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GeezerVison is showing github.com — gh-149584: `_remote_debugging`: Stop rereading the base frame sentinel by maurycy · Pull Request #159124 · python/cpython

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maurycy commented Oct 10, 2026 •

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Basically, process_frame_chain() reads frames until previous is NULL:

cpython/Modules/_remote_debugging/frames.c

Lines 345 to 348 in 565f76f

	parse_result = parse_frame_object(
	unwinder, &frame, frame_addr,
	&address_of_code_object, &next_frame_addr);
	}

The last one is always tstate->base_frame that is the sentinel set up once per thread with previous == NULL and owner == FRAME_OWNED_BY_INTERPRETER:

cpython/Python/pystate.c

Lines 1589 to 1599 in 565f76f

	_tstate->base_frame.previous = NULL;
	_tstate->base_frame.f_executable = PyStackRef_None;
	_tstate->base_frame.f_funcobj = PyStackRef_NULL;
	_tstate->base_frame.f_globals = NULL;
	_tstate->base_frame.f_builtins = NULL;
	_tstate->base_frame.f_locals = NULL;
	_tstate->base_frame.frame_obj = NULL;
	_tstate->base_frame.instr_ptr = NULL;
	_tstate->base_frame.stackpointer = _tstate->base_frame.localsplus;
	_tstate->base_frame.return_offset = 0;
	_tstate->base_frame.owner = FRAME_OWNED_BY_INTERPRETER;

Its address is already in the thread state buffer:

cpython/Modules/_remote_debugging/threads.c

Line 535 in 565f76f

uintptr_t base_frame_addr = GET_MEMBER(uintptr_t, ts, unwinder->debug_offsets.thread_state.base_frame);

so the remote read is not needed.

Tools/inspection/oracle_external_inspection.py with --runs 3, PGO+LTO, Ubuntu 24.04, AMD EPYC 4484PX, main at 182f323 vs the PR, on the same box one after another:

case	mode	main (`182f323`)	`182f323` + patch
flat_alternating	live-cache	8.04±0.03 us	7.13±0.02 us	-11.3%
gen_alternating	live-cache	7.95±0.02 us	7.36±0.03 us	-7.4%
flat_alternating	blocking-cache	14.59±0.06 us	13.73±0.02 us	-5.9%
gen_alternating	blocking-cache	15.11±0.05 us	14.31±0.09 us	-5.3%
basic	live-nocache	7.14±0.01 us	6.05±0.05 us	-15.3%
code_object_reuse	live-nocache	7.46±0.00 us	6.36±0.01 us	-14.7%
flat_alternating	live-nocache	7.45±0.01 us	6.37±0.01 us	-14.5%
shared_leaf	live-nocache	7.42±0.01 us	6.35±0.01 us	-14.4%
nested_alternating	live-nocache	7.45±0.01 us	6.47±0.09 us	-13.2%
deep_static	live-nocache	30.93±0.81 us	29.29±0.17 us	-5.3%
basic	blocking-nocache	17.55±0.08 us	16.63±0.03 us	-5.2%
basic	live-cache	3.16±0.01 us	3.28±0.05 us	+3.8%
deep_static	live-cache	5.59±0.07 us	5.82±0.08 us	+4.1%
shared_leaf	live-cache	3.36±0.01 us	3.52±0.01 us	+4.8%

I'm not exactly sure why basic or deep_static are worse here. My guess it's just PGO noise.

main (`182f323`)

maurycy@taut cpython-A (182f323) % ./python ./Tools/inspection/oracle_external_inspection.py --runs 3
3.16.0a0 (tags/v3.15.0b1-2187-g182f323154:182f323154, Oct 10 2026, 12:34:48) [GCC 13.3.0]
cases=async_running_task,asyncio,basic,code_object_reuse,deep_recursion,deep_static,flat_alternating,gen_alternating,nested_alternating,oversized_chunk,shared_leaf,threads runs=3 duration=3.0 rate_khz=100.0 warmup=0.7 poisson_sampling=False

async_running_task (get_async_stack_trace) ref_keys=47
mode                 samples    stacks     empty           µs      errors% impossible  impossible%
blocking-nocache      370014    740028      3733   21.74±0.03    0.00±0.00          0    0.00±0.00
live-nocache          473351    946654      2591   16.29±0.04    2.01±0.36      20791    2.20±0.50

asyncio (get_async_stack_trace) ref_keys=92
mode                 samples    stacks     empty           µs      errors%
blocking-nocache      453715    564504      1720   18.45±0.22    0.00±0.00
live-nocache          550817    647128      2595   15.01±1.02    0.51±0.17

basic (get_stack_trace) ref_keys=31
mode                 samples           µs      errors%      raw_tvd   tvd_excess  tvd_floor
blocking-nocache      479886   17.55±0.08    0.00±0.00          ref          ref          -
blocking-cache        618009   13.36±0.06    0.00±0.00  0.001±0.000  0.000±0.000      0.000
live-cache            899992    3.16±0.01    0.00±0.00  0.010±0.001  0.009±0.001      0.000
live-nocache          899993    7.14±0.01    0.00±0.00  0.011±0.001  0.011±0.001      0.000

code_object_reuse (get_stack_trace) ref_keys=18
mode                 samples           µs      errors% impossible  impossible%      raw_tvd   tvd_excess  tvd_floor
blocking-nocache      573963   14.00±0.03    0.00±0.00          0    0.00±0.00          ref          ref          -
blocking-cache        773891    9.95±0.01    0.00±0.00          0    0.00±0.00  0.001±0.000 -0.000±0.000      0.001
live-cache            899996    3.36±0.01    0.00±0.00          0    0.00±0.00  0.405±0.007  0.404±0.007      0.001
live-nocache          899991    7.46±0.00    0.00±0.00          0    0.00±0.00  0.005±0.000  0.004±0.000      0.001

deep_recursion (get_stack_trace) ref_keys=737
mode                 samples           µs      errors% impossible  impossible%      raw_tvd   tvd_excess  tvd_floor
blocking-nocache      222480   19.69±0.22    0.00±0.00          0    0.00±0.00          ref          ref          -
blocking-cache        215715   20.80±0.26    0.00±0.00          0    0.00±0.00  0.050±0.014  0.034±0.013      0.016
live-cache             96744  62.16±14.76   23.14±4.85      26342   28.18±7.20  0.754±0.044  0.731±0.046      0.022
live-nocache          142593  48.35±36.21   12.05±1.11      18435   14.94±7.70  0.802±0.022  0.782±0.026      0.019

deep_static (get_stack_trace) ref_keys=1
mode                 samples           µs      errors%      raw_tvd   tvd_excess  tvd_floor
blocking-nocache       56214   43.02±2.50    0.00±0.00          ref          ref          -
blocking-cache         65313   16.96±0.87    0.00±0.00  0.000±0.000  0.000±0.000      0.000
live-cache             76858    5.59±0.07    0.00±0.00  0.000±0.000  0.000±0.000      0.000
live-nocache           62809   30.93±0.81    0.00±0.00  0.000±0.000  0.000±0.000      0.000

flat_alternating (get_stack_trace) ref_keys=6
mode                 samples           µs      errors% impossible  impossible%      raw_tvd   tvd_excess  tvd_floor
blocking-nocache      573954   13.98±0.05    0.00±0.00          0    0.00±0.00          ref          ref          -
blocking-cache        552072   14.59±0.06    0.00±0.00          0    0.00±0.00  0.085±0.031  0.083±0.031      0.002
live-cache            899990    8.04±0.03    0.00±0.00       4228    0.47±0.03  0.069±0.004  0.067±0.004      0.002
live-nocache          899996    7.45±0.01    0.00±0.00       6388    0.71±0.07  0.136±0.015  0.135±0.015      0.002

gen_alternating (get_stack_trace) ref_keys=19
mode                 samples           µs      errors% impossible  impossible%      raw_tvd   tvd_excess  tvd_floor
blocking-nocache      558000   14.79±0.06    0.00±0.00          0    0.00±0.00          ref          ref          -
blocking-cache        545789   15.11±0.05    0.00±0.00          0    0.00±0.00  0.034±0.007  0.030±0.007      0.004
live-cache            713190    7.95±0.02   20.76±1.35     154784   21.67±3.13  0.282±0.043  0.278±0.043      0.004
live-nocache          723803    7.83±0.05   19.58±1.13     336208   46.43±4.08  0.488±0.100  0.483±0.099      0.005

nested_alternating (get_stack_trace) ref_keys=14
mode                 samples           µs      errors% impossible  impossible%      raw_tvd   tvd_excess  tvd_floor
blocking-nocache      546604   14.08±0.03    0.00±0.00          0    0.00±0.00          ref          ref          -
blocking-cache        724309   10.10±0.01    0.00±0.00          0    0.00±0.00  0.020±0.016  0.018±0.016      0.002
live-cache            899995    3.54±0.02    0.00±0.00          0    0.00±0.00  0.063±0.049  0.061±0.049      0.002
live-nocache          899997    7.45±0.01    0.00±0.00         11    0.00±0.00  0.067±0.038  0.065±0.038      0.002

oversized_chunk (get_stack_trace) ref_keys=17
mode                 samples           µs      errors% impossible  impossible%      raw_tvd   tvd_excess  tvd_floor
blocking-nocache      563868   14.25±0.19    0.00±0.00          0    0.00±0.00          ref          ref          -
blocking-cache        709827   10.92±0.01    0.00±0.00          0    0.00±0.00  0.026±0.014  0.023±0.014      0.002
live-cache            899991    4.71±0.00    0.00±0.00      12734    1.41±1.48  0.076±0.001  0.073±0.001      0.002
live-nocache          899991    7.62±0.05    0.00±0.00          0    0.00±0.00  0.109±0.006  0.107±0.006      0.002

shared_leaf (get_stack_trace) ref_keys=13
mode                 samples           µs      errors% impossible  impossible%      raw_tvd   tvd_excess  tvd_floor
blocking-nocache      575934   14.03±0.05    0.00±0.00          0    0.00±0.00          ref          ref          -
blocking-cache        779477    9.98±0.02    0.00±0.00          0    0.00±0.00  0.011±0.009  0.009±0.009      0.001
live-cache            899990    3.36±0.01    0.00±0.00        464    0.05±0.02  0.056±0.009  0.055±0.009      0.001
live-nocache          899996    7.42±0.01    0.00±0.00          0    0.00±0.00  0.044±0.028  0.043±0.028      0.001

threads (get_stack_trace) ref_keys=14
mode                 samples    stacks           µs      errors%      raw_tvd   tvd_excess  tvd_floor
blocking-nocache       47805    544044 187.12±59.02    0.00±0.00          ref          ref          -
blocking-cache         66936    809486 128.93±34.61    0.00±0.00  0.033±0.008  0.032±0.008      0.001
live-cache            321457   2789805   21.69±7.90    0.00±0.00  0.055±0.030  0.054±0.030      0.001
live-nocache          124833   1209809  66.76±20.70    0.00±0.00  0.047±0.003  0.046±0.003      0.001

PR (`182f323` + patch)

maurycy@taut cpython-B (182f323*) % ./python ./Tools/inspection/oracle_external_inspection.py --runs 3
3.16.0a0 (tags/v3.15.0b1-2187-g182f323154-dirty:182f323154, Oct 10 2026, 14:15:59) [GCC 13.3.0]
cases=async_running_task,asyncio,basic,code_object_reuse,deep_recursion,deep_static,flat_alternating,gen_alternating,nested_alternating,oversized_chunk,shared_leaf,threads runs=3 duration=3.0 rate_khz=100.0 warmup=0.7 poisson_sampling=False

async_running_task (get_async_stack_trace) ref_keys=46
mode                 samples    stacks     empty           µs      errors% impossible  impossible%
blocking-nocache      348511    697022      3379   23.11±0.11    0.00±0.00          0    0.00±0.00
live-nocache          460515    921009      3251   16.70±0.05    1.92±0.17      14318    1.56±0.31

asyncio (get_async_stack_trace) ref_keys=95
mode                 samples    stacks     empty           µs      errors%
blocking-nocache      443576    553293      1683   18.90±0.10    0.00±0.00
live-nocache          594948    700929      2211   13.79±0.52    0.40±0.04

basic (get_stack_trace) ref_keys=32
mode                 samples           µs      errors%      raw_tvd   tvd_excess  tvd_floor
blocking-nocache      504897   16.63±0.03    0.00±0.00          ref          ref          -
blocking-cache        617583   13.39±0.14    0.00±0.00  0.001±0.000  0.001±0.000      0.000
live-cache            899992    3.28±0.05    0.00±0.00  0.012±0.000  0.012±0.000      0.000
live-nocache          899995    6.05±0.05    0.00±0.00  0.013±0.000  0.013±0.000      0.000

code_object_reuse (get_stack_trace) ref_keys=22
mode                 samples           µs      errors% impossible  impossible%      raw_tvd   tvd_excess  tvd_floor
blocking-nocache      620199   12.81±0.22    0.00±0.00          0    0.00±0.00          ref          ref          -
blocking-cache        767626   10.07±0.25    0.00±0.00          0    0.00±0.00  0.001±0.000 -0.001±0.000      0.001
live-cache            899994    3.47±0.06    0.00±0.00          0    0.00±0.00  0.349±0.025  0.347±0.025      0.001
live-nocache          899991    6.36±0.01    0.00±0.00          0    0.00±0.00  0.005±0.000  0.004±0.000      0.001

deep_recursion (get_stack_trace) ref_keys=769
mode                 samples           µs      errors% impossible  impossible%      raw_tvd   tvd_excess  tvd_floor
blocking-nocache      228284   18.90±0.24    0.00±0.00          0    0.00±0.00          ref          ref          -
blocking-cache        220124   20.12±0.41    0.00±0.00          0    0.00±0.00  0.036±0.010  0.020±0.010      0.016
live-cache             83004   71.18±6.10   25.86±5.92      31325   38.01±2.84  0.693±0.046  0.666±0.048      0.028
live-nocache          192223  28.96±16.09    7.58±3.68      16518    9.73±6.10  0.850±0.019  0.832±0.017      0.017

deep_static (get_stack_trace) ref_keys=1
mode                 samples           µs      errors%      raw_tvd   tvd_excess  tvd_floor
blocking-nocache       56808   42.14±2.09    0.00±0.00          ref          ref          -
blocking-cache         65545   17.24±0.94    0.00±0.00  0.000±0.000  0.000±0.000      0.000
live-cache             76141    5.82±0.08    0.00±0.00  0.000±0.000  0.000±0.000      0.000
live-nocache           63351   29.29±0.17    0.00±0.00  0.000±0.000  0.000±0.000      0.000

flat_alternating (get_stack_trace) ref_keys=6
mode                 samples           µs      errors% impossible  impossible%      raw_tvd   tvd_excess  tvd_floor
blocking-nocache      614231   12.92±0.03    0.00±0.00          0    0.00±0.00          ref          ref          -
blocking-cache        582285   13.73±0.02    0.00±0.00          0    0.00±0.00  0.024±0.005  0.021±0.005      0.002
live-cache            899991    7.13±0.02    0.00±0.00       4204    0.47±0.14  0.077±0.013  0.076±0.013      0.002
live-nocache          899999    6.37±0.01    0.00±0.00       5548    0.62±0.04  0.111±0.017  0.110±0.017      0.002

gen_alternating (get_stack_trace) ref_keys=19
mode                 samples           µs      errors% impossible  impossible%      raw_tvd   tvd_excess  tvd_floor
blocking-nocache      590747   13.84±0.05    0.00±0.00          0    0.00±0.00          ref          ref          -
blocking-cache        573287   14.31±0.09    0.00±0.00          0    0.00±0.00  0.036±0.011  0.032±0.011      0.004
live-cache            724332    7.36±0.03   19.52±0.68     154090   21.29±4.24  0.287±0.025  0.283±0.025      0.004
live-nocache          725428    7.02±0.03   19.40±0.96     372262   51.34±4.62  0.638±0.059  0.633±0.059      0.005

nested_alternating (get_stack_trace) ref_keys=12
mode                 samples           µs      errors% impossible  impossible%      raw_tvd   tvd_excess  tvd_floor
blocking-nocache      584517   13.01±0.01    0.00±0.00          0    0.00±0.00          ref          ref          -
blocking-cache        714155   10.23±0.03    0.00±0.00          0    0.00±0.00  0.030±0.006  0.028±0.006      0.002
live-cache            899992    3.65±0.01    0.00±0.00         30    0.00±0.01  0.037±0.040  0.035±0.040      0.002
live-nocache          899998    6.47±0.09    0.00±0.00          4    0.00±0.00  0.067±0.030  0.065±0.030      0.002

oversized_chunk (get_stack_trace) ref_keys=17
mode                 samples           µs      errors% impossible  impossible%      raw_tvd   tvd_excess  tvd_floor
blocking-nocache      608009   13.03±0.04    0.00±0.00          0    0.00±0.00          ref          ref          -
blocking-cache        714629   10.83±0.09    0.00±0.00          0    0.00±0.00  0.024±0.011  0.021±0.011      0.002
live-cache            899993    4.59±0.01    0.00±0.00      24505    2.72±0.28  0.066±0.013  0.064±0.013      0.002
live-nocache          899989    6.52±0.01    0.00±0.00          1    0.00±0.00  0.095±0.010  0.092±0.010      0.002

shared_leaf (get_stack_trace) ref_keys=10
mode                 samples           µs      errors% impossible  impossible%      raw_tvd   tvd_excess  tvd_floor
blocking-nocache      617170   12.96±0.02    0.00±0.00          0    0.00±0.00          ref          ref          -
blocking-cache        767359   10.12±0.02    0.00±0.00          0    0.00±0.00  0.032±0.024  0.031±0.024      0.001
live-cache            899993    3.52±0.01    0.00±0.00        685    0.08±0.06  0.071±0.047  0.070±0.047      0.001
live-nocache          899991    6.35±0.01    0.00±0.00          0    0.00±0.00  0.063±0.006  0.062±0.006      0.001

threads (get_stack_trace) ref_keys=8
mode                 samples    stacks           µs      errors%      raw_tvd   tvd_excess  tvd_floor
blocking-nocache       62150    576377 159.09±86.15    0.00±0.00          ref          ref          -
blocking-cache         76231    810134 113.85±35.56    0.00±0.00  0.035±0.037  0.034±0.037      0.001
live-cache            318426   2714810   19.70±2.44    0.00±0.00  0.042±0.017  0.041±0.017      0.001
live-nocache          127896   1394382   60.32±9.68    0.00±0.00  0.064±0.023  0.063±0.022      0.001

Issue: _remote_debugging: reading whole pages over and over #149584

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maurycy requested a review from pablogsal as a code owner

October 10, 2026 15:17

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_remote_debugging: reading whole pages over and over #149584

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Oct 10, 2026

maurycy changed the title ~~gh-149584: _remote_debugging: Stop reading the base frame sentinel on every sample~~ gh-149584: _remote_debugging: Stop reading the base frame sentinel

Oct 10, 2026

maurycy changed the title ~~gh-149584: _remote_debugging: Stop reading the base frame sentinel~~ gh-149584: _remote_debugging: Stop rereading the base frame sentinel

Oct 10, 2026

maurycy commented

Oct 10, 2026

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