Android 内存分配跟踪工具(Allocation tracker)

Track memory allocations

Despite the impressive hardware of the first Android phones (T-Mobile G1 and ADP1) writing efficient mobile applications is not always straightforward. Android applications rely on automatic memory management handled by Dalvik's garbage collector which can sometimes cause performance issues if you are not careful with memory allocations.

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Android 的APP主要靠Dalvik的垃垃回收机制去管理内存，如果你不注意，那你就完蛋了！

In a performance sensitive code path, like the layout or drawing method of a view or the logic code of a game, any allocation comes at a price. After too many allocations, the garbage collector will kick in and stop your application to let it free some memory. Most of the time, garbage collections happen fast enough for you not to notice. However, if a collection happens while you are scrolling through a list of items or while you are trying to defeat a foe in a game, you may suddenly see a drop in performance/responsiveness of the application. It's not unusual for a garbage collection to take 100 to 200 ms. For comparison, a smooth animation needs to draw each frame in 16 to 33 ms. If the animation is suddenly interrupted for 10 frames, you can be certain that your users will notice.

Most of the time, garbage collection occurs because of tons of small, short-lived objects and some garbage collectors, like generational garbage collectors, can optimize the collection of these objects so that the application does not get interrupted too often. The Android garbage collector is unfortunately not able to perform such optimizations and the creation of short-lived objects in performance critical code paths is thus very costly for your application.

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任何的内存分配都是有代价的，分配越多，回收越频繁，搞不好就影响性能。

To help you avoid frequent garbage collections, the Android SDK ships with a very useful tool called  allocation tracker . This tool is part of DDMS, which you must have already used for debugging purposes. To start using the allocation tracker, you must first launch the standalone version of DDMS, which can be found in the  tools/  directory of the SDK. The version of DDMS included in the Eclipse plugin does not offer you ability to use the allocation tracker yet.

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你的救星是：allocation tracker！！！下面讲了这个东东怎么用！！

但是，各位，终极的解决方案并不是这个，真正能够拯救你的，是NDK！

Once DDMS is running, simply select your application process and then click the Allocation Tracker tab. In the new view, clickStart Tracking and then use your application to make it execute the code paths you want to analyze. When you are ready, clickGet Allocations. A list of allocated objects will be shown in the first table. By clicking on a line you can see, in the second table, the stack trace that led to the allocation. Not only you will know what type of object was allocated, but also in which thread, in which class, in which file and at which line. The following screenshot shows the allocations performed by Shelves while scrolling a ListView.

Even though it is not necessary, or sometimes not possible, to remove all allocations for your performance critical code paths. the allocation tracker will help you identify important issues in your code. For instance, a common mistake I have seen in many applications is to create a new Paint object on every draw. Moving the paint into an instance field is a simple fix that helps performance a lot. I highly encourage you to peruse the Android source code to see how we reduce allocations in performance critical code paths. You will also thus discover the APIs Android provide to help you reuse objects.