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GH-111843: Tier 2 exponential backoff (GH-111850)
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@ -45,6 +45,8 @@ typedef int (*optimize_func)(_PyOptimizerObject* self, PyCodeObject *code, _Py_C
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typedef struct _PyOptimizerObject {
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PyObject_HEAD
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optimize_func optimize;
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/* These thresholds are treated as signed so do not exceed INT16_MAX
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* Use INT16_MAX to indicate that the optimizer should never be called */
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uint16_t resume_threshold;
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uint16_t backedge_threshold;
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/* Data needed by the optimizer goes here, but is opaque to the VM */
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@ -76,6 +78,8 @@ PyAPI_FUNC(PyObject *)PyUnstable_Optimizer_NewCounter(void);
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PyAPI_FUNC(PyObject *)PyUnstable_Optimizer_NewUOpOptimizer(void);
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#define OPTIMIZER_BITS_IN_COUNTER 4
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/* Minimum of 16 additional executions before retry */
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#define MINIMUM_TIER2_BACKOFF 4
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#ifdef __cplusplus
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}
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@ -0,0 +1,2 @@
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Use exponential backoff to reduce the number of failed tier 2 optimization
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attempts by over 99%.
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@ -2335,10 +2335,12 @@ dummy_func(
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JUMPBY(-oparg);
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#if ENABLE_SPECIALIZATION
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this_instr[1].cache += (1 << OPTIMIZER_BITS_IN_COUNTER);
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if (this_instr[1].cache > tstate->interp->optimizer_backedge_threshold &&
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// Double-check that the opcode isn't instrumented or something:
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this_instr->op.code == JUMP_BACKWARD)
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{
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/* We are using unsigned values, but we really want signed values, so
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* do the 2s complement comparison manually */
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uint16_t ucounter = this_instr[1].cache + (1 << 15);
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uint16_t threshold = tstate->interp->optimizer_backedge_threshold + (1 << 15);
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// Double-check that the opcode isn't instrumented or something:
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if (ucounter > threshold && this_instr->op.code == JUMP_BACKWARD) {
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OPT_STAT_INC(attempts);
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int optimized = _PyOptimizer_BackEdge(frame, this_instr, next_instr, stack_pointer);
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ERROR_IF(optimized < 0, error);
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@ -2346,8 +2348,19 @@ dummy_func(
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// Rewind and enter the executor:
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assert(this_instr->op.code == ENTER_EXECUTOR);
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next_instr = this_instr;
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this_instr[1].cache &= ((1 << OPTIMIZER_BITS_IN_COUNTER) - 1);
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}
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else {
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int backoff = this_instr[1].cache & ((1 << OPTIMIZER_BITS_IN_COUNTER) - 1);
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if (backoff < MINIMUM_TIER2_BACKOFF) {
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backoff = MINIMUM_TIER2_BACKOFF;
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}
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else if (backoff < 15 - OPTIMIZER_BITS_IN_COUNTER) {
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backoff++;
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}
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assert(backoff <= 15 - OPTIMIZER_BITS_IN_COUNTER);
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this_instr[1].cache = ((1 << 16) - ((1 << OPTIMIZER_BITS_IN_COUNTER) << backoff)) | backoff;
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}
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this_instr[1].cache &= ((1 << OPTIMIZER_BITS_IN_COUNTER) - 1);
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}
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#endif /* ENABLE_SPECIALIZATION */
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}
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23
Python/generated_cases.c.h
generated
23
Python/generated_cases.c.h
generated
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@ -3409,10 +3409,12 @@
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JUMPBY(-oparg);
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#if ENABLE_SPECIALIZATION
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this_instr[1].cache += (1 << OPTIMIZER_BITS_IN_COUNTER);
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if (this_instr[1].cache > tstate->interp->optimizer_backedge_threshold &&
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// Double-check that the opcode isn't instrumented or something:
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this_instr->op.code == JUMP_BACKWARD)
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{
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/* We are using unsigned values, but we really want signed values, so
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* do the 2s complement comparison manually */
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uint16_t ucounter = this_instr[1].cache + (1 << 15);
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uint16_t threshold = tstate->interp->optimizer_backedge_threshold + (1 << 15);
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// Double-check that the opcode isn't instrumented or something:
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if (ucounter > threshold && this_instr->op.code == JUMP_BACKWARD) {
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OPT_STAT_INC(attempts);
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int optimized = _PyOptimizer_BackEdge(frame, this_instr, next_instr, stack_pointer);
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if (optimized < 0) goto error;
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@ -3420,8 +3422,19 @@
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// Rewind and enter the executor:
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assert(this_instr->op.code == ENTER_EXECUTOR);
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next_instr = this_instr;
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this_instr[1].cache &= ((1 << OPTIMIZER_BITS_IN_COUNTER) - 1);
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}
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else {
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int backoff = this_instr[1].cache & ((1 << OPTIMIZER_BITS_IN_COUNTER) - 1);
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if (backoff < MINIMUM_TIER2_BACKOFF) {
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backoff = MINIMUM_TIER2_BACKOFF;
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}
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else if (backoff < 15 - OPTIMIZER_BITS_IN_COUNTER) {
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backoff++;
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}
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assert(backoff <= 15 - OPTIMIZER_BITS_IN_COUNTER);
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this_instr[1].cache = ((1 << 16) - ((1 << OPTIMIZER_BITS_IN_COUNTER) << backoff)) | backoff;
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}
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this_instr[1].cache &= ((1 << OPTIMIZER_BITS_IN_COUNTER) - 1);
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}
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#endif /* ENABLE_SPECIALIZATION */
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DISPATCH();
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@ -107,6 +107,7 @@ error_optimize(
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_PyExecutorObject **exec,
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int Py_UNUSED(stack_entries))
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{
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assert(0);
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PyErr_Format(PyExc_SystemError, "Should never call error_optimize");
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return -1;
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}
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@ -122,8 +123,8 @@ PyTypeObject _PyDefaultOptimizer_Type = {
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_PyOptimizerObject _PyOptimizer_Default = {
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PyObject_HEAD_INIT(&_PyDefaultOptimizer_Type)
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.optimize = error_optimize,
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.resume_threshold = UINT16_MAX,
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.backedge_threshold = UINT16_MAX,
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.resume_threshold = INT16_MAX,
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.backedge_threshold = INT16_MAX,
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};
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_PyOptimizerObject *
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@ -309,7 +310,7 @@ PyUnstable_Optimizer_NewCounter(void)
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return NULL;
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}
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opt->base.optimize = counter_optimize;
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opt->base.resume_threshold = UINT16_MAX;
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opt->base.resume_threshold = INT16_MAX;
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opt->base.backedge_threshold = 0;
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opt->count = 0;
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return (PyObject *)opt;
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@ -915,7 +916,7 @@ PyUnstable_Optimizer_NewUOpOptimizer(void)
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return NULL;
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}
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opt->optimize = uop_optimize;
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opt->resume_threshold = UINT16_MAX;
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opt->resume_threshold = INT16_MAX;
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// Need at least 3 iterations to settle specializations.
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// A few lower bits of the counter are reserved for other flags.
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opt->backedge_threshold = 16 << OPTIMIZER_BITS_IN_COUNTER;
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