问题
Have profiled an app on an iPhone 4 using "Time Profiler" and "CPU Monitor" and trying to make sense of it.
Given that execution time is 8 minutes, CPU "Running Time" is around 2 minutes. About 67% of that is on the main thread, out of which 52% is coming from "own code".
Now, I can see the majority of time being spent in enumerating over arrays (and associated work), UIKit operations, etc.
The problem is, how do I draw any meaningful conclusions out of this data? i.e. there is something wrong going on here that needs fixing.
I can see a lot of CPU load over that running time (median at 70%) that isn't "justifiable" given the nature of the app.
Having said that, there are some things that do stand out. Parsing HTTP responses on the main thread, creating objects eagerly (backed up by memory profiling as well).
However, what I am looking for here is offending code along with useful conclusions solely based on CPU running time. i.e. spending "too much" time here.
Update
Let me try and elaborate in order to give a better picture.
Based on the functional requirements of this app, I can't see why it shouldn't be able to run on an iPhone 3G. A median CPU usage of around 70%, with a peak of 97% only looks like a red flag on an iPhone 4.
The most obvious response to this is to investigate the code and draw conclusions from that.
What I am hoping for is a categorical answer of the following form
- if you spend anywhere between 25% - 50% of your time on CA, there is something wrong with your animations
- if you spend 1000ms on anything related to UIKit, better check your processing
Then again, maybe there aren't any answers only indications of things being off when it comes to running time and CPU usage.
回答1:
Answer for question "is there something wrong going on here that needs fixing" is simple: do you see the problem while using application? If yes (you see glitches in animation, or app hang for a while), you probably want to fix it. If not, you may be looking for premature optimization.
Nonetheless, parsing http responses in main thread, may be a bad idea.
回答2:
In dev presentations Apple have pointed out that whilst CPU usage is not an accurate indicator in the simulator it is something to hold stock of when profiling on device. Personally I would consider any thread that takes significant CPU time without good reason a problem that needs to be resolved. Find the time sinks, prioritise by percentage, and start working through them. These may not be visible problems now but they will begin to, if they have not already, degrade the user's experience of the app and potentially the device too.
Check out their documentation on how to effectively use CPU profiling for some handy hints.
If enumeration of arrays is taking a lot of time then I would suggest that dictionaries or other more effective caches could be appropriate, assuming you can spare some memory to ease CPU.
An effective approach may be to remove all business logic from the main thread (a given) and make a good boundary layer between the app and the parsing / business logic. From here you can better hook in some test suites that could better tell you if the code is at fault or if it's simply the significant requirements of the app UI itself...
回答3:
Eight minutes?
Without beating around the bush, you want to make your application faster, right?
Forget looking at CPU load and wondering if it's the right amount. Forget guessing if it's HTTP parsing. Maybe it is, but guessing won't tell you. Forget rummaging around in the code timing things in hopes that you will find the problem(s).
You can find out directly why it is spending so much time. Here's the method I use, and here's an (amateurish) video of it.
Here's what will happen if you do that. First you will find something you would never have guessed, and when you fix it you will lop a big chunk off that 8 minutes, like maybe down to 6 minutes. Then you do it again, and lop off another big chunk. You repeat until you can't find anything to fix, and then it will be much faster than your 8 minutes.
OK, now the ball is in your court.
来源：https://stackoverflow.com/questions/19935278/is-running-time-cpu-usage-a-useful-metric-under-instruments-to-draw-any-con