Document CopyToDeviceCache and MoveToDeviceCache in README

HeterogeneousCore/AlpakaCore/README.md

@@ -164,6 +164,26 @@ In the [`fillDescriptions()`](https://twiki.cern.ch/twiki/bin/view/CMSPublic/SWG
 
 Also note that the `fillDescription()` function must have the same content for all backends, i.e. any backend-specific behavior with e.g. `#ifdef` or `if constexpr` are forbidden.
 
+### Copy e.g. configuration data to all devices in EDProducer
+
+While the EventSetup can be used to handle copying data to all devices
+of an Alpaka backend, for data used only by one EDProducer a simpler
+way would be to use one of
+* `cms::alpakatools::MoveToDeviceCache<TQueue, THostObject>` (recommended)
+  * `#include "HeterogeneousCore/AlpakaCore/interface/MoveToDeviceCache.h"`
+  * Moves the `THostObject` to all devices using `cms::alpakatools::CopyToDevice<THostObject>` synchronously. On host backends the argument `THostObject` is moved around, but not copied.
+  * The `THostObject` must not be copyable
+    * This is to avoid easy mistakes with objects that follow copy semantics of `std::shared_ptr` (that includes Alpaka buffers), that would allow the source memory buffer to be used via another copy during the asynchronous data copy to the device.
+  * The constructor argument `THostObject` object may not be used, unless it is initialized again e.g. by assigning another `THostObject` into it.
+  * The corresponding device-side object can be obtained with `get()` member function using either alpaka Device or Queue object. It can be used immediately after the constructor returns.
+* `cms::alpakatools::CopyToDeviceCache<TQueue, THostObject>` (use only if **must** use copyable `THostObject`)
+  * `#include "HeterogeneousCore/AlpakaCore/interface/CopyToDeviceCache.h"`
+  * Copies the `THostObject` to all devices using `cms::alpakatools::CopyToDevice<THostObject>` synchronously. Also host backends do a copy.
+  * The constructor argument `THostObject` object can be used for other purposes immediately after the constructor returns
+  * The corresponding device-side object can be obtained with `get()` member function using either alpaka Device or Queue object. It can be used immediately after the constructor returns.
+
+For examples see [`HeterogeneousCore/AlpakaTest/plugins/alpaka/TestAlpakaGlobalProducerCopyToDeviceCache.cc`](../../HeterogeneousCore/AlpakaTest/plugins/alpaka/TestAlpakaGlobalProducerCopyToDeviceCache.cc) and [`HeterogeneousCore/AlpakaTest/plugins/alpaka/TestAlpakaGlobalProducerMoveToDeviceCache.cc`](../../HeterogeneousCore/AlpakaTest/plugins/alpaka/TestAlpakaGlobalProducerMoveToDeviceCache.cc).
+
 ## Guarantees
 
 * All Event data products in the device memory space are guaranteed to be accessible only for operations enqueued in the `Queue` given by `device::Event::queue()` when accessed through the `device::Event`.