var.global_memory_used;
   my_atomic_add64_explicit(ptr, size, MY_MEMORY_ORDER_RELAXED);
+  // This is slightly approximate, as we are not using atomic set_if_bigger().
+  set_if_bigger(global_status_var.max_global_memory_used,
+                global_status_var.global_memory_used);
 }
 
 

sql/sql_test.cc

@@ -626,7 +626,7 @@ Open streams:  %10lu\n",
   MEM_MAKE_DEFINED(&info, sizeof info);
 #endif
 #if defined(HAVE_MALLINFO) || defined(HAVE_MALLINFO2)
-  char llbuff[10][22];
+  char llbuff[11][22];
   printf("\nMemory status:\n\
 Non-mmapped space allocated from system: %s\n\
 Number of free chunks:                   %lu\n\
@@ -640,6 +640,7 @@ Total free space:                        %s\n\
 Top-most, releasable space:              %s\n\
 Estimated memory (with thread stack):    %s\n\
 Global memory allocated by server:       %s\n\
+Peak global memory allocated by server:  %s\n\
 Memory allocated by threads:             %s\n",
 	 llstr(info.arena,   llbuff[0]),
 	 (ulong) info.ordblks,
@@ -654,7 +655,8 @@ Memory allocated by threads:             %s\n",
          llstr((count + thread_cache.size()) * my_thread_stack_size +
                info.hblkhd + info.arena, llbuff[7]),
          llstr(tmp.global_memory_used, llbuff[8]),
-         llstr(tmp.local_memory_used, llbuff[9]));
+         llstr(tmp.max_global_memory_used, llbuff[9]),
+         llstr(tmp.local_memory_used, llbuff[10]));
 
 #elif defined(HAVE_MALLOC_ZONE)
   malloc_statistics_t info;
@@ -665,11 +667,13 @@ Memory allocated by threads:             %s\n",
 Total allocated space:                   %s\n\
 Total free space:                        %s\n\
 Global memory allocated by server:       %s\n\
+Peak global memory allocated by server:  %s\n\
 Memory allocated by threads:             %s\n",
          llstr(info.size_allocated, llbuff[0]),
          llstr((info.size_allocated - info.size_in_use), llbuff[1]),
          llstr(tmp.global_memory_used, llbuff[2]),
-         llstr(tmp.local_memory_used, llbuff[3]));
+         llstr(tmp.max_global_memory_used, llbuff[3]),
+         llstr(tmp.local_memory_used, llbuff[4]));
 #endif
 
 #ifdef HAVE_EVENT_SCHEDULER