level-zero
level-zero copied to clipboard
oneapi-src
why no semaphores? events are a bit fine grained
While level 0 has fences and events it doesn't appear to have any semaphore support.
Events can be a messy solution from a scheduler point of view, as they pretty much block the GPU command processing hard at a point in time, some GPUs can reschedule other command streams, some can't. So while useful events can be a bit of a large hammer to hit a system with from a user application. Also GPU selected events invariably mean the CPU side is spinning on some memory location polling the event, again not really a great thing to have to depend on.
Vulkan recently added timeline semaphores and perhaps there should be some consideration of adding them to Level 0 as a better scheduling mechanism.
We had early drafts of the spec that did include the concept of timeline semaphores, but we have decided to pursue a different solution. We’re not quite ready to provide details but stay tuned.
I will respond to this issue again once we are ready to release the specification and we can discuss whether this addresses your concerns.
I'd just like to note that vulkan spec has removed the ability for a device to wait on an event and the host to signal it later. I'm not sure if doing in that in level 0 is allowed, but it might be good to explicitly call it as acceptable, since vulkan has gone the other way.
https://www.khronos.org/registry/vulkan/specs/1.2/html/chap6.html#commandbuffers-submission-progress for vulkan wording.