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gh-141498: Change backoff counter to use powers of 4 instead of powers of 2 #141589

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gh-141498: Change backoff counter to use powers of 4 instead of powers of 2 #141589

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86de63a
Some refactor
efimov-mikhail Nov 15, 2025
4eb3231
Changes in backoff bit pattern
efimov-mikhail Nov 15, 2025
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Changes in backoff bit pattern
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@efimov-mikhail
efimov-mikhail committed Nov 15, 2025
commit 4eb3231710cdeaf9a0dcf48b35784bb193a040cf
26 changes: 13 additions & 13 deletions Include/internal/pycore_backoff.h
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Original file line number Diff line number Diff line change
Expand Up @@ -22,20 +22,20 @@ extern "C" {
Another use is for the Tier 2 optimizer to decide when to create
a new Tier 2 trace (executor). Again, exponential backoff is used.

The 16-bit counter is structured as a 12-bit unsigned 'value'
and a 4-bit 'backoff' field. When resetting the counter, the
The 16-bit counter is structured as a 13-bit unsigned 'value'
and a 3-bit 'backoff' field. When resetting the counter, the
backoff field is incremented (until it reaches a limit) and the
value is set to a bit mask representing the value 2**backoff - 1.
The maximum backoff is 12 (the number of bits in the value).
value is set to a bit mask representing the value 2**(2*backoff+1) - 1.
The maximum backoff is 6, since 7 is an UNREACHABLE_BACKOFF.

There is an exceptional value which must not be updated, 0xFFFF.
*/

#define BACKOFF_BITS 4
#define BACKOFF_MASK 0xF
#define MAX_BACKOFF 12
#define UNREACHABLE_BACKOFF 15
#define MAX_VALUE 0xFFF
#define BACKOFF_BITS 3
#define BACKOFF_MASK 7
#define MAX_BACKOFF 6
#define UNREACHABLE_BACKOFF 7
#define MAX_VALUE 0x1FFF

static inline bool
is_unreachable_backoff_counter(_Py_BackoffCounter counter)
Expand Down Expand Up @@ -67,10 +67,10 @@ restart_backoff_counter(_Py_BackoffCounter counter)
assert(!is_unreachable_backoff_counter(counter));
int backoff = counter.value_and_backoff & BACKOFF_MASK;
if (backoff < MAX_BACKOFF) {
return make_backoff_counter((1 << (backoff + 1)) - 1, backoff + 1);
return make_backoff_counter((1 << (2 * backoff + 3)) - 1, backoff + 1);
}
else {
return make_backoff_counter((1 << MAX_BACKOFF) - 1, MAX_BACKOFF);
return make_backoff_counter((1 << (2 * MAX_BACKOFF + 1)) - 1, MAX_BACKOFF);
}
}

Expand Down Expand Up @@ -115,7 +115,7 @@ trigger_backoff_counter(void)
// as we always end up tracing the loop iteration's
// exhaustion iteration. Which aborts our current tracer.
#define JUMP_BACKWARD_INITIAL_VALUE 4000
#define JUMP_BACKWARD_INITIAL_BACKOFF 12
#define JUMP_BACKWARD_INITIAL_BACKOFF 6
static inline _Py_BackoffCounter
initial_jump_backoff_counter(void)
{
Expand All @@ -128,7 +128,7 @@ initial_jump_backoff_counter(void)
* otherwise when a side exit warms up we may construct
* a new trace before the Tier 1 code has properly re-specialized. */
#define SIDE_EXIT_INITIAL_VALUE 4000
#define SIDE_EXIT_INITIAL_BACKOFF 12
#define SIDE_EXIT_INITIAL_BACKOFF 6

static inline _Py_BackoffCounter
initial_temperature_backoff_counter(void)
Expand Down
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