Monitoring Memory

Introduction

System Monitor can monitor the current usage of memory (RAM, Random Access Memory) and memory contents which had been swapped out to hard disk due to lack of free memory (Swap Space). The total usage of main memory is displayed in the menu bar.

If you like to monitor main memory of the system, perform the following steps:

Ensure the control window of System Monitor is open.
Select the item Information Sources > Memory in the sidebar.
Make sure the check mark Monitor this item is set at the right side of the window.

The use of memory is divided by the system into the following categories:

Reserved: Memory which is used by the operating system and which is specialled marked not to be available for swapping.
Active: normal used memory.
Inactive: free memory which has not been cleaned by the system yet. This memory will be released and erased immediately when necessary, but it can also be “reactivated” when a program requests already released blocks with their former contents. Delaying clean-up of memory to the latest possible moment is a technique to improve efficiency. The buffers used by the operating system kernel to accelerate disk and network operations are also part of the inactive memory.
Free: normal free memory.

As a result of the definition of inactive memory, this is memory available on one hand, but it contains usable, recyclable contents on the other hand. So it can be considered free or being used at the same time. If you like inactive memory to be seen as free memory, set a check mark at Show inactive memory as free memory in the monitor section Memory.

The data shown is calculated live and updated about once per second.

Menu Bar

The box Menu Bar controls in which form usage of memory should be displayed. System Monitor offers a variety of icons and functions that you can combine in any order. The following elements can be used:

The label MEM (memory) in small, vertical print to designate memory values and to separate them from other icons in the menu bar.
The label MEM in large, horizontal print.
The display of used (U) and free (F) memory as plain text in two lines using small print.
The display of used memory as text in percent.
The display of used memory as bar chart.
The display of used memory as pie chart.
The display of memory usage history for the last 30 seconds.

Within the box, the upper bar simulates as a sample how this monitor section should be presented in the menu bar. You can grab the individual items by mouse and move them into a different order. You can remove an element by pulling it out of the bar. The lower bar contains the stock of items available. If you like to add a certain item to the menu bar, use the mouse to drag it from the lower to the upper bar. Pressing the button Default causes your current settings to be deleted, replacing it by a default suggestion for the menu bar.

If you mistakenly have removed all items of this monitoring section from the menu bar, a three-star-icon (⁂, asterism) will appear as a placeholder. This ensures that you still can open the associated menu.

You cannot drag items directly into the real menu bar because Mac OS X does not allow this.

Menu Items

At least the following items will be shown as part of the menu associated with this monitor section:

The percentages of used, active, inactive and free memory, as explained above.
The number of page-ins from disk storage to main memory since the system has been started.
The number of page-outs from main memory to disk storage since the system has been started.
The number of bytes being used by the swap space on disk.

You can additionally make the following choices for the menu:

Show top 5 memory consuming processes: When setting a check mark here, System Monitor additionally collects information which currently running processes are responsible for the major part of memory usage. The process names will be listed in one section of the menu, sorted by their contributions to the total memory consumption. The corresponding percentages will be listed as well.

It is optionally possible to replace the display of percentage values by readings for absolute consumption of real memory. You can select between one of the items with percentage values and with resident memory sizes (real memory).

Retrieving the process list is a costly operation for technical reasons. The load caused by System Monitor itself will slightly increase when you enable this feature.

If you let processes display with their readings for real memory, you'll have to consider that the shown values also include shared memory areas used by multiple processes. If certain processes are using the Mac OS X technology “Cocoa” for example, the system library for Cocoa is resident only once in RAM memory, although it is being used by all these processes simultaneously. The affected memory is part of each of the processes memory areas, so it will be counted multiple times although it was allocated once only. Apple security policy for Apps does not allow that System Monitor can distinguish between private real memory and shared real memory.

Show history graph: After enabling this item, the history of memory usage within the last 30 seconds will be added to the menu.

Using history graphs to visualize memory usage typically does not make much sense. The objective of a good operating system is to utilize available main memory almost completely (slightly below 100 %) in order not to let valuable RAM be unexploited. After the system has reached its typical workload after start, the usage of memory should stop varying greatly if possible. The size of memory of your computer is optimal if memory is being used almost at 100 % and no swap space is being used.

Show paging operations as number of swapped bytes: In case this option is selected, the readings for page-ins and page-outs will not be shown as counters in the menu, but as total amounts of transferred memory sizes.