[SYCL][L0] Programming guide for multi-tile and multi-card under Level-Zero backend. (#3872)

Signed-off-by: Sergey V Maslov <[email protected]>

Author: smaslov-intel

sycl/doc/MultiTileCardWithLevelZero.md

@@ -0,0 +1,175 @@
+# Considerations for programming to multi-tile and multi-card under Level-Zero backend
+
+## 1 Devices discovery
+### 1.1 Root-devices
+
+Intel GPUs are represented as SYCL GPU devices, root-devices.
+The discovery of root-devices is best with "sycl-ls" tool, for example:
+	
+```
+$ sycl-ls
+[opencl:0] GPU : Intel(R) OpenCL HD Graphics 3.0 [21.19.19792]
+[opencl:0] CPU : Intel(R) OpenCL 2.1 [2020.11.11.0.03_160000]
+[level_zero:0] GPU : Intel(R) Level-Zero 1.1 [1.1.19792]
+[host:0] HOST: SYCL host platform 1.2 [1.2]
+```
+
+Note that "sycl-ls" shows all devices from all platforms of all SYCL backends that are seen
+by SYCL runtime. Thus in the example above there is CPU (managed by OpenCL backend) and 2!
+GPUs corresponding to the single physical GPU (managed by either OpenCL or Level-Zero backend).
+There are few ways to filter observable root-devices.
+	
+One is using environment variable SYCL_DEVICE_FILTER described in [EnvironmentVariables.md](https://github.com/intel/llvm/blob/sycl/sycl/doc/EnvironmentVariables.md)
+```
+$ SYCL_DEVICE_FILTER=level_zero sycl-ls
+[level_zero:0] GPU : Intel(R) Level-Zero 1.1 [1.1.19792]
+```
+Another way is to use similar SYCL API described in [FilterSelector.md](https://github.com/intel/llvm/blob/sycl/sycl/doc/extensions/FilterSelector/FilterSelector.adoc)
+E.g. `filter_selector("level_zero")` will only see Level-Zero operated devices.
+
+If there are multiple GPUs in a system then they will be seen as multiple different root-devices.
+On Linux these would be multiple SYCL root-devices of the same SYCL platform (representing Level-Zero driver).
+On Windows these would appear as root-devices of multiple different SYCL platforms (Level-Zero drivers).
+
+`CreateMultipleRootDevices=N NEOReadDebugKeys=1` evironment variables can be used to emulate multiple GPU cards, e.g.
+```
+$ CreateMultipleRootDevices=2 NEOReadDebugKeys=1 SYCL_DEVICE_FILTER=level_zero sycl-ls
+[level_zero:0] GPU : Intel(R) Level-Zero 1.1 [1.1.19792]
+[level_zero:1] GPU : Intel(R) Level-Zero 1.1 [1.1.19792]
+```
+	
+### 1.2 Sub-devices
+	
+Some Intel GPU HW is composed of multiple tiles, e.g. 4 tile ATS.
+The root-device in such cases can be partitioned to sub-devices, each corresponding to the physical tiles.
+
+``` C++	
+try {
+  vector<device> SubDevices = RootDevice.create_sub_devices<
+  cl::sycl::info::partition_property::partition_by_affinity_domain>(
+  cl::sycl::info::partition_affinity_domain::next_partitionable);
+}
+```
+
+Each call to `create_sub_devices` will return exactly the same sub-devices and in the persistent order.
+To control what sub-devices are exposed by Level-Zero UMD one can use ZE_AFFINITY_MASK environment variable.
+
+NOTE: The `partition_by_affinity_domain` is the only partitioning supported for Intel GPU.
+Similar `next_partitionable` and `numa` are the only partitioning properties supported (both doing the same thing).
+
+`CreateMultipleSubDevices=N NEOReadDebugKeys=1` environment variables can be used to emulate multiple tiles of a GPU.
+
+## 2 Context	
+	
+Contexts are used for resources isolation and sharing. A SYCL context may consist of one or multiple devices.
+Both root-devices and sub-devices can be within single context, but they all should be of the same SYCL platform.
+A SYCL program (kernel_bundle) created against a context with multiple devices will be built to each of the root-devices in the context.
+For context that consists of multiple sub-devices of the same root-device only single build (to that root-device) is needed.
+	
+## 3 Memory
+### 3.1 USM
+
+There are multiple ways to allocate memory:
+
+`malloc_device`:
+- Allocation can only be accessed by the specified device but not by other devices in the context nor by host.
+- The data stays on the device all the time and thus is the fastest available for kernel execution.
+- Explicit copy is needed for transfer data to the host or other devices in the context.
+				   
+`malloc_host`:
+- Allocation can be accessed by the host and any other device in the context.
+- The data stays on the host all the time and is accessed via PCI from the devices.
+- No explicit copy is needed for synchronizing of the data with the host or devices.
+			   
+`malloc_shared`:
+- Allocation can be accessed by the host and the specified device only.
+- The data can migrate (operated by the Level-Zero driver) between the host and the device for faster access.
+- No explicit copy is necessary for synchronizing between the host and the device, but it is needed for other devices in the context.
+				   
+NOTE: Memory allocated against a root-device is accessible by all of its sub-devices (tiles).
+So if operating on a context with multiple sub-devices of the same root-device then you can use `malloc_device` on that root-device instead of using the slower `malloc_host`.
+Remember that if using `malloc_device` you'd need an explicit copy out to the host if it necessary to see data there.
+					   
+### 3.2 Buffer
+	
+SYCL buffers are also created against a context and under the hood are mapped to the Level-Zero USM allocation discussed above.
+The current mapping is following:
+
+- For integrated device the allocations are made on host, and are accessible by the host and the device without any copying.
+- Memory buffers for context with sub-devices of the same root-device (possibly including the root-device itself) are allocated on that root-device.
+   Thus they are readily accessible by all the devices in such context. The synchronization with the host is performed by SYCL RT with map/unmap doing implicit copies when necessary.
+- Memory buffers for context with devices from different root-devices in it are allocated on host (thus made accessible to all devices).
+	
+## 4 Queue
+
+SYCL queue is always attached to a single device in a possibly multi-device context.
+Some typical scenarios are the following (from most performant to least performant):
+
+**A.** Context with a single sub-device in it and the queue is attached to that sub-device (tile)
+- The execution/visibility is limited to the single sub-device only
+- Expected to offer the best performance per tile
+- Example:
+``` C++	
+try {
+  vector<device> SubDevices = ...;
+  for (auto &D : SubDevices) {
+    // Each queue is in its own context, no data sharing across them.
+    auto Q = queue(D);
+    Q.submit([&](handler& cgh) {...});
+  }
+}
+```
+
+**B.** Context with multiple sub-devices of the same root-device (multi-tile)
+- Queues are to be attached to the sub-devices effectively implementing "explicit scaling"
+- The root-device should not be passed to such context for better performance
+- Example:
+``` C++	
+try {
+  vector<device> SubDevices = ...;
+  auto C = context(SubDevices);
+  for (auto &D : SubDevices) {
+    // All queues share the same context, data can be shared across queues.
+    auto Q = queue(C, D);
+    Q.submit([&](handler& cgh) {...});
+  }
+}
+```
+	
+**C.** Context with a single root-device in it and the queue is attached to that root-device
+- The work will be automatically distributed across all sub-devices/tiles via "implicit scaling" by the driver
+- The most simple way to enable multi-tile HW but doesn't offer possibility to target specific tiles
+- Example:
+``` C++	
+try {
+  // The queue is attached to the root-device, driver distributes to sub-devices, if any.
+  auto D = device(gpu_selector{});
+  auto Q = queue(D);
+  Q.submit([&](handler& cgh) {...});
+}
+```
+		
+**D.** Contexts with multiple root-devices (multi-card)
+- The most unrestrictive context with queues attached to different root-devices
+- Offers most sharing possibilities at the cost of slow access through host memory or explicit copies needed
+- Example:
+``` C++	
+try {
+  auto P = platform(gpu_selector{});
+  auto RootDevices = P.get_devices();
+  auto C = context(RootDevices);
+  for (auto &D : RootDevices) {
+    // Context has multiple root-devices, data can be shared across multi-card (requires explict copying)
+    auto Q = queue(C, D);
+    Q.submit([&](handler& cgh) {...});
+  }
+}
+```
+
+Depending on the chosen programming model (A,B,C,D) and algorithm used make sure to do proper memory allocation/synchronization.
+				
+## 5 Examples
+	
+These are few examples of programming to multiple tiles and multiple cards:
+- https://github.com/jeffhammond/PRK/blob/dpct/Cxx11/dgemm-multigpu-onemkl.cc
+- https://github.com/pvelesko/PPP/tree/master/languages/c%2B%2B/sycl/gpu2gpu

sycl/doc/index.rst

@@ -34,4 +34,5 @@ Developing oneAPI DPC++ Compiler
    LinkedAllocations
    Assert
    SharedLibraries
+   MultiTileCardWithLevelZero