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GH-111843: Tier 2 exponential backoff (GH-111850)

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Mark Shannon 2023-11-09 13:49:51 +00:00 committed by GitHub
parent 25c4956488
commit 34a03e951b
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GPG Key ID: 4AEE18F83AFDEB23
5 changed files with 47 additions and 14 deletions
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4
Include/cpython/optimizer.h
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@ -45,6 +45,8 @@ typedef int (*optimize_func)(_PyOptimizerObject* self, PyCodeObject *code, _Py_C
typedef struct _PyOptimizerObject {
PyObject_HEAD
optimize_func optimize;
/* These thresholds are treated as signed so do not exceed INT16_MAX
* Use INT16_MAX to indicate that the optimizer should never be called */
uint16_t resume_threshold;
uint16_t backedge_threshold;
/* Data needed by the optimizer goes here, but is opaque to the VM */
@ -76,6 +78,8 @@ PyAPI_FUNC(PyObject *)PyUnstable_Optimizer_NewCounter(void);
PyAPI_FUNC(PyObject *)PyUnstable_Optimizer_NewUOpOptimizer(void);
#define OPTIMIZER_BITS_IN_COUNTER 4
/* Minimum of 16 additional executions before retry */
#define MINIMUM_TIER2_BACKOFF 4
#ifdef __cplusplus
}

2
Misc/NEWS.d/next/Core and Builtins/2023-11-05-06-40-35.gh-issue-111843.c045cB.rst Normal file
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@ -0,0 +1,2 @@
Use exponential backoff to reduce the number of failed tier 2 optimization
attempts by over 99%.

23
Python/bytecodes.c
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@ -2335,10 +2335,12 @@ dummy_func(
JUMPBY(-oparg);
#if ENABLE_SPECIALIZATION
this_instr[1].cache += (1 << OPTIMIZER_BITS_IN_COUNTER);
if (this_instr[1].cache > tstate->interp->optimizer_backedge_threshold &&
// Double-check that the opcode isn't instrumented or something:
this_instr->op.code == JUMP_BACKWARD)
{
/* We are using unsigned values, but we really want signed values, so
* do the 2s complement comparison manually */
uint16_t ucounter = this_instr[1].cache + (1 << 15);
uint16_t threshold = tstate->interp->optimizer_backedge_threshold + (1 << 15);
// Double-check that the opcode isn't instrumented or something:
if (ucounter > threshold && this_instr->op.code == JUMP_BACKWARD) {
OPT_STAT_INC(attempts);
int optimized = _PyOptimizer_BackEdge(frame, this_instr, next_instr, stack_pointer);
ERROR_IF(optimized < 0, error);
@ -2346,8 +2348,19 @@ dummy_func(
// Rewind and enter the executor:
assert(this_instr->op.code == ENTER_EXECUTOR);
next_instr = this_instr;
this_instr[1].cache &= ((1 << OPTIMIZER_BITS_IN_COUNTER) - 1);
}
else {
int backoff = this_instr[1].cache & ((1 << OPTIMIZER_BITS_IN_COUNTER) - 1);
if (backoff < MINIMUM_TIER2_BACKOFF) {
backoff = MINIMUM_TIER2_BACKOFF;
}
else if (backoff < 15 - OPTIMIZER_BITS_IN_COUNTER) {
backoff++;
}
assert(backoff <= 15 - OPTIMIZER_BITS_IN_COUNTER);
this_instr[1].cache = ((1 << 16) - ((1 << OPTIMIZER_BITS_IN_COUNTER) << backoff)) | backoff;
}
this_instr[1].cache &= ((1 << OPTIMIZER_BITS_IN_COUNTER) - 1);
}
#endif /* ENABLE_SPECIALIZATION */
}

23
Python/generated_cases.c.h generated
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@ -3409,10 +3409,12 @@
JUMPBY(-oparg);
#if ENABLE_SPECIALIZATION
this_instr[1].cache += (1 << OPTIMIZER_BITS_IN_COUNTER);
if (this_instr[1].cache > tstate->interp->optimizer_backedge_threshold &&
// Double-check that the opcode isn't instrumented or something:
this_instr->op.code == JUMP_BACKWARD)
{
/* We are using unsigned values, but we really want signed values, so
* do the 2s complement comparison manually */
uint16_t ucounter = this_instr[1].cache + (1 << 15);
uint16_t threshold = tstate->interp->optimizer_backedge_threshold + (1 << 15);
// Double-check that the opcode isn't instrumented or something:
if (ucounter > threshold && this_instr->op.code == JUMP_BACKWARD) {
OPT_STAT_INC(attempts);
int optimized = _PyOptimizer_BackEdge(frame, this_instr, next_instr, stack_pointer);
if (optimized < 0) goto error;
@ -3420,8 +3422,19 @@
// Rewind and enter the executor:
assert(this_instr->op.code == ENTER_EXECUTOR);
next_instr = this_instr;
this_instr[1].cache &= ((1 << OPTIMIZER_BITS_IN_COUNTER) - 1);
}
else {
int backoff = this_instr[1].cache & ((1 << OPTIMIZER_BITS_IN_COUNTER) - 1);
if (backoff < MINIMUM_TIER2_BACKOFF) {
backoff = MINIMUM_TIER2_BACKOFF;
}
else if (backoff < 15 - OPTIMIZER_BITS_IN_COUNTER) {
backoff++;
}
assert(backoff <= 15 - OPTIMIZER_BITS_IN_COUNTER);
this_instr[1].cache = ((1 << 16) - ((1 << OPTIMIZER_BITS_IN_COUNTER) << backoff)) | backoff;
}
this_instr[1].cache &= ((1 << OPTIMIZER_BITS_IN_COUNTER) - 1);
}
#endif /* ENABLE_SPECIALIZATION */
DISPATCH();

9
Python/optimizer.c
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@ -107,6 +107,7 @@ error_optimize(
_PyExecutorObject **exec,
int Py_UNUSED(stack_entries))
{
assert(0);
PyErr_Format(PyExc_SystemError, "Should never call error_optimize");
return -1;
}
@ -122,8 +123,8 @@ PyTypeObject _PyDefaultOptimizer_Type = {
_PyOptimizerObject _PyOptimizer_Default = {
PyObject_HEAD_INIT(&_PyDefaultOptimizer_Type)
.optimize = error_optimize,
.resume_threshold = UINT16_MAX,
.backedge_threshold = UINT16_MAX,
.resume_threshold = INT16_MAX,
.backedge_threshold = INT16_MAX,
};
_PyOptimizerObject *
@ -309,7 +310,7 @@ PyUnstable_Optimizer_NewCounter(void)
return NULL;
}
opt->base.optimize = counter_optimize;
opt->base.resume_threshold = UINT16_MAX;
opt->base.resume_threshold = INT16_MAX;
opt->base.backedge_threshold = 0;
opt->count = 0;
return (PyObject *)opt;
@ -915,7 +916,7 @@ PyUnstable_Optimizer_NewUOpOptimizer(void)
return NULL;
}
opt->optimize = uop_optimize;
opt->resume_threshold = UINT16_MAX;
opt->resume_threshold = INT16_MAX;
// Need at least 3 iterations to settle specializations.
// A few lower bits of the counter are reserved for other flags.
opt->backedge_threshold = 16 << OPTIMIZER_BITS_IN_COUNTER;