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Am 28.05.20 um 18:06 schrieb Marek
Olšák:
Am 28.05.20 um 12:06
schrieb Michel Dänzer:
> On 2020-05-28 11:11 a.m., Christian König wrote:
>> Well we still need implicit sync [...]
> Yeah, this isn't about "we don't want implicit sync",
it's about "amdgpu
> doesn't ensure later jobs fully see the effects of
previous implicitly
> synced jobs", requiring userspace to do pessimistic
flushing.
Yes, exactly that.
For the background: We also do this flushing for explicit
syncs. And
when this was implemented 2-3 years ago we first did the
flushing for
implicit sync as well.
That was immediately reverted and then implemented
differently because
it caused severe performance problems in some use cases.
I'm not sure of the root cause of this performance problems.
My
assumption was always that we then insert to many pipeline
syncs, but
Marek doesn't seem to think it could be that.
On the one hand I'm rather keen to remove the extra handling
and just
always use the explicit handling for everything because it
simplifies
the kernel code quite a bit. On the other hand I don't want
to run into
this performance problem again.
Additional to that what the kernel does is a "full" pipeline
sync, e.g.
we busy wait for the full hardware pipeline to drain. That
might be
overkill if you just want to do some flushing so that the
next shader
sees the stuff written, but I'm not an expert on that.
Do we busy-wait on the CPU or in WAIT_REG_MEM?
WAIT_REG_MEM is what UMDs do and should be faster.
We use WAIT_REG_MEM to wait for an EOP fence value to reach memory.
We use this for a couple of things, especially to make sure that the
hardware is idle before changing VMID to page table associations.
What about your idea of having an extra dw in the shared BOs
indicating that they are flushed?
As far as I understand it an EOS or other event might be sufficient
for the caches as well. And you could insert the WAIT_REG_MEM
directly before the first draw using the texture and not before the
whole IB.
Could be that we can optimize this even more than what we do in the
kernel.
Christian.
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