Tag Archives: Barriers

Buffer management with Vulkan : transfer, Staging buffer

Hi guys !
I keep my promise and I am coming with explanations and implementation on how to use and manage one (or several) buffer in Vulkan application.

How I manage my resources ?


Firstly, I have a way to manage Vulkan resource that is a bit weird. The idea is to “emulate” the behaviour of shared_ptr and enable copy / move.
So, if you do that :

b1 and b2 are exactly the same Vulkan’s Image.

A counter?

To emulate the behaviour of a shared_ptr, I created one class that is simply a Counter.

A Vulkan Resource

A Vulkan resource lives through a device. So I wrote this little class that represents a Vulkan Resource :

Buffer in Vulkan

Unlike OpenGL, buffers in Vulkan are separated from memory. You must bind the memory to them. Since you can choose if you want the memory on the device_local heap or in the host_visible, you can chose which heap your buffer will use.

So what are buffer made with ?

Buffers are made with a size, a usage (vertex? uniform ?), one block of memory, one ptr if the buffer is HOST_VISIBLE etc.
My buffer class is :

It may be is a bit complicate, but it is not really that difficult. A buffer will be created with an usage, one size and one boolean to put or not this buffer in device_local memory.
The creation of the buffer is quite simple. You just have to give the size and the usage :

The last line is to get the memory requirements. It will give you the real size you need (padding or other things) and list of memory types that can be used with the buffer.
To get the memory type index, I developed this function which cares about device local memory or host visible memory :

This code was made with the specifications themselves.
Now we should allocate memory for our buffers :

As you can see, you allocate the memory, and you bind the memory. If the memory is host visible, you can map it.

Now we have a class to manage our buffers. But it is not finished at all !

Staging resources

We cannot write directly to the device_local memory. We must use something that we call a staging resource. Staging resources can be buffers or images. The idea is to bind a host visible memory to a staging resource, and transfer the memory through the staging resource to a resource with memory that resides in device_local memory.

staging buffer

Command Buffers submitting

Before to transfer anything, I wanted to have a class that manages the submitting of command buffers. When the work is done, the command submitter should notify transferer object that use it. I used an observer pattern :

The code is not difficult, it allocates if needed one command buffer and return it and use fencing to know if works are completed.

Buffer transferer

You guessed that our Buffer transferer must implement the abstract class :

The idea is to have several buffers ready to transfer data. Why this idea? Because users may don’t care about the CPU buffer and only want a GPU Buffer ! Thanks to that, if he wants to transfer data like glBufferSubData, he actually can !

The code to transfer a buffer is not complicated at all. However, you just have to be careful about the memory barrier. Personally, I use one from Transfer to ALL_Commands in this case.

I do not manage the reallocation if the dst buffer is too small, or loop / recursion the transfer when our staging buffer is too small, but with our architecture, It would not be difficult to manage these cases !

How to use it??

Simply like that !

I saw that a high number of my visits comes from twitter. If you want to follow me: it is here.

Kisses and see you soon to see how to load / manage images !

Barriers in Vulkan : They are not that difficult

Hi !
Yes, I know, I lied, I said that my next article will be about buffers or images, but, finally, I’d prefer to talk about barriers first. However, barriers are, IMHO, a really difficult thing to well understand, so, this article might countain some mistakes.
In that case, please, let me know it by mail, or by one comment.
By the way, this article could remind you in some parts the article on GPU Open : Performance Tweets series: Barriers, fences, synchronization and Vulkan barriers explained

What memory barriers are for?

Memory barriers are source of bugs.
More seriously, barriers are used for three (actually four) things.

  1. Execution Barrier (synchronization) : To ensure that prior commands has finished
  2. Memory Barrier (memory visibility / availability): To ensure that prior writes are visible
  3. Layout Transitioning (useful for image) : To Optimize the usage of the resource
  4. Reformatting

I am not going to talk about reformating because (it is a shame) I am not very confident with it.

What exactly is an execution barrier ?

An execution barrier could remind you mutex on CPU thread. You write something in one resource. When you want to read what you write in, you must wait the write is finished.

What exactly is a memory barrier ?

When you write something from one thread, it could write it on some caches and you must flush them to ensure the visibility where you want to read that data. That is what memory barriers are for.
They ensure as well layout transition for image to get the best performance your graphic card can.

How it is done in Vulkan

Now that we understand why barriers are so important, we are going to see how can we use them in Vulkan.

Vulkan’s Pipeline

Vulkan Pipeline

To be simple, the command enters in the top_of_pipe stage and end at bottom_of_pipe stage.
It exists an extra stage that refers to the host.

Barriers between stages

We are going to see two examples (that are inspired from GPU Open).
We will begin with the worse case : your first command writes at each stage everywhere it is possible, your second command reads at each stage everywhere it is possible.
It simply means that you want to wait for the first command totally finish before the second one begin.

To be simple, with a scheme it means that :

  • In gray : All the stages that need to be executed before or after the barrier (or the ones that are never reached)
  • In red : Above the barrier, it means where the data are produced. Below the barrier, it means where the data are consumed.
  • In green : They are unblocked stages. You should try to have the maximum green stages as possible.

As you can see, here, you don’t have any green stages, so it is not good at all for performances.

In Vulkan C++, you should have something like that:

Some people use BOTTOM_OF_PIPE as source and TOP_OF_PIPE as the destination. It is not false, but it is useful only for execution barrier. These stages do not access memory, so they can’t make memory access visible or even available!!!! You should not (must not?) issue a memory barrier on these stages, but we are going to see that later.

Now, we are going to see a better case
Imagine your first command fills an image or one buffer (SSBO or imageStore) through the VERTEX_SHADER. Now imagine you want to use these data in EVALUATION_SHADER.
The prior scheme, after modification, is :
barriers in the good way

As you can see, there is a lot of green stages and it is very good!
The Vulkan C++ code should be:

By Region or not?

This part may contain errors, so please, let me know if you disagree with me
To begin, what does by region means?
A region is a little part of your framebuffer. If you specify to use by region dependency, it means that (in fragment buffer space) operations need to be finished only in the region (that is specific to the implementation) and not in the whole image.
Well, it is not clear what is a fragment buffer space. In my opinion, and after reading the documentation, it could be from the EARLY_TEST (or at least FRAGMENT_SHADER if early depth is not enabled) to the COLOR_ATTACHMENT.

Actually, to me this flag lets the driver to optimize a bit. However, it must be used only (and should not be useful elsewhere IMHO) between subpasses for subpasses input attachments).
But I may be wrong !

Everything above about is wrong, if you want a plain explanation, see the comment from devsh. To make it simple, it means that the barrier will operate only on “one pixel” of the image. It could be used for input attachment or pre depth pass for example

Memory Barriers

Okay, now that we have seen how make a pure execution barrier (that means without memory barriers).
Memory barriers ensure the availability for the first half memory dependency and the visibility for the second one. We can see them as a “flushing” and “invalidation”. Make information available does not mean that it is visible.
In each kind of memory barrier you will have a srcAccessMask and a dstAccessMask.
How do they work?

Access and stage are somewhat coupled. For each stage of srcStage, all memory accesses using the set of access types defined in srcAccessMask will be made available. It can be seen as a flush of caches defined by srcAccessMask in all stages.

For dstStage / dstAccess, it is the same thing, but instead to make information available, the information is made visible for these stages and these accesses.

That’s why using BOTTOM/TOP_OF_PIPELINE is meaningless for memory barrier.

For buffer and image barriers, you could as well perform a “releasing of ownership” from a queue to another of the resource you are using.
An example, you transfer the image in your queue that is only used for transfers. At the end, you must perform a releasing from the transfer queue to the compute (or graphic) queue.

Global Memory Barriers

These kind of memory barriers applies to all memory objects that exist at the time of its execution.
I do not have any example of when to use this kind of memory barrier. Maybe if you have a lot of barriers to do, it is better to use global memory barriers.
An example:

Buffer Memory Barriers

Here, accessesMask are valid only for the buffer we are working on through the barrier.
Here is the example :

Image Memory Barriers

Image memory barriers have another kind of utility. They can perform layout transitions.

I want to create mipmaps associated to one image (we will see the complete function in another article) through vkCmdBlitImage.
After a vkCmdBlitImage, I want use the mipmap I just wrote as a source for the next mipmap level.

oldLayout must be DST_TRANSFER and newLayout must be SRC_TRANSFER.
Which kind of access I made and which kind of access I will do?
That is easy, I performed a TRANSFER_WRITE and I want to perform a TRANSFER_READ.
At each stage my last command “finish” and at each stage my new command “begin”? Both in TRANSFER_STAGE.

In C++ it is done by something like that:

I hope that you enjoyed that article and that you have learned some things. Synchronization through Vulkan is not as easy to handle and all I wrote may (surely?) contains some errors.


Memory barriers on TOP_OF_PIPE #128