Android重量级开发之--提高android启动速度研究

作者：Tigertang2@gmail.com

大家都知道启动速度慢是智能操作系统的一个通病，Android也不例外，启动速度大概在1分钟左右，虽然日本有一个叫quick boot的一秒启动android的产品，但是毕竟是旁门左道。所以从常规来提高android的启动速度成了大家研究的重点，也是难点。下面将初步研究的一下经验跟大家分享一下。

首先看一下android系统的启动流程：

bootloader
引导程序

kernel
内核

init
init初始化（这个大家都比较熟悉了，不要多说）

loads several daemons and services, including zygote
see /init.rc and init.<platform>.rc

zygote

这个是占用时间最多的，重点修理对象

preloads classes
装载了一千多个类，妈呀！！！
starts package manager扫描package(下面详细介绍)

service manager

start services（启动多个服务）

从实际的测试数据来看，有两个地方时最耗时间的，一个是zygote的装载一千多个类和初始化堆栈的过程，用了20秒左右。另一个是扫描

/system/app,
/system/framework,
/data/app,
/data/app-private.

这几个目录下面的package用了大概10秒，所以我们重点能够修理的就是这两个老大的。

一、首先是调试工具的使用，可以测试哪些类和那些过程占用了多少时间，

主要工具为

stopwatch

Message loggers

grabserial
参考http://elinux.org/Grabserial
printk times参考http://elinux.org/Printk_Times
logcat
Android自带

bootchart参考http://elinux.org/Bootchart和

http://elinux.org/Bootchart

strace

AOSP的一部分（Eclair及以上版本）

使用例子

在init.rc中为了调试zygote

service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server改为
service zygote /system/xbin/strace -tt -o/data/boot.strace /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server

method tracer*

ftrace*

详细使用可看提供的文档和网页介绍

上面的工具如果不用详细的分析不一定都用到，也可以使用logcat就可以,在代码中加一点计算时间和一些类的调试信息也可以达到很好效果。

二、zygote装载1千多个类

首先，我们可以添加一点调试信息，以获得具体转载情况。

diff --git a/core/java/com/android/internal/os/ZygoteInit.java b/core/java/com/android/internal/os/ZygoteInit.java
index 404c513..f2b573c 100644
--- a/core/java/com/android/internal/os/ZygoteInit.java
+++ b/core/java/com/android/internal/os/ZygoteInit.java
@@ -259,6 +259,8 @@ public class ZygoteInit {
} else {
Log.i(TAG, "Preloading classes...");
long startTime = SystemClock.uptimeMillis();
+ long lastTime = SystemClock.uptimeMillis();
+ long nextTime = SystemClock.uptimeMillis();

// Drop root perms while running static initializers.
setEffectiveGroup(UNPRIVILEGED_GID);
@@ -292,12 +294,24 @@ public class ZygoteInit {
if (Config.LOGV) {
Log.v(TAG, "Preloading " + line + "...");
}
+ //if (count%5==0) {
+ // Log.v(TAG, "Preloading " + line + "...");
+ //}
+ Log.v(TAG, "Preloading " + line + "...");
Class.forName(line);
+ nextTime = SystemClock.uptimeMillis();
+ if (nextTime-lastTime >50) {
+ Log.i(TAG, "Preloading " + line + "... took " + (nextTime-lastTime) + "ms.");
+ }
+ lastTime = nextTime;
+
if (Debug.getGlobalAllocSize() > PRELOAD_GC_THRESHOLD) {
if (Config.LOGV) {
Log.v(TAG,
" GC at " + Debug.getGlobalAllocSize());
}
+ Log.i(TAG,
+ " GC at " + Debug.getGlobalAllocSize());
runtime.gcSoftReferences();
runtime.runFinalizationSync();
Debug.resetGlobalAllocSize();

上面+代表添加的代码，这样就可以很容易的得到在装载类的过程中具体装载了哪些类，耗费了多久。具体装载的类在文件platform/frameworks/base/ preloaded-classes

内容类似：

android.R$styleable
android.accounts.AccountMonitor
android.accounts.AccountMonitor$AccountUpdater
android.app.Activity
android.app.ActivityGroup
android.app.ActivityManager$MemoryInfo$1
android.app.ActivityManagerNative
android.app.ActivityManagerProxy
android.app.ActivityThread
android.app.ActivityThread$ActivityRecord
android.app.ActivityThread$AppBindData
android.app.ActivityThread$ApplicationThread
android.app.ActivityThread$ContextCleanupInfo
android.app.ActivityThread$GcIdler
android.app.ActivityThread$H
android.app.ActivityThread$Idler

而这个文件是由文件WritePreloadedClassFile.java中的WritePreloadedClassFile类自动生成

/**

* Writes /frameworks/base/preloaded-classes. Also updates

* {@link LoadedClass#preloaded} fields and writes over compiled log file.

publicclassWritePreloadedClassFile

/**

* Preload any class that take longer to load than MIN_LOAD_TIME_MICROS us.

static final int MIN_LOAD_TIME_MICROS = 1250;//这个代表了装载时间小于1250us即1.25ms的类将不予装载，也许可以改这个参数减少一下类的装载

//这里可以看到什么样的类会被装载

A:启动必须装载的类，比如系统级的类

B：刚才说的装载时间大于1.25ms的类

C：被使用一次以上或被应用装载的类

仔细看看筛选类的具体实现，可以帮助我们认识哪些类比较重要，哪些可以去掉。

筛选规则是

第一isPreloadable，

/**Reports if the given class should be preloaded. */
public static boolean isPreloadable(LoadedClass clazz) {

return clazz.systemClass && !EXCLUDED_CLASSES.contains(clazz.name);

}

意思是指除了EXCLUDED_CLASSES包含的类之外的所有系统装载的类。

EXCLUDED_CLASSES包含

/**
* Classes which we shouldn't load from the Zygote.
*/
private static final Set<String> EXCLUDED_CLASSES
= new HashSet<String>(Arrays.asList(
// Binders
"android.app.AlarmManager",
"android.app.SearchManager",
"android.os.FileObserver",
"com.android.server.PackageManagerService$AppDirObserver",

// Threads
"android.os.AsyncTask",
"android.pim.ContactsAsyncHelper",
"java.lang.ProcessManager"
));

目前是跟Binders跟Threads有关的不会被预装载。

第二 clazz.medianTimeMicros() > MIN_LOAD_TIME_MICROS装载时间大于1.25ms。

第三names.size() > 1 ，既是被processes一次以上的。

上面的都是指的system class,另外还有一些application class需要被装载

规则是fromZygote而且不是服务

proc.fromZygote() && !Policy.isService(proc.name)

fromZygote指的除了com.android.development的zygote类

public boolean fromZygote() {
return parent != null && parent.name.equals("zygote")
&& !name.equals("com.android.development");
}

/除了常驻内存的服务

/**
* Long running services. These are restricted in their contribution to the
* preloader because their launch time is less critical.
*/
// TODO: Generate this automatically from package manager.
private static final Set<String> SERVICES = new HashSet<String>(Arrays.asList(
"system_server",
"com.google.process.content",
"android.process.media",
"com.android.bluetooth",
"com.android.calendar",
"com.android.inputmethod.latin",
"com.android.phone",
"com.google.android.apps.maps.FriendService", // pre froyo
"com.google.android.apps.maps:FriendService", // froyo
"com.google.android.apps.maps.LocationFriendService",
"com.google.android.deskclock",
"com.google.process.gapps",
"android.tts"
));

好了。要转载的就是这些类了。虽然preloaded-classes是在下载源码的时候已经确定了的，也就是对我们来说WritePreloadedClassFile类是没用到的，我们可以做的就是在preloaded-classes文件中，把不预装载的类去掉，试了把所有类去掉，启动确实很快跳过那个地方，但是启动HOME的时候就会很慢了。所以最好的方法就是只去掉那些没怎么用到的，不过要小心处理。至于该去掉哪些，还在摸索，稍后跟大家分享。有兴趣的朋友可以先把preloaded-classes这个文件里面全部清空，启动快了很多，但在启动apk的时候会慢了点。当然了，也可以把android相关的类全部去掉，剩下java的类，试过了也是可以提高速度。

三，系统服务初始化和package扫描

在启动系统服务的init2()时会启动应用层（Java层）的所有服务。

public static void main(String[] args) {

System.loadLibrary("android_servers");
init1(args); //init1 初始化，完成之后会回调init2()
}

在init2()中会启动一个线程来启动所有服务

public static final void init2() {
Log.i(TAG, "Entered the Android system server!");
Thread thr = new ServerThread();
thr.setName("android.server.ServerThread");
thr.start();
}

class ServerThread extends Thread {

。。。

public void run() {

。。。

关键服务：

ServiceManager.addService("entropy", new EntropyService());

ServiceManager.addService(Context.POWER_SERVICE, power);

context = ActivityManagerService.main(factoryTest);

ServiceManager.addService("telephony.registry", new TelephonyRegistry(context));

PackageManagerService.main(context,
factoryTest != SystemServer.FACTORY_TEST_OFF);//apk扫描的服务

ServiceManager.addService(Context.ACCOUNT_SERVICE,
new AccountManagerService(context));

ContentService.main(context,
factoryTest == SystemServer.FACTORY_TEST_LOW_LEVEL);

battery = new BatteryService(context);
ServiceManager.addService("battery", battery);

hardware = new HardwareService(context);
ServiceManager.addService("hardware", hardware);

AlarmManagerService alarm = new AlarmManagerService(context);
ServiceManager.addService(Context.ALARM_SERVICE, alarm);

ServiceManager.addService(Context.SENSOR_SERVICE, new SensorService(context));

WindowManagerService.main(context, power,
factoryTest != SystemServer.FACTORY_TEST_LOW_LEVEL);
ServiceManager.addService(Context.WINDOW_SERVICE, wm);

上面这些都是关键服务，不建议进行裁剪。

下面的这些不是很关键，可以进行裁剪，当是必须相应的修改framework部分的代码，工作量比较大和复杂。我去掉了20个服务，大概需要相应修改大概20多个文件。

statusBar = new StatusBarService(context);
ServiceManager.addService("statusbar", statusBar);

ServiceManager.addService("clipboard", new ClipboardService(context));

imm = new InputMethodManagerService(context, statusBar);
ServiceManager.addService(Context.INPUT_METHOD_SERVICE, imm);

ServiceManager.addService("netstat", new NetStatService(context));

connectivity = ConnectivityService.getInstance(context);

ServiceManager.addService(Context.CONNECTIVITY_SERVICE, connectivity);
ServiceManager.addService(Context.ACCESSIBILITY_SERVICE,
new AccessibilityManagerService(context));

notification = new NotificationManagerService(context, statusBar, hardware);
ServiceManager.addService(Context.NOTIFICATION_SERVICE, notification);

ServiceManager.addService("mount", new MountService(context));

ServiceManager.addService(DeviceStorageMonitorService.SERVICE,
new DeviceStorageMonitorService(context));

ServiceManager.addService(Context.LOCATION_SERVICE, new LocationManagerService(context));

ServiceManager.addService( Context.SEARCH_SERVICE, new SearchManagerService(context) );

if (INCLUDE_DEMO) {
Log.i(TAG, "Installing demo data...");
(new DemoThread(context)).start();
}

Intent intent = new Intent().setComponent(new ComponentName(
"com.google.android.server.checkin",
"com.google.android.server.checkin.CheckinService"));

ServiceManager.addService("checkin", new FallbackCheckinService(context));

wallpaper = new WallpaperManagerService(context);
ServiceManager.addService(Context.WALLPAPER_SERVICE, wallpaper);

ServiceManager.addService(Context.AUDIO_SERVICE, new AudioService(context));

headset = new HeadsetObserver(context);

dock = new DockObserver(context, power);

ServiceManager.addService(Context.BACKUP_SERVICE, new BackupManagerService(context));

ServiceManager.addService(Context.APPWIDGET_SERVICE, appWidget);

package 扫描部分，整个流程为下图所示：

最终的zip文件（apk）读取是在下面这两个函数：

/*
* Open the specified file read-only.We memory-map the entire thing and
* close the file before returning.
*/
status_t ZipFileRO::open(const char* zipFileName)
{
int fd = -1;
off_t length;

assert(mFileMap == NULL);

LOGD("opening zip '%s'\n", zipFileName);

/*
* Open and map the specified file.
*/

fd = ::open(zipFileName, O_RDONLY);

if (fd < 0) {
LOGW("Unable to open zip '%s': %s\n", zipFileName, strerror(errno));
return NAME_NOT_FOUND;
}

length = lseek(fd, 0, SEEK_END);
if (length < 0) {
close(fd);
return UNKNOWN_ERROR;
}

mFileMap = new FileMap();

if (mFileMap == NULL) {
close(fd);
return NO_MEMORY;
}

if (!mFileMap->create(zipFileName, fd, 0, length, true)) {
LOGW("Unable to map '%s': %s\n", zipFileName, strerror(errno));
close(fd);
return UNKNOWN_ERROR;
}

mFd = fd;

/*
* Got it mapped, verify it and create data structures for fast access.
*/
if (!parseZipArchive()) {
mFileMap->release();
mFileMap = NULL;
return UNKNOWN_ERROR;
}

LOGD("done opening zip\n");

return OK;
}

/*
* Parse the Zip archive, verifying its contents and initializing internal
* data structures.
*/
bool ZipFileRO::parseZipArchive(void)
{

#define CHECK_OFFSET(_off) { \
if ((unsigned int) (_off) >= maxOffset) { \
LOGE("ERROR: bad offset %u (max %d): %s\n", \
(unsigned int) (_off), maxOffset, #_off); \
goto bail; \
} \
}

const unsigned char* basePtr = (const unsigned char*)mFileMap->getDataPtr();
const unsigned char* ptr;
size_t length = mFileMap->getDataLength();
bool result = false;
unsigned int i, numEntries, cdOffset;
unsigned int val;

/*
* The first 4 bytes of the file will either be the local header
* signature for the first file (kLFHSignature) or, if the archive doesn't
* have any files in it, the end-of-central-directory signature
* (kEOCDSignature).
*/
val = get4LE(basePtr);
if (val == kEOCDSignature) {
LOGI("Found Zip archive, but it looks empty\n");
goto bail;
} else if (val != kLFHSignature) {
LOGV("Not a Zip archive (found 0x%08x)\n", val);
goto bail;
}

/*
* Find the EOCD.We'll find it immediately unless they have a file
* comment.
*/
ptr = basePtr + length - kEOCDLen;

while (ptr >= basePtr) {
if (*ptr == (kEOCDSignature & 0xff) && get4LE(ptr) == kEOCDSignature)
break;
ptr--;
}
if (ptr < basePtr) {
LOGI("Could not find end-of-central-directory in Zip\n");
goto bail;
}

/*
* There are two interesting items in the EOCD block: the number of
* entries in the file, and the file offset of the start of the
* central directory.
*
* (There's actually a count of the #of entries in this file, and for
* all files which comprise a spanned archive, but for our purposes
* we're only interested in the current file.Besides, we expect the
* two to be equivalent for our stuff.)
*/
numEntries = get2LE(ptr + kEOCDNumEntries);
cdOffset = get4LE(ptr + kEOCDFileOffset);

/* valid offsets are [0,EOCD] */
unsigned int maxOffset;
maxOffset = (ptr - basePtr) +1;

LOGV("+++ numEntries=%d cdOffset=%d\n", numEntries, cdOffset);
if (numEntries == 0 || cdOffset >= length) {
LOGW("Invalid entries=%d offset=%d (len=%zd)\n",
numEntries, cdOffset, length);
goto bail;
}

/*
* Create hash table.We have a minimum 75% load factor, possibly as
* low as 50% after we round off to a power of 2.
*/
mNumEntries = numEntries;
mHashTableSize = roundUpPower2(1 + ((numEntries * 4) / 3));
mHashTable = (HashEntry*) calloc(1, sizeof(HashEntry) * mHashTableSize);

/*
* Walk through the central directory, adding entries to the hash
* table.
*/
ptr = basePtr + cdOffset;
for (i = 0; i < numEntries; i++) {
unsigned int fileNameLen, extraLen, commentLen, localHdrOffset;
const unsigned char* localHdr;
unsigned int hash;

if (get4LE(ptr) != kCDESignature) {
LOGW("Missed a central dir sig (at %d)\n", i);
goto bail;
}
if (ptr + kCDELen > basePtr + length) {
LOGW("Ran off the end (at %d)\n", i);
goto bail;
}

localHdrOffset = get4LE(ptr + kCDELocalOffset);
CHECK_OFFSET(localHdrOffset);
fileNameLen = get2LE(ptr + kCDENameLen);
extraLen = get2LE(ptr + kCDEExtraLen);
commentLen = get2LE(ptr + kCDECommentLen);

//LOGV("+++ %d: localHdr=%d fnl=%d el=%d cl=%d\n",
// i, localHdrOffset, fileNameLen, extraLen, commentLen);
//LOGV(" '%.*s'\n", fileNameLen, ptr + kCDELen);

/* add the CDE filename to the hash table */
hash = computeHash((const char*)ptr + kCDELen, fileNameLen);
addToHash((const char*)ptr + kCDELen, fileNameLen, hash);

//localHdr = basePtr + localHdrOffset;
//if (get4LE(localHdr) != kLFHSignature) {
// LOGW("Bad offset to local header: %d (at %d)\n",
// localHdrOffset, i);
//goto bail;
// }

ptr += kCDELen + fileNameLen + extraLen + commentLen;
CHECK_OFFSET(ptr - basePtr);
}

result = true;

bail:
return result;
#undef CHECK_OFFSET
}

红色部分是修改后的代码，大家可以对比一下。(未完。。。)

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