What does memory pressure actually mean?
Q I use a 2009, 27-inch iMac, with a 4GB memory. It’s been rather slow since installing Mavericks. The 1TB hard drive was only 20% full, so I suspected memory. Sure enough, Activity Monitor showed that memory used was usually about 3.95GB of the 4GB available, despite Memory Pressure being in the low green. I upgraded to 8GB and find that memory used has gone up to around 7.95GB, with Memory Pressure still a low green. Can you explain please? Andrew Frost A When you open an application in OS X it’s loaded into free RAM. The other open apps are left in memory, even if they aren’t being used. Once you get to the point where there isn’t any free memory left, the OS starts actively managing the memory in use. Initially it will request that apps dump unneeded data and it compresses the data in memory pages that haven’t been accessed for a while. When it’s doing this, the Memory Pressure graph shows yellow. If that isn’t enough, memory pages will start getting swapped out to the disk and the graph changes to red. Unless Activity Monitor consistently shows Memory Pressure in yellow or red, adding more physical RAM won’t help. On a six-year-old Mac, it’s more likely that the graphics chip and CPU are lagging behind the requirements of modern apps. It’s also possible dust has built up inside. If your fans are normally running full blast, consider opening the case for some dusting.
Dust gets sucked into your iMac through the air intakes, so be sure to give them a clean now and again.