Xecto: 80% of xectors
This is a library for regular array parallelism in CL. It's also an "80%" library: the goal isn't to extract all the performance there is, but to allow programmers to easily express computations with a decent overhead compared to hand-written loops, in common cases. Uncommon cases should either be handled specially, or rewritten to better fit Xecto's simplistic (and simple) approach.
- Structured data
- Typed arrays
- Views (later)
- Bulk operations
- Copy on write
Very lazily sketched out; this looks more like an interface design issue
Xecto provides homogeneously, but dynamically-typed bulk structured data. Arrays of structures are actually always structures of specialised, homogeneous arrays.
[more stuff, when it's better thought out]
Think data frames: it's not just a bunch of vectors, there's a level of semantic typing.
Prototyped; just more of the same
Arrays in Xecto are typed: currently, we only have arrays of double floats, but we'll also have arrays of single floats, machine integers, etc. What's the stance on T arrays? No clue.
Arrays can be of arbitrary rank and size; plans include allocating them from the foreign heap (or directly via
As with many similar libraries, each array is represented as a flat data vector and shape information; shape information includes the dimensions, and an affine transformation from the coordinates to an index in the data vector.
Data vectors are immutable, but reference counted to help with copy-on-write. However, arrays themselves are mutable: the copying (if any) is transparent, and references to data vectors are updated as needed.
This means that operations like slicing or transposition are nearly free and do not directly entail copying or allocation.
Usual stuff: map, reduce, scan.
Some amount of recycling rule: single element is replicated as needed, but nothing more.
Reduce and scan work on the first dimension of the single input; the reduced/scanned function is then applied, map-like on the remaining dimensions.
Copy on write
Not yet, but it's a SMOP
Mention ! variants (foo-into). Expresses partial writing, but also storage reuse.
tmpfs for TLB-level copying.
The library is "just" a minimally smart interpreter; the upside is that the primitives are chosen so they execute efficiently.
Rather than working with scalars, the primitives compute on strided spans of vectors (for inputs and outputs), much like level 1 BLAS operations. This allows for SIMD-level parallelism
This also means that each operation boils down to a large number of specialized primitive calls; that's where thread-level parallelism comes in.
Hard part done
Each map operation boils down to a perfect nest of for-loops. The nesting is reordered for locality: we attempt to get monotonous address sequences as much as possible, especially in the result vector. Loops are also merged when possible to increase trip counts and reduce nesting depth. Finally, we attempt to ensure a trip count in the innermost loop, to better exploit the primitives.
Primitives are pre-compiled and specialised for some key trip count and stride values. That's how we get SIMD.
Execution then proceeds by first finding the primitive corresponding to the operation and the inner loop's stride and trip count, and interprets the remainder of the loop nest.
As shown earlier, each operation is executed as a perfect loop nest that gives rise to a number of primitive operations. The key to exploiting threads is that operations are implemented as futures (with a thread pool and task stealing).
Each outer loop is executed a couple times to yield a small number of tasks. Tasks then note dependencies, which gives us pipelining.
-> Note: probably want early dealloc.
Futures are triggered via a stack and task stealing, so we get locality for free.
NUMA awareness via hashing on middle bits of written addresses.
- Paul Khuong
- 3-clause BSD