This is an pretty good session about iOS memory. iOS Memory Deep Dive - WWDC 2018 - Videos - Apple Developer. I saw it and took some notes here.
Not all memory is created equal.
There are dirty memory, clean memory, compressed memory in iOS system. We have to know the differences between them.
Page is typically 16KB in size and operating system gives it to you when your app requests memory.
memory in use = number of pages x page size
Some pages can hold multiple objects, and some objects
can span multiple pages.
- Data that can be paged out of memory
- Memory mapped files
- memory written by an app
- all heap allocations
- decoded image buffers
There is no traditional disk swap in iOS
The system will do the compression and decompression for you by memory compressor.
What does Memory compressor do?
- Compresses unaccessed pages
- Decompresses pages upon access
Before being compressed:
After being compressed:
When you got Memory warning, you App is not always the cause. Maybe because the compressor freeing memory. Like, you receiving a phone call while using the App.
After being decompressed:
After removing objects in
- Trade-offs between CPU and memory. Caching can reduce the CPU usage and time complexity, but it costs memory.
- Remember the compressor. When decompressing, the used memory will be increased.
- Prefer NSCache over dictionary.
It is the dirty size + the compressed size that the system uses to determine how much memory the app is really using.
We should mainly focuse on these two part, dirty and compressed memory when analyzing the memory profile.
Tools for Profiling Footprint
Xcode memory gauge
- VM Tracker
- providing profiles for dirty and compressed memories.
- Virtual memory trace
- memory graph
working with memory graph using commands
vmmap helps to show some dirty memory info of your app. In general, we should look for the big number for the size.
swapped size columns here.
According to this session, we can ignore the
virtual size, because it is memory requested by the app, while not neccessarily be used.
swapped size is related to compressed memory. So we should care more about
dirty size and
An example of using vmmap to debug a memory issue
First, we can use summary info to look for the big numbers in
virtual size and
swapped size colomn. Here, we find
CG Image takes much more memory than others.
vmmap --summary PlanetPics.memgraph
Then, we use
grep to get more info about
There are two regions here, the last row is summary info. The secong CG Image region takes more takes more dirty and swapped memory. So we have to see more infomation of this region by using
All these commands can work with other shell commands, like redirecting the output stream a
vmmap --verbose PlanetPics.memgraph | grep " CG image" | > output.txt
And we will more regions.
It turns out that vmmap, by default, if it finds contiguous regions, it collapses. A general rule. the later region was created, the later my app’s life cycle it happened. Chance are this later region is more closely tied to whatever caused that memory pike.
So, we start to look at the last region. We can use the start memory address of the last region and search it in the memory graph in XCode.
leak to get the trace tree. By scanning these info, we would find more clues.
malloc_history to see the back trace for this object, we found the related code creating this particular VM memory.
- vmmap and AWK
This command can work with other commands.
It not only shows the cycle, but also the root object of the cycle.
- Shows objects allocated on the heap;
- useful for indentifying large objects in memory adn what allocated it.
heap App.memgraph -addresses all | <classes-pattern>
heap command shows the class name in
CLASS_NAME column, the num of the class in
COUNT column, the average size of the object in the
AVG column, the total size in the
heap App.memgraph -sortBySize | > ~/output.txt
In some cases, we not only want to know the memory size, but also want to know the how it created. So, here comes the
- enable the malloc_stack logging
malloc_history App.memgraph [address]
Which tool to pick
heap to find some objects or regions with big number, use
leak to see references between objects, like finding circular reference; use
malloc_history to see how it is created.
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