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GH-93354: Use exponential backoff to avoid excessive specialization attempts. (GH-93355)

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Mark Shannon 2022-05-31 11:58:26 +01:00 • committed by GitHub
parent a565ab0fd5
commit eb618d5ff0
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4 changed files with 93 additions and 46 deletions
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47
Include/internal/pycore_code.h
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@ -227,9 +227,6 @@ extern void _PyLineTable_InitAddressRange(
extern int _PyLineTable_NextAddressRange(PyCodeAddressRange *range);
extern int _PyLineTable_PreviousAddressRange(PyCodeAddressRange *range);
#define ADAPTIVE_CACHE_BACKOFF 64
/* Specialization functions */
extern int _Py_Specialize_LoadAttr(PyObject *owner, _Py_CODEUNIT *instr,
@ -423,6 +420,50 @@ write_location_entry_start(uint8_t *ptr, int code, int length)
}
/** Counters
* The first 16-bit value in each inline cache is a counter.
* When counting misses, the counter is treated as a simple unsigned value.
*
* When counting executions until the next specialization attempt,
* exponential backoff is used to reduce the number of specialization failures.
* The high 12 bits store the counter, the low 4 bits store the backoff exponent.
* On a specialization failure, the backoff exponent is incremented and the
* counter set to (2**backoff - 1).
* Backoff == 6 -> starting counter == 63, backoff == 10 -> starting counter == 1023.
*/
/* With a 16-bit counter, we have 12 bits for the counter value, and 4 bits for the backoff */
#define ADAPTIVE_BACKOFF_BITS 4
/* The initial counter value is 31 == 2**ADAPTIVE_BACKOFF_START - 1 */
#define ADAPTIVE_BACKOFF_START 5
#define MAX_BACKOFF_VALUE (16 - ADAPTIVE_BACKOFF_BITS)
static inline uint16_t
adaptive_counter_bits(int value, int backoff) {
return (value << ADAPTIVE_BACKOFF_BITS) |
(backoff & ((1<<ADAPTIVE_BACKOFF_BITS)-1));
}
static inline uint16_t
adaptive_counter_start(void) {
unsigned int value = (1 << ADAPTIVE_BACKOFF_START) - 1;
return adaptive_counter_bits(value, ADAPTIVE_BACKOFF_START);
}
static inline uint16_t
adaptive_counter_backoff(uint16_t counter) {
unsigned int backoff = counter & ((1<<ADAPTIVE_BACKOFF_BITS)-1);
backoff++;
if (backoff > MAX_BACKOFF_VALUE) {
backoff = MAX_BACKOFF_VALUE;
}
unsigned int value = (1 << backoff) - 1;
return adaptive_counter_bits(value, backoff);
}
#ifdef __cplusplus
}
#endif