Using Instruments to Work Through Low Memory Warnings

Question

I am trying to work through some low memory conditions using instruments. I can watch memory consumption in the Physical Memory Free monitor drop down to a couple of MB, eve

Answer 1

The difference between physical memory from VM Tracker and allocated memory from "Allocations" is due to the major differences of how these instruments work:

• Allocations traces what your app does by installing a tap in the functions that allocate memory (malloc, NSAllocateObject, ...). This method yields very precise information about each allocation, like position in code (stack), amount, time, type. The downside is that if you don't trace every function (like vm_allocate) that somehow allocates memory, you lose this information.

• VM Tracker samples the state of the system's virtual memory in regular intervals. This is a much less precise method, as it just gives you an overall view of the current state. It operates at a low frequency (usually something like every three seconds) and you get no idea of how this state was reached.

A known culprit of invisible allocations is CoreGraphics: It uses a lot of memory when decompressing images, drawing bitmap contexts and the like. This memory is usually invisible in the Allocations instrument. So if your app handles a lot of images it is likely that you see a big difference between the amount of physical memory and the overall allocated size.

Spikes in physical memory might result from big images being decompressed, downsized and then only used in screen resolution in some view's or layer's contents. All this might happen automatically in UIKit without your code being involved.