Introduction

There are a lot of articles discussing about Fence mechanism in Android. But I could not find a simple explanation.

What Problem Fence try to resolve?

Problem 1: Producer pass buffer to consumer before consumer could use the buffer

In Android, there is an object called GraphicBuffer, which is passed in between different kind of producuers and consumers. When a producer pass a GraphicBuffer to a consumer, it is possible that the producer is still doing some job on that buffer in background, e.g. because the producer may have told GPU to render the buffer (but CPU has done the job). For performance consideration, CPU could not wait for GPU to finish the job before hand over the buffer to consumer.

Fence was invented to allow to pass the ownership before the buffer is ready to be consumed.

Problem 2: The Producer would dequeue the buffer before it could overwrite the buffer

The same problem exists when the consumer finishes using the buffer by releasing a buffer to buffer queue, but there are some background jobs ongoing which is using the buffer. The producer could dequeue the buffer but have to wait for the fence to be signaled before it could actually overwritten the buffer.

See following description in frameworks/native/include/gui/ConsumerBase.h

        // mFence is a fence which will signal when the buffer associated with
        // this buffer slot is no longer being used by the consumer and can be
        // overwritten. The buffer can be dequeued before the fence signals;
        // the producer is responsible for delaying writes until it signals.
        sp<Fence> mFence;

How it works

When a consumer acquireBuffer(), it will also copy/get the fence that in the future it could wait on.

status_t ConsumerBase::acquireBufferLocked(BufferItem *item,
        nsecs_t presentWhen, uint64_t maxFrameNumber) {
...

    status_t err = mConsumer->acquireBuffer(item, presentWhen, maxFrameNumber);
...

    mSlots[item->mSlot].mFence = item->mFence;
...
}

Above code acquire a buffer from buffer queue and save the fence