CODEWITHSUNDEEP
multithreadingvulkan
Maik Klein
Maik Klein

Reputation: 16148

How to synchronize `vkMapMemory`?

vkMapMemory states:

vkMapMemory does not check whether the device memory is currently in use before returning the host-accessible pointer. The application must guarantee that any previously submitted command that writes to this range has completed before the host reads from or writes to that range, and that any previously submitted command that reads from that range has completed before the host writes to that region

It links to this site which sadly doesn't seem to exist yet. I am wondering how I would synchronize this?

Basically I need to worry about two things

The only real way that I see to synchronize this is with a thread safe list. Every time you want to write/read to/from that buffer you have to add the memory range that you are currently trying to read or write into that thread safe list.

That means when when you want to access that buffer you need to lock that list and search for the range that you are trying to access.

Is that how you would synchronize vkMapMemory or are there other ways to do this?

Upvotes: 4

Views: 2135

Answers (1)

ratchet freak
ratchet freak

Reputation: 48196

The only time that the gpu will try to access the mapped memory is when a command buffer accessing that memory has been submitted. That memory will be in use until the associated vkFence has been signaled.

A fully general solution would be to track every memory access by the gpu and surround each CPU mapped memory access with a begin/end pair that will wait on the appropriate fences and call flush/invalidate as needed. This is a lot of state tracking and a plethora of potentially blocking calls.

However for persistent mesh/texture data you will only need to write to memory to a staging buffer and then copy to a device-local non-host-visible buffer. You shouldn't need this often so a single fence to track whether a copy from it is in flight is enough. Or for data that needs to survive only for a single frame (per object transforms) you can use a ring buffer. Readback of GPU occlusion test or compute results, you can use a ring buffer.

I hope you can see the pattern emerging. Use just a few mapped ring-buffers and be very conscious about when they are used by the gpu and then you just need to keep a small array of vkFence+offset+size per ring buffer to ensure no data hazard occurs.

Upvotes: 5

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