背景：
做opengl 跨线程渲染的时候，避免了不了使用Fence机制其使用也很简单，今天我们就从源码角度看下Fence到底做了啥
Fence
| C++
GL_APICALL GLsync GL_APIENTRY glFenceSync(GLenum condition, GLbitfield flags)
GL_APICALL void GL_APIENTRY glDeleteSync(GLsync sync)
GL_APICALL GLenum GL_APIENTRY glClientWaitSync(GLsync sync, GLbitfield flags, GLuint64 timeout)
|
今天就从这个3个方法来看下源码里具体的实现
glFenceSync
| C++
GL_APICALL GLsync GL_APIENTRY glFenceSync(GLenum condition, GLbitfield flags)
{
TRACE("(GLenum condition = 0x%X, GLbitfield flags = %X)", condition, flags);
es2::Context *context = es2::getContext();
if(context)
{
return context->createFenceSync(condition, flags);
}
return nullptr;
} |
createFenceSync
| C++
GLsync Context::createFenceSync(GLenum condition, GLbitfield flags)
{
GLuint handle = mResourceManager->createFenceSync(condition, flags);
return reinterpret_cast(static_cast(handle));
}
GLuint ResourceManager::createFenceSync(GLenum condition, GLbitfield flags)
{
GLuint name = mFenceSyncNameSpace.allocate();
FenceSync *fenceSync = new FenceSync(name, condition, flags);
fenceSync->addRef();
mFenceSyncNameSpace.insert(name, fenceSync);
return name;
}
|
可以看的出来这个fenc指针最终存储在mResourceManager这个对象里边，而mResourceManager是可以通过共享context被另外一个线程context持有的，所以fence指针另外线程是可以访问到的
glClientWaitSync
一般AddFence后会在另外一个线程调用glClientWaitSync 等待完成，那我们就看下它是怎么实现的
| C++
GL_APICALL GLenum GL_APIENTRY glClientWaitSync(GLsync sync, GLbitfield flags, GLuint64 timeout)
{
TRACE("(GLsync sync = %p, GLbitfield flags = %X, GLuint64 timeout = %llu)", sync, flags, timeout);
if((flags & ~(GL_SYNC_FLUSH_COMMANDS_BIT)) != 0)
{
error(GL_INVALID_VALUE);
}
es2::Context *context = es2::getContext();
if(context)
{
es2::FenceSync *fenceSyncObject = context->getFenceSync(sync);
if(fenceSyncObject)
{
return fenceSyncObject->clientWait(flags, timeout);
}
else
{
return error(GL_INVALID_VALUE, GL_FALSE);
}
}
return GL_FALSE;
} |
接下来看下fenceSyncObject->clientWait这个实现
| C++
GLenum FenceSync::clientWait(GLbitfield flags, GLuint64 timeout)
{
// The current assumtion is that no matter where the fence is placed, it is
// done by the time it is tested, which is similar to Context::flush(), since
// we don't queue anything without processing it as fast as possible.
return GL_ALREADY_SIGNALED;
}
|
我看到这个开源库是没实现这个方法的，这里比较遗憾
glDeleteSync
这个就很简单了
| C++
GL_APICALL void GL_APIENTRY glDeleteSync(GLsync sync)
{
TRACE("(GLsync sync = %p)", sync);
es2::Context *context = es2::getContext();
if(context)
{
context->deleteFenceSync(sync);
}
}
void Context::deleteFenceSync(GLsync fenceSync)
{
// The spec specifies the underlying Fence object is not deleted until all current
// wait commands finish. However, since the name becomes invalid, we cannot query the fence,
// and since our API is currently designed for being called from a single thread, we can delete
// the fence immediately.
mResourceManager->deleteFenceSync(static_cast(reinterpret_cast(fenceSync)));
}
void ResourceManager::deleteFenceSync(GLuint fenceSync)
{
FenceSync *fenceObject = mFenceSyncNameSpace.remove(fenceSync);
if(fenceObject)
{
fenceObject->release();
}
}
void Object::release()
{
if(dereference() == 0)
{
delete this;
}
} |
当引用计数是0的时候，会清楚这个资源，也就是一旦调用Delete共享context线程，与原线程都不用使用了Fence了