gdb: Extend 'scylla memory' to show memory used by large allocations

Adds new columns to the "Page spans" table named "large [B]" and
"[spans]", which shows how much memory is allocated in spans of given
size. Excludes spans used by small pools.

Useful in determining what is the size of large allocations which
consume the memory.

Example output:

Page spans:
index      size [B]      free [B]     large [B] [spans]
    0          4096          4096          4096       1
    1          8192         32768             0       0
    2         16384         16384             0       0
    3         32768         98304       2785280      85
    4         65536         65536       1900544      29
    5        131072        524288     471597056    3598
...
   31 8796093022208             0             0       0
Large allocations: 484675584 [B]
Message-Id: <[email protected]>

scylla-gdb.py

@@ -651,8 +651,22 @@ def invoke(self, arg, from_tty):
             gdb.write('{objsize:5} {span_size:6} {use_count:10} {memory:12} {wasted_percent:5.1f}\n'
                       .format(objsize=object_size, span_size=span_size, use_count=use_count, memory=memory, wasted_percent=wasted_percent))
 
+        idx = 0
+        large_allocs = defaultdict(int) # key: span size [B], value: span count
+        nr_pages = int(cpu_mem['nr_pages'])
+        pages = cpu_mem['pages']
+        while idx < nr_pages:
+            page = pages[idx]
+            span_size = int(page['span_size'])
+            if span_size == 0:
+                span_size = 1
+            if not page['pool'] and not page['free']:
+                large_allocs[span_size * page_size] += 1
+            idx += span_size
+
         gdb.write('Page spans:\n')
-        gdb.write('{index:5} {size:>13} {total}\n'.format(index="index", size="size [B]", total="free [B]"))
+        gdb.write('{index:5} {size:>13} {total:>13} {allocated_size:>13} {allocated_count:>7}\n'.format(
+            index="index", size="size [B]", total="free [B]", allocated_size="large [B]", allocated_count="[spans]"))
         for index in range(int(cpu_mem['nr_span_lists'])):
             span_list = cpu_mem['free_spans'][index]
             front = int(span_list['_front'])
@@ -662,7 +676,11 @@ def invoke(self, arg, from_tty):
                 span = pages[front]
                 total += int(span['span_size'])
                 front = int(span['link']['_next'])
-            gdb.write('{index:5} {size:13} {total}\n'.format(index=index, size=(1 << index) * page_size, total=total * page_size))
+            span_size = (1 << index) * page_size
+            allocated_size = large_allocs[span_size] * span_size
+            gdb.write('{index:5} {size:13} {total:13} {allocated_size:13} {allocated_count:7}\n'.format(index=index, size=span_size, total=total * page_size,
+                                                                allocated_count=large_allocs[span_size],
+                                                                allocated_size=allocated_size))
 
 
 class TreeNode(object):