I am often asked about perfmon’s Performance tab’s PF Usage and Physical Memory (K) Available readings. Pagefile usage is always high, but I’m not sure how to explain it.
by Ari Weil
There is a lot to talk about here…since this is a database blog, I’m going to provide the links to Microsoft’s published documentation so anyone who wants to get some good background information can do so. It’s certainly worth reading if you want to bone up on your Windows internals knowledge:
Hope that helps!
Share this post with friends
April 3rd, 2009 at 10:08 am
I believe the author is refering to Task manager, not Performance Monitor. These two items are poorly understood and the source of much confusion.
PF Usage: This is as abbreviation for pagefile usage. But the label is misleading because that is not really what it is showing. This is actually the commit charge. This is more like potential pagefile usage: if everything in RAM that could be sent to the pagefile actually was, this would be the space required. Actual pagefile usage, as shown in performance monitor will usually be a fraction of this value.
In any event the amount of data in the pagefile has relatively little to do with performance. It is frequency of access, not size of data, that matters here. Unfortunately, there is nothing in either Task Manager or Performance Monitor that will tell you this.
Available memory: This is commonly believed to be the same as free memory. Wrong again. This is actually the total of memory on the free lists and the standby list. The standby list is a pool of memory that is in use by applications but is immediately available for reassignment if needed. This would be unmodified data or modified data that has been copied to the pagefile. The amount of memory that is really free will typically be a quite small. At all times the system will attempt to keep free memory low and available memory high.
A high available memory number usually means that the system is not excessively loaded. It does NOT mean that the system is wasting precious memory.
Always remember this: Windows designers know more about memory management than you do. The system knows what it is doing with it’s resources, even if you do not.
Larry Miller
Microsoft MCSA