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markshannon merged 3 commits into from
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Add file describing how to add or modify specialized families of instructions. #26954

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Add file describing how to add or modify specialized families of inst…
markshannon Jun 29, 2021
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Stick to 80 char limit and add reference.
markshannon Jun 30, 2021
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Add example and fix a couple of grammatical errors.
markshannon Jun 30, 2021
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Stick to 80 char limit and add reference.
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markshannon committed Jun 30, 2021
commit b0c9e51408c801487ea1ef75ae7b6d54bbf9af3f
96 changes: 59 additions & 37 deletions Python/adaptive.md
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## Families of instructions

The core part of PEP 659 (specializing adaptive interpreter) is the families of instructions that perform the adaptive specialization.
The core part of PEP 659 (specializing adaptive interpreter) is the families
of instructions that perform the adaptive specialization.

A family of instructions has the following fundamental properties:

* It corresponds to a single instruction in the code generated by the bytecode compiler.
* It corresponds to a single instruction in the code
generated by the bytecode compiler.
* It has a single adaptive instruction that records an execution count and,
at regular intervals, attempts to specialize itself. If not specializing, it executes
the non-adaptive instruction.
* It has at least one specialized form of the instruction that is tailored for a particular value or set of values at runtime.
at regular intervals, attempts to specialize itself. If not specializing,
it executes the non-adaptive instruction.
* It has at least one specialized form of the instruction that is tailored
for a particular value or set of values at runtime.
* All members of the family have access to same number of cache entries.
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@iritkatriel iritkatriel Jun 30, 2021

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* All members of the family have access to same number of cache entries.
* All members of the family have access to the same number of cache entries.

Individual family members do not need to use all of the entries.

The current implementation also requires the following, although these are not fundamental and may change:
The current implementation also requires the following,
although these are not fundamental and may change:

* If a family uses one or more entries, then the first entry must be a `_PyAdaptiveEntry` entry.
* If a family uses no cache entries, then the `oparg` is used as the counter for the adaptive instruction.
* All instruction names should start with the name of the non-adaptive instruction.
* If a family uses one or more entries, then the first entry must be a
`_PyAdaptiveEntry` entry.
* If a family uses no cache entries, then the `oparg` is used as the
counter for the adaptive instruction.
* All instruction names should start with the name of the non-adaptive
instruction.
* The adaptive instruction should end in `_ADAPTIVE`.
* Specialized forms should have names describing their specialization.

Expand All @@ -32,66 +39,81 @@ and `Tadaptive` is the mean time to execute the specialized and adaptive forms.

`Tadaptive = (sum(Ti*Ni) + Tmiss*Nmiss)/(sum(Ni)+Nmiss)`

`Ti` is the time to execute the `i`th instruction in the family and `Ni` is the number of times that instruction is executed.
`Tmiss` is the time to process a miss, including de-optimzation and the time to execute the base instruction.
`Ti` is the time to execute the `i`th instruction in the family and `Ni` is
the number of times that instruction is executed.
`Tmiss` is the time to process a miss, including de-optimzation
and the time to execute the base instruction.

The ideal situation is where misses are rare and the specialized forms are much faster than the base instruction.
The ideal situation is where misses are rare and the specialized
forms are much faster than the base instruction.
`LOAD_GLOBAL` is near ideal, `Nmiss/sum(Ni) ≈ 0`.
In which case we have `Tadaptive ≈ sum(Ti*Ni)`.
Since we can expect the specialized forms `LOAD_GLOBAL_MODULE` and `LOAD_GLOBAL_BUILTIN` to be much faster than the adaptive base instruction, we would expect the specialization of `LOAD_GLOBAL` to be profitable.
Since we can expect the specialized forms `LOAD_GLOBAL_MODULE` and
`LOAD_GLOBAL_BUILTIN` to be much faster than the adaptive base instruction,
we would expect the specialization of `LOAD_GLOBAL` to be profitable.

## Design considerations

While `LOAD_GLOBAL` may be ideal, instructions like `LOAD_ATTR` and `CALL_FUNCTION` are not.
For maximum performance we want to keep `Ti` low for all specialized instructions and `Nmiss` as low as possible.
While `LOAD_GLOBAL` may be ideal, instructions like `LOAD_ATTR` and
`CALL_FUNCTION` are not. For maximum performance we want to keep `Ti`
low for all specialized instructions and `Nmiss` as low as possible.

Keeping `Nmiss` low means that there should be specializations for almost
all values seen by the base instruction. Keeping `sum(Ti*Ni)` low means keeping `Ti`
low which means minimizing branches and dependent memory accesses (pointer chasing).
These two objectives may be in conflict, requiring judgement and experimentation to
design the family of instructions.
all values seen by the base instruction. Keeping `sum(Ti*Ni)` low means
keeping `Ti` low which means minimizing branches and dependent memory
accesses (pointer chasing). These two objectives may be in conflict,
requiring judgement and experimentation to design the family of instructions.

### Gathering data

Before choosing how to specialize an instruction, it is important to gather some data. What are the pattern of usage of the base instruction?
Data can best be gathered by instrumenting the interpreter.
Since a specialization function and adaptive instruction are going to be required,
Before choosing how to specialize an instruction, it is important to gather
some data. What are the pattern of usage of the base instruction?
Data can best be gathered by instrumenting the interpreter. Since a
specialization function and adaptive instruction are going to be required,
instrumentation can most easily be added in the specialization function.

### Choice of specializations

The performance of the specializing adaptive interpreter relies on the quality of
specialization and keeping the overhead of specialization low.
The performance of the specializing adaptive interpreter relies on the
quality of specialization and keeping the overhead of specialization low.

Specialized instructions must be fast. In order to be fast, specialized instructions should be tailored
for a particular set of values that allows them to:
Specialized instructions must be fast. In order to be fast,
specialized instructions should be tailored for a particular
set of values that allows them to:
1. Verify that incoming value is part of that set with low overhead.
2. Perform the operation quickly.

This requires that the set of values is chosen such that membership can be tested quickly and
that membership is sufficient to allow the operation to performed quickly.
This requires that the set of values is chosen such that membership can be
tested quickly and that membership is sufficient to allow the operation to
performed quickly.

For example, `LOAD_GLOBAL_MODULE` is specialized for `globals()` dictionaries that have a keys with the expected version.
For example, `LOAD_GLOBAL_MODULE` is specialized for `globals()`
dictionaries that have a keys with the expected version.

This can be tested quickly:
* `globals->keys->dk_version == expected_version`

and the operation can be performed quickly:
* `value = globals->keys->entries[index].value`.

Because it is impossible to measure the performance of an instruction without also
measuring unrelated factors, the assessment of the quality of a specialization will require some judgement.
Because it is impossible to measure the performance of an instruction without
also measuring unrelated factors, the assessment of the quality of a
specialization will require some judgement.

As a general rule, specialized instructions should be much faster than the base instruction.
As a general rule, specialized instructions should be much faster than the
base instruction.

### Implementation of specialized instructions

In general, specialized instructions should be implemented in two parts:
1. A sequence of guards, each of the form `DEOPT_IF(guard-condition-is-false, BASE_NAME)`,
1. A sequence of guards, each of the form
`DEOPT_IF(guard-condition-is-false, BASE_NAME)`,
followed by a `record_cache_hit()`.
2. The operation, which should ideally have no branches and a minimum number of dependent memory accesses.
2. The operation, which should ideally have no branches and
a minimum number of dependent memory accesses.

In practice, the parts may overlap, as data required for guards can be re-used in the operation.
In practice, the parts may overlap, as data required for guards
can be re-used in the operation.

If there are branches in the operation, then consider further specialization to eliminate
the branches.
If there are branches in the operation, then consider further specialization
to eliminate the branches.
4 changes: 4 additions & 0 deletions Python/specialize.c
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#include "opcode.h"
#include "structmember.h" // struct PyMemberDef, T_OFFSET_EX

/* For guidance on adding or extending families of instructions see
* ./adaptive.md
*/


/* We layout the quickened data as a bi-directional array:
* Instructions upwards, cache entries downwards.
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