Requesting a Repeat Explanation of the Performance Tab

[George Tyndall]

I'm pretty sure Bob explained in the past the meaning of the various categories of Physicial Memory and Kernel Memory, but I can't find the thread, so I am requesting that the explanation be repeated.

Also, what is the meaning of Physical Memory: 41%?

Task Manager Performance.JPG

[Bob]

OK. I'm a bit short on time right now, but I'll try to post something soon. Remind me if I don't post something by this weekend.

Regarding Physical Memory = 41%. That's: ((Total - Available) / Total) * 100 rounded to the nearest percent. It's a measure of the minimum amount of physical memory required by the OS and all executing processes at that instant. It's not a measure of how much physical memory is actually being used as the OS will utilize available memory to improve operation and performance.

[George Tyndall]

OK. I'm a bit short on time right now....

No urgency whatsoever, Bob.

for what you posted so far.

[Bob]

The Task Manager Performance tab differs between the versions of Windows, so I'll just address the Windows 7 one (which is what you posted in the screen capture).

Windows uses two types of memory: Physical memory and Virtual memory. Physical memory is RAM and only the operating system can manage that directly. Applications use Virtual memory which is a logical mapping and independent of the amount of RAM. Virtual memory can either reside in physical memory (such as when it needs to be executed or accessed) or reside in the system page file on a storage device when it isn't needed in RAM. The amount of memory that an application uses that must to be in physical memory at any given instant is called the "working set". The Windows Memory Manager moves Virtual memory into and out of Physical memory as needed. The location of the Virtual memoy is totally transparent to the application which only sees a contiguous address space.

The operating system (OS) itself uses both Physical memory and Virtual memory to do its job. The memory used by the OS is detailed in the Kernal Memory section. The memory that must always reside in Physical memory is shown by the "Nonpaged" entry. The virtual memory being used by the OS is shown in the "Paged" entry.

[Bob]

When an application or process asks the OS for memory, it can either reserve the memory or commit the memory. Reserve is a place holder. It only tells the OS to make sure that the application can get that much memory, it doesn't actually make it available. Commit on the other hand makes that virtual memory available and it is backed up in the system page file. The "System" section contains a "Commit" entry. The first number in the line is the currently used amount of committed memory. The second number is the system commit limit. Essentially the maximum amount of commited memory that can be allocated. It's the sum of the size of the page file and the amount of RAM. If the system commit limit is reached bad things happen and you can crash or freeze the system.

The system can automatically expand and contract the system page unless you prevented the page file from expanding or you do not have sufficient free space. If you reach the system commit limit and you can't expand the page file you are toast.

The "Physical Memory" section describes the use of real memory. The "Total" figure is supposed to be the amount of installed RAM. It's not quite, for reasons I'm not privy to. If you were to start the resource manager and look at the Memory tab all the installed memory is accounted for. It's a task manager quirk. "Cached" used to be called "System Cache" in Windows XP. It's a little quirky also. It contains the commited memory that has been modified but not yet written to the page file, the working set of the OS, and cached pages that have either been recently accessed or loaded in anticipation of use for performance reasons. "Available" also contains the same cached pages included in "Cached" (but not including the OS working set or modified pages) and the unused or free pages. And, "Free" is the currently unused or free pages.

"Available" is where the cached data resides and is important for performance. If the workload drives down the available amount, the data will have to be read from the hdd and performance will go down. If the data that needs to be fetched resides in the page file, paging will also increase and performance will further degrade. If Available is low and the performance manger shows high page file reads, you need more RAM.

[George Tyndall]

If you reach the system commit limit and you can't expand the page file you are toast.

Bob, you really have a way with words.

I'm guessing you have been published.

Is that correct?

Thanks so much for that detailed explanation, which will take some time for me to digest.