CP013: Add initial proposal for this_system::discover_topology (#103)

* Add initial proposal for `this_system::discover_topology`
* Add initial wording for system topology term of art.
* Add initial proposal for `system_topology`
* Add initial proposal for `this_system::discover_topology`.
* Introduce terms of art for system resource and topology discovery
policy.
* Introduce minimal `system_resource` class.
* Introduce free function `traverse_topology` for traversing a
`system_topology` according to a topology traversal policy.
* Update the changelog.
* Update link to P0443 to point to latest revision.
* Make the return type of `traverse_topology` be to be defined and add
note about potential options.
* Add more detail to open questions section.

affinity/cpp-23/d1795r1.md

@@ -1,21 +1,29 @@
-# P1795r0: System topology discovery for heterogeneous & distributed computing
+# P1795r1: System topology discovery for heterogeneous & distributed computing
 
-**Date: 2019-06-03**
+**Date: 2019-10-07**
 
-**Audience: SG1, SG14, LEWG**
+**Audience: SG1, SG14**
 
-**Authors: Gordon Brown, Ruyman Reyes, Michael Wong, Mark Hoemmen, Jeff Hammond, Tom Scogland**
+**Authors: Gordon Brown, Ruyman Reyes, Michael Wong, Mark Hoemmen, Jeff Hammond, Tom Scogland, Domagoj Šarić**
 
-**Emails: [email protected], [email protected], [email protected], [email protected], [email protected], [email protected]**
+**Emails: [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected]**
 
 **Reply to: [email protected]**
 
 # Acknowledgements
 
-This paper is the result of discussions from man contributors within the heterogeneous C\+\+ group, including H. Carter Edwards, Thomas Rodgers, Patrice Roy, Carl Cook, Jeff Hammond, Hartmut Kaiser, Christian Trott, Paul Blinzer, Alex Voicu, Nat Goodspeed and Tony Tye.
+This paper is the result of discussions from many contributors within the heterogeneous C\+\+ group, including H. Carter Edwards, Thomas Rodgers, Patrice Roy, Carl Cook, Jeff Hammond, Hartmut Kaiser, Christian Trott, Paul Blinzer, Alex Voicu, Nat Goodspeed and Tony Tye.
 
 # Changelog
 
+### P1437r1 (BEL 2019)
+
+* Introduce terms of art for *system topology*, *system resource* and *topology traversal policy*.
+* Introduce minimal design for `system_topology` class.
+* Introduce minimal design for `system_resource` class.
+* Introduce free function `this_system::discover_topology` for performing  runtime system topology discovery.
+* Introduce free function `traverse_topology` for traversing a `system_topology` using a *topology traversal policy* to return a collection of `execution_resource`s,
+
 ### P1437r0 (COL 2019)
 
 * Split off from [[17]][p0796], focussing on a mechanism for discovering the topology and affinity properties of a given system.
@@ -30,29 +38,29 @@ For the earlier changelogs from prior to the split from P0796 see Appendix A.
 
 This paper is the result of a request from SG1 at the 2018 San Diego meeting to split [[17]][p0796] into two separate papers, one for the high-level interface and one for the low-level interface. This paper focusses on the low-level interface; a mechanism for discovering the topology and affinity properties of a given system. [[18]][p1436] focusses on the high-level interface, a series of properties for querying affinity relationships and requesting affinity on work being executed.
 
-# Background
+# 1. Background
 
 Computer systems are no longer homogeneous platforms. From desktop workstations to high-performance supercomputers, and from mobile devices to purpose-built embedded SoCs, every system has some form of co-processor along side the traditional multi-core CPU, and often more than one. Furthermore, the architectures of these co-processors range from many-core CPUs, GPUs, FPGAs and DSPs to specifically designed vision and machine learning processors. In larger supercomputer systems there are thousands of these processors in some configuration of nodes, connected physically or via network adapters.
 
 The way these processors access memory is also far from homogeneous. For example, the system may present a single shared virtual address space [[21]][hmm] [[22]][opencl-svm], or it may have different address spaces mutually inaccessible other than through special functions [[4]][opencl-2-2]. Different memory regions may have different levels of consistency, cache coherency, and support for atomic operations. Different parts of the system may have different access latencies or bandwidths to different memory regions (so-called "NUMA affinity regions") [[2]][hwloc]. Some parts of memory may be persistent. Different systems may configure the same types of memory in different ways around the processors.
 
 In order to program these new systems and the architectures that inhabit them, it's vital that applications are capable of understating both what architectures are available and the properties of those architectures, namely their observable behaviors, capabilities and limitations. However, the current C\+\+ standard provides no way to achieve this, so developers have to rely entirely on third party and operating system libraries.
 
-# Goals: what this paper is, and what it is not
+# 2. Goals: what this paper is, and what it is not
 
 This paper seeks to define, within C\+\+, a facility for discovering execution resources available to a system that are capable of executing work, and for querying their properties.
 
 However, it is not the goal of this proposal to introduce support in the C\+\+ language or the standard library for all of the various heterogeneous architectures available today. The authors of this paper recognize that this is unrealistic as it would require significant changes to the C\+\+ machine model and would be extremely volatile to future developments in architecture and system design.
 
 Instead, it seeks to define a single, unified, and stable layer in the C\+\+ Standard Library. Applications, libraries, and programming models (such as SYCL [[3]][sycl-1-2-1], Kokkos [[19]][kokkos], HPX [[13]][hpx] or TBB [[12]][tbb]) can build on this layer; hardware vendors can support it via standards such as OpenCL [[4]][opencl-2-2], CUDA [[20]][cuda], OpenMP [[6]][openmp-5], MPI [[16]][mpi], Hwloc [[2]][hwloc], HSA [[5]][HSA] and HMM [[21]][hmm]; and it can be extended when necessary.
 
-This layer will not be characterized in terms of specific categories of hardware such as CPUs, GPUs and FPGAs as these are broad concepts that are subject to change over time and have no foundation in the C\+\+ machine model. It will instead define a number of abstract properties of system architectures that are not tied to any specific hardward.
+This layer will not be characterized in terms of specific categories of hardware such as CPUs, GPUs and FPGAs as these are broad concepts that are subject to change over time and have no foundation in the C\+\+ machine model. It will instead define a number of abstract properties of system architectures that are not tied to any specific hardware.
 
 The initial set of properties that this paper would propose be defined in the C\+\+ standard library would reflect a generalization of the observable behaviors, capabilities and limitations of common architectures available in heterogeneous and distributed systems today. However the intention is that the interface be extensible so that that vendors can provide their own extensions to provide visibility into the more niche characteristics of certain architectures.
 
-It is intended that this layer be defined as a natural extension of the Executors proposal, a unified interface for execution. The current executors proposal [[14]][p0443] already provides a route to supporting heterogeneous and distributed systems, however it is missing a way to identify what architectures a system has.
+It is intended that this layer be defined as a natural extension of the Executors proposal, a unified interface for execution. The current executors proposal [[14]][p0443r11] already provides a route to supporting heterogeneous and distributed systems, however it is missing a way to identify what architectures a system has.
 
-# Motivation
+# 3. Motivation
 
 There are many reasons why such a feature within C\+\+ would benefit developers and the C\+\+ ecosystem as a whole, and those can differ from one domain to another. We've attempted to outline some of these benefits here.
 
@@ -98,11 +106,11 @@ For example, a unified C\+\+ interface for topology discovery could provide acce
 
 Another example of this is that while Hwloc is highly used in many domains, it now does not always accurately represent existing systems. This is because Hwloc presents their topology as strictly hierarchical, which no longer accurately describes many systems. A unified C\+\+ interface does not need to be bound to the limitations of a single library, and can provide a much broader representation of a system's execution resource topology.
 
-# Proposed direction
+# 5. Proposed direction
 
-Below we outline a proposed direction:
+This paper aims to build on the unified executors proposal, detailed in P0443 [[14]][p0443r11], so this proposal and any others that stem from it will target P0443 as a baseline, and aim to integrate with its direction as closely as possible.
 
-* Align with the direction of the unified executors proposal [[14]][p0443].
+Below we outline a proposed direction:
 
 * Propose an abstract definition of an execution resource, as a hardware or software abstraction capable of creating execution agents.
 
@@ -124,11 +132,116 @@ As a result of the above this paper may also:
 * Propose a lifetime model for execution agents.
 * Propose some additions to the C\+\+ machine model to facilitate describing these additional properties.
 
-# Suggested straw polls
+# 6. Proposal
+
+## Header `<system>` synopsis
+
+```cpp
+namespace experimental {
+
+/* system_topology */
+
+class system_topology {
+
+  system_topology() = delete;
+
+};
+
+/* system_resource */
+
+class system_resource {
+
+  /* to be defined */
+
+};
+
+/* traverse_topology */
+
+template <class T>
+to-be-decided<system_resource> traverse_topology(const system_topology &, const T &) noexcept;
+
+/* this_system::discover_topology */
+
+namespace this_system {
+
+system_topology discover_topology();
+
+}  // namespace this_system
+
+}  // experimental
+```
+
+## Terms of art
+
+The term *system resource* refers to a hardware or software abstraction of an execution, memory, network or I/O resource within a system.
+
+The term *system topology* refers to a possibly cyclic graph of *execution resources* connected to the abstract machine, and their various properties.
+
+> [*Note:* The current definition of *system topology* is currently incomplete and will be developed over the course of this proposal as the various C\+\+ domains are represented. *--end note*]
+
+The term *topology traversal policy* refers to a policy that describes the way in which a *system topology* is traversed in order to to produce a collection of *system resources*.
+
+## Class `system_topology`
+
+The `system_topology` class provides an abstraction of a read-only snapshot of the *system topology* at a particular point in time. A `system_topology` object may not maintain or otherwise be associated with the lifetime of operating system or third party library resources.
+
+### `system_topology` constructors
+
+```cpp
+system_topology() = delete;
+```
+
+*Effects:* Explicitly deleted.
+
+## Class `system_resource`
+
+The `system_resource` class provides an abstraction of a read-only snapshot of a *system resource* from the *system topology* at a particular point in time. A `system_resource` object may not maintain or otherwise be associated with the lifetime of operating system or third party library resources.
+
+> [*Note:* The `system_resource` class is intended to reflect the properties of a *system resource* and it's relationships with other *system resources*, however the precise definition is still to be decided. *--end note*]
+
+## Free functions
+
+### `this_system::discover_topology`
+
+The free function `this_system::discover_topology` performs runtime discovery of the *system topology* and returns a `system_topology` object.
+
+```cpp
+namespace this_system {
+  system_topology discover_topology();
+}  // namespace this_system
+```
+
+*Returns:* A `system_topology` object representing a snapshot of the *system topology* at the current point in time.
+
+*Requires:* Calls to `this_system::discover_topology()` may not introduce a data race with any other call to `this_system::discover_topology()`.
+
+*Effects:* Performs runtime discovery of the system topology and constructs a `system_topology` object. May invoke the operating system or third party libraries in discovering topology information, but must release any resources acquired for this purpose before returning.
+
+*Throws:* Any exception thrown as a result of performing runtime discovery of the system topology.
+
+### `traverse_topology`
+
+The free function `traverse_topology` performs a traversal of a `system_topology` object using a *topology traversal policy* specified by the tag type `T` and returns a sequence of `system_resource` objects.
+
+```cpp
+template <class T>
+to-be-decided<system_resource> traverse_topology(const system_topology &, const T &) noexcept;
+```
+
+*Returns:* A sequence of `system_resource` objects representing the *system resources* matching the criteria of the *topology traversal policy*.
+
+*Effects:* Traverses the `system_topology` object provided and identifies any *system resources* which match the criteria of the *topology traversal policy*, adding a single `system_resource` to the sequence returned for each match found.
+
+*Throws:* May not throw.
+
+> [*Note:* The exact representation of *system resources* returned by `traverse_topology` is still to be decided as this will have implications on lifetimes. One option is to return a container of *system_resource* objects by-value such as a `vector`, however this would require some form of reference counting. Another option is to return a reference to a reference to the *system_resource* objects via a   `span ` or a `ranges::view`, however this would require the `system_topology` object to remain alive. *--end note*]
 
-Would SG1 like to see a continued effort to pursue the goals outlined in this paper?
+# 7. Open questions
 
-Does SG1 believe the proposed direction laid out in this paper is suitable to achieve those goals?
+* How granular should topology discovery be, should they whole topology be discovered in a single operation or should it be done in multiple nested operations, only discovering what is needed at each layer?
+* What kind of *topology traversal policies* would people list to see standardized?
+* How should we support notification of a topology update, polling or callback?
+* Should we also provide an interface for compile-time topology discovery?
 
 # References
 
@@ -171,9 +284,9 @@ Does SG1 believe the proposed direction laid out in this paper is suitable to ac
 [hpx]: https://github.com/STEllAR-GROUP/hpx
 [[13]][hpx] HPX
 
-[p0443]:
-http://wg21.link/p0443
-[[14]][p0443] A Unified Executors Proposal for C\+\+
+[p0443r11]:
+http://wg21.link/p0443r11
+[[14]][p0443r11] A Unified Executors Proposal for C\+\+
 
 [exposing-locality]: https://docs.google.com/viewer?a=v&pid=sites&srcid=bGJsLmdvdnxwYWRhbC13b3Jrc2hvcHxneDozOWE0MjZjOTMxOTk3NGU3
 [[15]][exposing-locality] Exposing the Locality of new Memory Hierarchies to HPC Applications