On 12/06/2017 03:25 AM, Nicolai Hähnle wrote:
On 06.12.2017 08:07, James Jones wrote:
[snip]
So lets say you have a setup where both display and GPU supported
FOO/tiled, but only GPU supported compressed (FOO/CC) and cached
(FOO/cached). But the GPU supported the following transitions:
trans_a: FOO/CC -> null
trans_b: FOO/cached -> null
Then the sets for each device (in order of preference):
GPU:
1: caps(FOO/tiled, FOO/CC, FOO/cached);
constraints(alignment=32k)
2: caps(FOO/tiled, FOO/CC); constraints(alignment=32k)
3: caps(FOO/tiled); constraints(alignment=32k)
Display:
1: caps(FOO/tiled); constraints(alignment=64k)
Merged Result:
1: caps(FOO/tiled, FOO/CC, FOO/cached);
constraints(alignment=64k);
transition(GPU->display: trans_a, trans_b; display->GPU: none)
2: caps(FOO/tiled, FOO/CC); constraints(alignment=64k);
transition(GPU->display: trans_a; display->GPU: none)
3: caps(FOO/tiled); constraints(alignment=64k);
transition(GPU->display: none; display->GPU: none)
We definitely don't want to expose a way of getting uncached rendering
surfaces for radeonsi. I mean, I think we are supposed to be able
to program
our hardware so that the backend bypasses all caches, but (a) nobody
validates that and (b) it's basically suicide in terms of
performance. Let's
build fewer footguns :)
sure, this was just a hypothetical example. But to take this case as
another example, if you didn't want to expose uncached rendering (or
cached w/ cache flushes after each draw), you would exclude the entry
from the GPU set which didn't have FOO/cached (I'm adding back a
cached but not CC config just to make it interesting), and end up
with:
trans_a: FOO/CC -> null
trans_b: FOO/cached -> null
GPU:
1: caps(FOO/tiled, FOO/CC, FOO/cached); constraints(alignment=32k)
2: caps(FOO/tiled, FOO/cached); constraints(alignment=32k)
Display:
1: caps(FOO/tiled); constraints(alignment=64k)
Merged Result:
1: caps(FOO/tiled, FOO/CC, FOO/cached); constraints(alignment=64k);
transition(GPU->display: trans_a, trans_b; display->GPU: none)
2: caps(FOO/tiled, FOO/cached); constraints(alignment=64k);
transition(GPU->display: trans_b; display->GPU: none)
So there isn't anything in the result set that doesn't have GPU cache,
and the cache-flush transition is always in the set of required
transitions going from GPU -> display
Hmm, I guess this does require the concept of a required cap..
Which we already introduced to the allocator API when we realized we
would need them as we were prototyping.
Note I also posed the question of whether things like cached (and
similarly compression, since I view compression as roughly an
equivalent mechanism to a cache) in one of the open issues on my XDC
2017 slides because of this very problem of over-pruning it causes.
It's on slide 15, as "No device-local capabilities". You'll have to
listen to my coverage of it in the recorded presentation for that
slide to make any sense, but it's the same thing Nicolai has laid out
here.
As I continued working through our prototype driver support, I found I
didn't actually need to include cached or compressed as capabilities:
The GPU just applies them as needed and the usage transitions make it
transparent to the non-GPU engines. That does mean the GPU driver
currently needs to be the one to realize the allocation from the
capability set to get optimal behavior. We could fix that by
reworking our driver though. At this point, not including
device-local properties like on-device caching in capabilities seems
like the right solution to me. I'm curious whether this applies
universally though, or if other hardware doesn't fit the "compression
and stuff all behaves like a cache" idiom.
Compression is a part of the memory layout for us: framebuffer
compression uses an additional "meta surface". At the most basic level,
an allocation with loss-less compression support is by necessity bigger
than an allocation without.
We can allocate this meta surface separately, but then we're forced to
decompress when passing the surface around (e.g. to a compositor.)
Consider also the example I gave elsewhere, where a cross-vendor tiling
layout is combined with vendor-specific compression:
Device 1, rendering: caps(BASE/foo-tiling, VND1/compression)
Device 2, sampling/scanout: caps(BASE/foo-tiling, VND2/compression)
Some more thoughts on caching or "device-local" properties below.
Compression requires extra resources for us as well. That's probably
universal. I think the distinction between the two approaches is
whether the allocating driver deduces that compression can be used with
a given capability set and hence adds the resources implicitly, or
whether the capability set indicates it explicitly. My theory is that
the implicit path is possible, but it has downsides. The explicit path
is attractive due to its exact nature, as I alluded to in my talk: You
can tell the exact properties of an allocation given the capability set
used to allocate it. If that can be made to work, I prefer that path as
well. Agreed that your path also works better for the
multi-vendor+device example.
[snip]
I think I like the idea of having transitions being part of the
per-device/engine cap sets, so that such information can be used upon
merging to know which capabilities may remain or have to be dropped.
I think James's proposal for usage transitions was intended to work
with flows like:
1. App gets GPU caps for RENDER usage
2. App allocates GPU memory using a layout from (1)
3. App now decides it wants use the buffer for SCANOUT
4. App queries usage transition metadata from RENDER to SCANOUT,
given the current memory layout.
5. Do the transition and hand the buffer off to display
No, all usages the app intends to transition to must be specified up
front when initially querying caps in the model I assumed. The app
then specifies some subset (up to the full set) of the specified
usages as a src and dst when querying transition metadata.
The problem I see with this is that it isn't guaranteed that there will
be a chain of transitions for the buffer to be usable by display.
I hadn't thought hard about it, but my initial thoughts were that it
would be required that the driver support transitioning to any single
usage given the capabilities returned. However, transitioning to
multiple usages (E.g., to simultaneously rendering and scanning out)
could fail to produce a valid transition, in which case the app would
have to fall back to a copy in that case, or avoid that simultaneous
usage combination in some other way.
Adding transition metadata to the original capability sets, and using
that information when merging could give us a compatible memory layout
that would be usable by both GPU and display.
I'll look into extending the current merging logic to also take into
account transitions.
Yes, it'll be good to see whether this can be made to work. I agree
Rob's example outcomes above are ideal, but it's not clear to me how
to code up such an algorithm. This also all seems unnecessary if
"device local" capabilities aren't needed, as posited above.
although maybe the user doesn't need to know every possible transition
between devices once you have more than two devices..
We should be able to infer how buffers are going to be moved around
from the list of usages, shouldn't we?
Maybe we are missing some bits of information there, but I think the
allocator should be able to know what transitions the app will care
about and provide only those.
The allocator only knows the requested union of all usages currently.
The number of possible transitions grows combinatorially for every
usage requested I believe. I expect there will be cases where ~10
usages are specified, so generating all possible transitions all the
time may be excessive, when the app will probably generally only care
about 2 or 3 states, and in practice, there will probably only
actually be 2 or 3 different underlying possible combinations of
operations.
Exactly. So I wonder if we can't just "cut through the bullshit" somehow?
I'm looking for something that would also eliminate another part of the
design that makes me uncomfortable: the metadata for transitions. This
makes me uncomfortable for a number of reasons. Who computes the
metadata? How is the representation of the metadata? With cross-device
usages (which is the whole point of the exercise), this quickly becomes
infeasible.
So instead as a thought experiment, let's just use what we already have:
capabilities and constraints (or properties/attributes).
I kind of already outlined this with the long example in my email here
https://lists.freedesktop.org/archives/mesa-dev/2017-December/179055.html
Let me try to summarize the transition algorithm. Its inputs are:
- the current (source) capability set
- the desired new usages
- the capability sets associated with these usages, as queried when the
surface was allocated
Steps of the algorithm:
1. Compute the merged capability set for the new usages (the destination
capability set).
2. Compute the transition capability set, which is the merger of the
source and destination sets.
3. Determine whether a "release" transition is required on the source
device(s):
3a. For global properties, a transition is required if the source
capability set is a superset of the transition set.
3b. For device-local properties, a transition is required if there is
some destination device for which the device-local properties are a
subset of the source set.
4. Determine whether an "acquire" transition is required on the
destination device(s) in a similar way.
Finally, execute the transitions using corresponding APIs, where the
APIs simply receive the computed capability sets.
For example, release transitions would receive the source capability set
(and perhaps the source usages), the transition capability set, and the
set difference of device-local capabilities, and nothing else.
The point is that all steps of the algorithm can be implemented in a
device-agnostic way in libdevicealloc, without calling into any
device/driver callbacks.
I'm pretty sure this or something like it can be made to work. We need
to think through a lot of example cases, but at least we'll have thought
them through, which is better than relying on some opaque metadata thing
and then finding out later that there are some new cross-device cases
where things don't work out because the piece of (presumably
device-specific driver) code that computes the metadata isn't aware of
them.
This sounds pretty good. I'd like to see more detailed pseudo-code of a
full cycle (cap query, allocation, transition to and from a few usages),
but it seems pretty solid. I very much like that it enables the
explicit capability sets, but I'm mildly worried it might add API
complexity overall rather than reduce it.
I think in the end our two proposals are very similar: Yours just moves
the conversion from high-level properties -> device commands to the
driver applying the transition. That's fine in theory, though it shifts
some minor overhead to the time of the transition. We could design the
APIs such that it's possible to cache/pre-bake the device commands for a
given transition though to alleviate that if it proves meaningful.
To make it clearer what the "metadata" is in my version and hence
perhaps make it clearer how similar the two are, a few notes:
Transitions are queried per device in my proposal. Note this means you
need to query two different sets of transition metadata for a
cross-device transition, one from the source that would be applied on
that device in the source API, and one from the destination that would
be applied on that device in the destination API. APIs/engines that
don't require transitions would return some NULL metadata indicating no
required transition on that side.
Some examples of the metadata approach:
1) transition from NVIDIA dev rendering -> NVIDIA dev texturing both in
Vulkan, same device:
-Query transition. You'd get some metadata representing very simple
cache management stuff if anything. You'd apply it using some form of
pipeline barrier on the relevant image.
2) transition from NVIDIA dev rendering -> NVIDIA dev texturing both in
Vulkan, different device:
-Query transition from each device. You'd get some metadata
representing more complex cache management, and potentially a decompress
depending on the compatibility of the two devices. The driver is the
same for both devices in this case, so it can calculate the similarities
exactly by examining the capability set and each device's properties.
You'd apply it using some form of pipeline barrier with the respective
metadata on the relevant image on each device.
3) transition from NVIDIA dev rendering -> AMD dev texturing both in Vulkan:
-Query transition from each device. NVIDIA driver would see the
destination usage is a foreign device it has no knowledge of and perform
a complete cache flush and decompress. AMD driver would see the source
usage is something it doesn't recognize and perform a full cache
invalidate (and compression surface invalidate, if any?). You'd apply
it using some form of pipeline barrier with the respective metadata on
the relevant image on each device.
4) transition from NVIDIA dev rendering -> NVIDIA encoder with cache
coherence
-Query source transition on GPU dev. Query destination transition on
video encoder dev. GPU recognizes the destination is a device it is
aware has certain properties and hence returns a decompress only since
it knows it has cache coherence. Video encoder dev returns NULL
transition. Apply source transition on source graphics API. Note this
case requires some careful coordination across a vendor's various driver
stacks to perform optimally. It would automatically degrade to the
foreign device case for naive/incomplete drivers though.
[snip]
One final note: When I initially wrote up the capability merging
logic, I treated "layout" as a sort of "special" capability, basically
like Nicolai originally outlined above. Miguel suggested I add the
"required" bit instead to generalize things, and it ended up working
out much cleaner. Besides the layout, there is at least one other
obvious candidate for a "required" capability that became obvious as
soon as I started coding up the prototype driver: memory location. It
might seem like memory location is a simple device-agnostic constraint
rather than a capability, but it's actually too complicated (we need
more memory locations than "device" and "host"). It has to be vendor
specific, and hence fits in better as a capability.
Could you give more concrete examples of what you'd like to see, and why
having this as constraints is insufficient?
We have more than one "device local" memory with different capabilities
on some devices. I think you guys have this situation as well with your
cards with an SSD on them or something if I'm interpretting the
marketing stuf right. I'd like to be able to express those all without
needing to code them into the device-agnostic portion of the allocator
library ahead of time. That way, if we come up with any new clever
ones, we don't need to wait for everyone to update their allocator
library to make use of them.
Additionally, with things like SLI/Crossfire, we end up with a sort of
NUMA memory architecture, where memory on a "remote" card might have
similar but not exactly the same capabilities as device-local memory.
This would be rather complex to represent in the generic constraints as
well.
I think if possible, we should try to keep the design generalized to
as few types of objects and special cases as possible. The more we
can generalize the solutions to our existing problem set, the better
the mechanism should hold up as we apply it to new and unknown
problems as they arise.
I'm coming around to the fact that those things should perhaps live in a
single list/array, but I still don't like the term "capability".
I admit it's a bit of bike-shedding, but I'm starting to think it would
be better to go with the generic term "property" or "attribute", and
then add flags/adjectives to that based on how merging should work.
This would include the constraints as well -- it seems arbitrary to me
that those would be singled out into their own list.
Basically, the underlying principle is that a good API would have either
one list that includes all the properties, or one list per
merging-behavior. And I think one single list is easier on the API
consumer and easier to extend.
Agreed with Rob. Constraints are different for a reason: They're
non-extensible and hence can merge in more complex ways. Capabilities
are extensible, but must be merged by simple memcmp()-style operations,
currently more or less simple intersection.
However, I also don't care about naming. "Constraints" was chosen
because it connotates negatively since they "limit" what an allocation
created from a capability set can do, and similarly "capabilities"
connotates positively because it indicates things that are built up
additively to describe abilities of an allocation. However, I don't
know that that metaphor held up entirely as the design was realized, so
it might be a good time to bikeshed new names anyway.
Thanks,
-James
Cheers,
Nicolai
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