RENDERING BATTLEFIELD 4
WITH MANTLE
Johan Andersson – Electronic Arts
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DX11
Mantle
Avg: 78 fps
Min: 42 fps
Avg: 120 fps
Min: 94 fps
+58%!
Core i7-3970x, AMD Radeon R9 290x, 1080p ULTRA
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BF4 MANTLE GOALS
Goals:
– Significantly improve CPU performance
Non-goals:
– Design new renderer from scratch for Mantle
– More consistent & stable performance
– Improve GPU performance where possible
– Add support for a new Mantle rendering
backend in a live game
 Minimize changes to engine interfaces
 Compatible with built PC content
– Work on wide set of hardware
 APU to quad-GPU
 But x64 only (32-bit Windows needs to die)
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– Take advantage of asymmetric MGPU
(APU+discrete)
– Optimize video memory consumption
BF4 MANTLE STRATEGIC GOALS
 Prove that low-level graphics APIs work outside of consoles
 Push the industry towards low-level graphics APIs everywhere
 Build a foundation for the future that we can build great games on
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SHADERS
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SHADERS
 Shader resource bind points replaced with a resource table object - descriptor set
– This is how the hardware accesses the shader resources
– Flat list of images, buffers and samplers used by any of the shader stages
– Vertex shader streams converted to vertex shader buffer loads
 Engine assign each shader resource to specific slot in the descriptor set(s)
– Can share slots between shader stages = smaller descriptor sets
– The mapping takes a while to wrap one’s head around
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SHADER CONVERSION
 DX11 bytecode shaders gets converted to AMDIL & mapping applied using ILC tool
– Done at load time
– Don’t have to change our shaders!
 Have full source & control over the process
 Could write AMDIL directly or use other frontends if wanted
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DESCRIPTOR SETS
 Very simple usage in BF4: for each draw call write flat list of resources
–Essentially direct replacement of SetTexture/SetConstantBuffer/SetInputStream
 Single dynamic descriptor set object per frame
 Sub-allocate for each draw call and write list of resources
 ~15000 resource slots written per frame in BF4, still very fast
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DESCRIPTOR SETS
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DESCRIPTOR SETS – FUTURE OPTIMIZATIONS
 Use static descriptor sets when possible
 Reduce resource duplication by reusing & sharing more across shader stages
 Nested descriptor sets
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COMPUTE PIPELINES
 1:1 mapping between pipeline & shader
 No state built into pipeline
 Can execute in parallel with rendering
 ~100 compute pipelines in BF4
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GRAPHICS PIPELINES
 All graphics shader stages combined to a single pipeline object together with important graphics state
 ~10000 graphics pipelines in BF4 on a single level, ~25 MB of video memory
 Could use smaller working pool of active state objects to keep reasonable amount in memory
– Have not been required for us
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PRE-BUILDING PIPELINES
 Graphics pipeline creation is expensive operation, do at load time instead of runtime!
– Creating one of our graphics pipelines take ~10-60 ms each
– Pre-build using N parallel low-priority jobs
– Avoid 99.9% of runtime stalls caused by pipeline creation!
 Requires knowing the graphics pipeline state that will be used with the shaders
– Primitive type
– Render target formats
– Render target write masks
– Blend modes
 Not fully trivial to know all state, may require engine changes / pre-defining use cases
– Important to design for!
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PIPELINE CACHE
 Cache built pipelines both in memory cache and disk cache
– Improved loading times
– Max 300 MB
– Simple LRU policy
– LZ4 compressed (free)
 Database signature:
– Driver version
– Vendor ID
– Device ID
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MEMORY
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MEMORY MANAGEMENT
 Mantle devices exposes multiple memory heaps with characteristics
– Can be different between devices, drivers and OS:es
Type
Size
Page
CPU access
GPU
Read
GPU
Write
CPU
Read
CPU
Write
Local
256 MB
65535
CpuVisible|CpuGpuCoherent|CpuUncached|CpuWriteCombined
130
170
0.0058
2.8
Local
4096 MB
65535
130
180
0
0
Remote
16106 MB
65535
CpuVisible|CpuGpuCoherent|CpuUncached|CpuWriteCombined
2.6
2.6
0.1
3.3
Remote
16106 MB
65535
CpuVisible|CpuGpuCoherent
2.6
2.6
3.2
2.9
 User explicitly places resources in wanted heaps
– Driver suggests preferred heaps when creating objects, not a requirement
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FROSTBITE MEMORY HEAPS
 System Shared Mapped
– CPU memory that is GPU visible.
– Write combined & persistently mapped = easy
& fast to write to in parallel at any time
 System Shared Pinned
– CPU cached for readback.
– Not used much
 Video Shared
– GPU memory accessible by CPU. Used for
descriptor sets and dynamic buffers
– Max 256 MB (legacy constraint)
– Avoid keeping persistently mapped as WDMM
doesn’t like this and can decide to move it back
to CPU memory 
 Video Private
– GPU private memory.
– Used for render targets, textures and other
resources CPU does not need to access
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MEMORY REFERENCES
 WDDM needs to know which memory allocations are referenced for each command buffer
– In order to make sure they are resident and not paged out
– Max ~1700 memory references are supported
– Overhead with having lots of references
 Engine needs to keep track of what memory is referenced while building the command buffers
– Easy & fast to do
– Each reference is either read-only or read/write
– We use a simple global list of references shared for all command buffers.
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MEMORY POOLING
 Pooling memory allocations were required for us
– Sub allocate within larger 1 – 32 MB chunks
– All resources stored memory handle + offset
– Not as elegant as just void* on consoles
– Fragmentation can be a concern, not too much issues for us in practice
 GPU virtual memory mapping is fully supported, can simplify & optimize management
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OVERCOMMITTING VIDEO MEMORY
 Avoid overcommitting video memory!
– Will lead to severe stalls as VidMM moves blocks and moves memory back and forth
– VidMM is a black box 
– One of the biggest issues we ran into during development
 Recommendations
– Balance memory pools
– Make sure to use read-only memory references
– Use memory priorities
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MEMORY PRIORITIES
 Setting priorities on the memory allocations helps VidMM choose what to page out when it has to
 5 priority levels
– Very high = Render targets with MSAA
– High = Render targets and UAVs
– Normal = Textures
– Low = Shader & constant buffers
– Very low = vertex & index buffers
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MEMORY RESIDENCY FUTURE
 For best results manage which resources are in video memory yourself & keep only ~80% used
– Avoid all stalls
– Can async DMA in and out
 We are thinking of redesigning to fully avoid possibility of overcommitting
 Hoping WDDM’s memory residency management can be simplified & improved in the future
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RESOURCE MANAGEMENT
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RESOURCE LIFETIMES
 App manages lifetime of all resources
– Have to make sure GPU is not using an object or memory while we are freeing it on the CPU
– How we’ve always worked with GPUs on the consoles
– Multi-GPU adds some additional complexity that consoles do not have
 We keep track of lifetimes on a per frame granularity
– Queues for object destruction & free memory operations
– Add to queue at any time on the CPU
– Process queues when GPU command buffers for the frame are done executing
– Tracked with command buffer fences
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LINEAR FRAME ALLOCATOR
 We use multiple linear allocators with Mantle for both transient buffers & images
– Used for huge amount of small constant data and other GPU frame data that CPU writes
– Easy to use and very low overhead
– Don’t have to care about lifetimes or state
 Fixed memory buffers for each frame
– Super cheap sub-allocation from from any thread
– If full, use heap allocation (also fast due to pooling)
 Alternative: ring buffers
– Requires being able to stall & drain pipeline at any allocation if full, additional complexity for us
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TILING
 Textures should be tiled for performance
– Explicitly handled in Mantle, user selects linear or tiled
– Some formats (BC) can’t be accessed as linear by the GPU
 On consoles we handle tiling offline as part of our data processing pipeline
– We know the exact tiling formats and have separate resources per platform
 For Mantle
– Tiling formats are opaque, can be different between GPU architectures and image types
– Tile textures with DMA image upload from SystemShared to VideoPrivate
 Linear source, tiled destination
 Free
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COMMAND BUFFERS
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COMMAND BUFFERS
 Command buffers are the atomic unit of work dispatched to the GPU
– Separate creation from execution
– No “immediate context” a la DX11 that can execute work at any call
– Makes resource synchronization and setup significantly easier & faster
 Typical BF4 scenes have around ~50 command buffers per frame
– Reasonable tradeoff for us with submission overhead vs CPU load-balancing
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COMMAND BUFFER SOURCES
 Frostbite has 2 separate sources of command buffers
– World rendering
 Rendering the world with tons of objects, lots of draw calls. Have all frame data up front
 All resources except for render targets are read-only
 Generated in parallel up front each frame
– Immediate rendering (“the rest”)
 Setting up rendering and doing lighting, post-fx, virtual texturing, compute, etc
 Managing resource state, memory and running on different queues (graphics, compute, DMA)
 Sequentially generated in a single job, simulate an immediate context by splitting the command buffer
 Both are very important and have different requirements
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RESOURCE TRANSITIONS
 Key design in Mantle to significantly lower driver overhead & complexity
– Explicit hazard tracking by the app/engine
– Drives architecture-specific caches & compression
– AMD: FMASK, CMASK, HTILE
– Enables explicit memory management
 Examples:
– Optimal render target writes → Graphics shader read-only
– Compute shader write-only → DrawIndirect arguments
 Mantle has a strong validation layer that tracks transitions which is a major help
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MANAGING RESOURCE TRANSITIONS
 Engines need a clear design on how to handle state transitions
 Multiple approaches possible:
– Sequential in-order command buffers
 Generate one command buffer at the time in order
 Transition resources on-demand when doing operation on them, very simple
 Recommendation: start with this
– Out-of-order multiple command buffers
 Track state per command buffer, fix up transitions when order of command buffers is known
– Hybrid approaches & more
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MANAGING RESOURCE TRANSITIONS IN FROSTBITE
 Current approach in Frostbite is quite basic:
– We keep track of a single state for each resource (not subresource)
– The “immediate rendering” transition resources as needed depending on operation
– The out of order “world rendering” command buffers don’t need to transition states
 Already have write access to MRTs and read-access to all resources setup outside them
 Avoids the problem of them not knowing the state during generation
 Works now but as we do more general parallel rendering it will have to change
– Track resource state for each command buffer & fixup between command buffers
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DYNAMIC STATE OBJECTS
 Graphics state is only set with the pipeline object and 5 dynamic state objects
– State objects: color blend, raster, viewport, depth-stencil, MSAA
– No other parameters such as in DX11 with stencil ref or SetViewport functions
 Frostbite use case:
– Pre-create when possible
– Otherwise on-demand creation (hash map)
– Only ~100 state objects!
 Still possible to end up with lots of state objects
– Esp. with state object float & integer values (depth bounds, depth bias, viewport)
– But no need to store all permutations in memory, objects are fast to create & app manages lifetimes
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QUEUES
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QUEUES
 Universal queue can do both graphics, compute and presents
 We use also use additional queues to parallelize GPU operations:
– DMA queue – Improve perf with faster transfers & avoiding idling graphics will transfering
– Compute queue - Improve perf by utilizing idle ALU and update resources simultaneously with gfx
 More GPUs = more queues!
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QUEUES SYNCHRONIZATION
 Order of execution within a queue is sequential
 Synchronize multiple queues with GPU semaphores (signal & wait)
 Also works across multiple GPUs
Compute
Graphics
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Wait
S
S
W
QUEUES SYNCHRONIZATION CONT
 Started out with explicit semaphores
– Error prone to handle when having lots of different semaphores & queues
– Difficult to visualize & debug
 Switched to more representation more similar to a job graph
 Just a model on top of the semaphores
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GPU JOB GRAPH
 Each GPU job has list of dependencies (other command buffers)
 Dependencies has to finish first before job can run on its queue
 The dependencies can be from any queue
DMA
Compute
Graphics 1
Graphics 2
 Was easier to work with, debug and visualize
 Really extendable going forward
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Graphics 2
ASYNC DMA
 AMD GPUs have dedicated hardware DMA engines, let’s use them!
– Uploading through DMA is faster than on universal queue, even if blocking
– DMA have alignment restrictions, have to support falling back to copies on universal queue
 Use case: Frame buffer & texture uploads
– Used by resource initial data uploads and our UpdateSubresource
– Guaranteed to be finished before the GPU universal queue starts rendering the frame
 Use case: Multi-GPU frame buffer copy
– Peer-to-peer copy of the frame buffer to the GPU that will present it
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ASYNC COMPUTE
 Frostbite has lots of compute shader passes that could run in parallel with graphics work
– HBAO, blurring, classification, tile-based lighting, etc
 Running as async compute can improve GPU performance by utilizing ”free” ALU
– For example while doing shadowmap rendering (ROP bound)
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ASYNC COMPUTE – TILE-BASED LIGHTING
Compute
Graphics
Wait
Gbuffer S
TileZ
Cull lights
Shadowmaps
Reflection
 3 sequential compute shaders
– Input: zbuffer & gbuffer
– Output: HDR texture/UAV
 Runs in parallel with graphics pipeline that renders to other targets
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Lighting S
Distort
W Transp
ASYNC COMPUTE – TILE-BASED LIGHTING
Compute
Graphics
Wait
Gbuffer S
TileZ
Cull lights
Shadowmaps
Lighting S
Reflection
Distort
W Transp
 We manually prepare the resources for the async compute
– Important to not access the resources on other queues at the same time (unless read-only state)
– Have to transition resources on the queue that last used it
 Up to 80% faster in our initial tests, but not fully reliable
– But is a pretty small part of the frame time
– Not in BF4 yet
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MULTI-GPU
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MULTI-GPU
 Multi-GPU alternatives:
– AFR – Alternate Frame Rendering (1-4 GPUs of the same power)
– Heterogeneous AFR – 1 small + 1 big GPU (APU + Discrete)
– SFR – Split Frame Rendering
– Multi-GPU Job Graph – Primary strong GPU + slave GPUs helping
 Frostbite supports AFR natively
– No synchronization points within the frame
– For resources that are not rendered every frame: re-render resources for each GPU
 Example: sky envmap update on weather change
 With Mantle multi-GPU is explicit and we have to build support for it ourselves
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MULTI-GPU AFR WITH MANTLE
 All resources explicitly duplicated on each GPU with async DMA
– Hidden internally in our rendering abstraction
 Every frame alternate which GPU we build command buffers for and are using resources from
 Our UpdateSubresource has to make sure it updates resources on all GPU
 Presenting the screen has to in some modes copy the frame buffer to the GPU that owns the display
 Bonus:
– Can simulate multi-GPU mode even with single GPU!
– Multi-GPU works in windowed mode!
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MULTI-GPU ISSUES
 GPUs are independently rendering & presenting to the screen – can cause micro-stuttering
– Frames are not presented in a regular intervals
– Frame rate can be high but presentation & gameplay is not smooth
– FCAT is a good tool to analyse this
GPU0
GPU1
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Frame 0
Frame 1
P
Frame 2 P
P
Frame 3
P
Irregular
presentation
interval
MULTI-GPU ISSUES
 GPUs are independently rendering & presenting to the screen – can cause micro-stuttering
– Frames are not presented in a regular intervals
– Frame rate can be high but presentation & gameplay is not smooth
– FCAT is a good tool to analyse this
GPU0
GPU1
Frame 0
P
Frame 1
Frame 2
P
P
Frame 3
P
Ideal
presentation
interval
 We need to introduce dependency & dampening between the GPUs to alleviate this – frame pacing
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FRAME PACING
 Measure average frame rate on each GPU
– Short history (10-30 frames)
– Filter out spikes
 Insert delay on the GPU before each present
– Force the frame times to become more regular and GPUs to align
– Delay value is based on the calculate avg frame rate
GPU0
GPU1
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Frame 0
P
Frame 2
Frame 1
D P
Delay
P
Frame 3
P
CONCLUSION
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MANTLE DEV RECOMMENDATIONS
 The validation layer is a critical friend!
 You’ll end up with a lot of object & memory management code, try share with console code
 Make sure you have control over memory usage and can avoid overcommitting video memory
 Build a robust solution for resource state management early
 Figure out how to pre-create your graphics pipelines, can require engine design changes
 Build for multi-GPU support from the start, easier than to retrofit
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FUTURE
 Second wave of Frostbite Mantle titles
 Adapt Frostbite core rendering layer based on learnings from Mantle
– Refine binding & buffer updates to further reduce overhead
– Virtual memory management
– More async compute & async DMAs
– Multi-GPU job graph R&D
 Linux
– Would like to see how our Mantle renderer behaves with different memory management & driver model
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QUESTIONS?
Email: johan@frostbite.com
Web:
http://frostbite.com
Twitter: @repi
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