Add section about Haskell execution model

src/Measurement_Observation/haskell_model.rst

@@ -0,0 +1,129 @@
+
+Haskell Compilation and Execution Model
+=======================================
+
+
+GHC doesn't use a VM but compiles to native code (when a native code generator
+is used).
+So we can use tools to profile native code execution: e.g. perf.
+However the native code we execute doesn't look exactly like the one produced by
+imperative languages (C, C++, Rust, etc.) and this confuses the tools which make
+some assumptions.
+For example:
+
+1. we don't have clear procedure boundaries with call/return instruction pairs
+   (we use tail calls, i.e. jump instructions)
+
+2. we don't use the so-called C stack in the usual way. I.e. we don't use usual
+   stack registers (e.g. rsp/rbp on x86-64) as stack registers.
+
+3. GHC uses its own calling convention
+
+In addition, lazy evaluation makes control-flow and memory usage difficult to
+understand.
+
+In this chapter we describe the compilation pipeline and the execution model.
+For each stage of the pipeline we list what can be measured at runtime.
+
+
+Compilation pipeline overview
+-----------------------------
+
+GHC compiles Haskell codes in several stages, each with their own intermediate
+representation:
+
+Haskell: the functional language we love with its bazillion of extensions.
+
+Core: a simple and explicitly typed functional language. Types are basically
+Haskell types. Type-class dictionaries are passed explicitly as records,
+type-applications are used everywhere needed, coercions (proofs that we can
+convert a type into another) are first class values, etc.
+
+STG: a functional language closer to the execution model. It's still typed but
+with primitive types. E.g. it tracks if a value is a heap object or an
+unboxed word (unboxed = doesn't have its own heap object) but not the Haskell
+type of the heap object. Complex forms are lowered to simpler ones (e.g. unboxed
+sums are lowered into unboxed tuples); this is called unarisation.
+
+Additionally every heap allocation is made explicit with a let-binding. For
+example instead of ``foo (MkBar x y)`` you have ``let b = MkBar x y in foo b``.
+This is called `A-normal form<https://en.wikipedia.org/wiki/A-normal_form>`_.
+During code generation from STG we know that all functions are applied to
+"simple" arguments only (variables, constants, etc.).
+
+Following the approach pioneered with "super-combinators", every top-level STG
+binding is then compiled into imperative code. The idea is that executing this
+imperative code has the same result as interpreting the functional code. For
+example, ``let b = MkBar x y in foo b`` is basically compiled to:
+
+.. code-block:: C
+
+  b <- allocate heap object for `MkBar x y`
+  evaluate foo
+  apply `foo` to `b`
+
+Different GHC backends use different imperative representations. The interpreter
+uses ByteCode, the JavaScript backend uses some JavaScript-like form, all other
+backends use Cmm.
+
+Cmm: an imperative language that looks like LLVM IR. It supports expressions,
+statements, and it abstracts over machine primops, machine registers, stack
+usage, and calling conventions. A pass performs register assignment and stack
+layout for the target architecture. Then assembly code (textual) is generated
+for the target architecture and an external assembler program generates machine
+code (binary) for it. Finally an external linker is used to transform the
+resulting code objects into a single executable or library.
+
+Executables are linked against a runtime system (RTS) that provides primitives
+to manage:
+- memory: allocation, garbage collection...
+- scheduling: thread scheduling, blocking queues...
+- IOs: primitives to interact with the operating system
+- dynamic code loading: loading and unloading code objects at runtime.
+
+The RTS itself comes in different flavors: e.g. using multiple OS threads to execute
+Haskell code or not.
+
+All these compilation stages and the RTS provide knobs to tweak the generated
+code and the behavior of the runtime system. In particular, some probes can be
+optionally inserted in the generated code at various stages to produce different
+profiling information at runtime.
+
+
+Execution model overview
+------------------------
+
+The heap of a Haskell program contain objects that reflect its current execution
+state: suspended computations (thunks), partially applied functions (PAP),
+values (datacon and their payload), threads (TSO) and their stack, etc.
+When the computer executes the compiled code for a top-level STG binding
+(starting from ``main``), it asks for more objects to be allocated in the heap,
+for some thunks to be reduced to another heap object, or for existing objects to
+be used as arguments for function applications.
+
+The garbage collector is responsible for freeing space in the heap. It runs when
+there is not enough space left or depending on other heuristics. GHC provides
+several knobs to configure the garbage collector strategies to use and to tweak
+their properties.
+
+The garbage collector used has an impact on profiling. For an extreme example,
+if the heap size is configured to be large enough than your program never has to
+perfom garbage collection, you'll find that you spend 0% time doing garbage
+collection and 100% executing your program ("mutator" time): all the garbage
+collection occurs at once implicitly when your program exits. On the other hand,
+if you configure the heap to be very small, most of the time can be spent doing
+garbage collection even if you haven't changed anything in your program.
+It means that **you must be very aware of the RTS options you use when profiling**.
+
+Similarly, some profiling options have an impact on the size of the heap
+objects: e.g. the heap object that represents `10 :: Int64` uses 24 bytes
+instead of 16 bytes without profiling. Because of this, you may find that your
+code with this profiling enabled triggers more garbage collections than your
+code without this profiling enabled. **You have to be aware of the compilation
+options you use when you make some runtime measurements.**
+
+Consequences on Profiling
+-------------------------
+
+As a consequence of the Haskell compilation pipeline and of the Haskell execution model
+we can measure many different things at different levels.

src/Measurement_Observation/index.rst

@@ -6,6 +6,7 @@ Measurement, Profiling, and Observation
    :name: Heap_Ghc
 
    the_recipe
+   haskell_model
    Heap_Ghc/index
    Heap_Third/index
    Measurement_Ghc/index