Malloc Information

You can examine a process' heap through the Malloc Information view. This view illustrates heap usage and provides statistics such as allocation call counts and block counts for specific byte ranges.

Leaving this view open lets you monitor a process' heap activity and memory usage. The data come from the general-purpose process-level allocator, so there's no application overhead when the IDE retrieves them. To display the data, the view uses multiple panes, which are described below.

Screenshot of Malloc Information view showing bar graphs of heap distribution, allocation and free requests, and block statistics for various size ranges
Note: All three panes—Overview, Distribution, and Charts—are shown by default but you can hide any of them by deselecting the corresponding options in the dropdown in the upper right corner.

Overview

This top pane provides statistics about activity for the entire heap, in multiple panels.

Total Heap

This panel contains a bar that indicates the used, overhead, and free heap space, in color-coded format:
  • Used (purple)
  • Overhead (turquoise)
  • Free (lavender)

This bar graph quickly tells you whether the process is running low on heap space. You can hover the pointer over a section to see a tooltip stating the purpose and size of the corresponding memory area (e.g., used space for small blocks, overhead for metadata of large blocks).

The total heap size is displayed next to the title. Here, the numbers shown are exact measurements that come from the allocation library, unlike with the Memory Information and System Resources views, which display estimates.

Calls Made

The numbers of malloc(), free(), and realloc() calls made by the process are given in this panel.

Core Requests

This panel shows the number of allocations and deallocations (frees) made by the system allocator based on the program's memory requests. At startup, the allocation library reserves heap memory that isn't freed until the program terminates, so the number of allocations is always greater than the number of deallocations. Typically, the allocator dispenses memory in increments of 4 KB (one page).

Distribution

This pane shows the distribution of heap memory based on byte range, in a table with these columns:
Byte Range
Size range of the memory blocks
Allocations
Total number of calls that allocated memory
Deallocations
Total number of calls that freed memory
Outstanding
Remaining number of allocated blocks. This value equals the number of allocated blocks minus the number of deallocated blocks.
% Returned
Ratio of freed blocks to allocated blocks, expressed as a percentage. This value equals the number of deallocations divided by the number of allocations.
Usage (min/max) bytes
Calculated minimum and maximum memory usage for a byte range. These values equal the number of allocated blocks multiplied by the minimum and maximum sizes of the range. For example, if the 65–128 byte range had two blocks allocated, the usage would be 130/256. You should use these values for estimated memory usage only—the actual usage typically lies in between.

If the Allocations and Deallocations counts seem high for some ranges, it means the program is allocating and freeing memory frequently, which can slow it considerably. For small blocks, this activity makes the heap overhead substantial compared to the used memory. In this case, the program is using the heap inefficiently and should be optimized.

If any Outstanding counts are increasing steadily, it means the program is allocating memory faster than freeing it, which can degrade performance. To learn about preventing leaks and inefficient heap usage, see the Heap Analysis chapter of the Programmer's Guide.

Charts

This bottom pane contains five charts illustrating different aspects of the process' historical heap usage:
Overview History
Displays a timeline of the heap usage snapshots shown in the Total Heap panel. This bar graph automatically rescales as the process increases its total heap size.
Outstanding Allocations
Shows the total outstanding (remaining) blocks over time. Individual byte ranges are represented by distinctly colored lines. You can compare this graph with the Overview History graph to understand memory allocation trends.
Allocation Deltas
Illustrates the changes in allocation call counts over time. This line graph lets you observe which byte ranges have the most newly allocated memory.
Deallocation Deltas
Illustrates the changes in deallocation call counts over time. This line graph lets you observe which byte ranges have the most newly freed memory.
Outstanding Allocations Deltas
Illustrates the differences between allocation and deallocation call counts over time, for each byte range. This line graph helps you recognize when some ranges are leaking memory and by how much.