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pythongh-149584: Fix excessive overhead in the Tachyon profiler regarding the cache behavior (python#149649)
Use exact remote reads for interpreter state, thread state, and interpreter frame structs instead of pulling full remote pages into the profiler page cache. This matches the core change from python#149585. The profiler clears the page cache between samples, so live entries are always packed at the front. Track the live count and only clear/search that prefix instead of scanning all 1024 slots on the hot path. Use the frame cache to predict the next thread state and top frame address, then batch interpreter/thread/frame reads with process_vm_readv when profiling a Linux target. Reuse prefetched frame buffers in the frame walker when the prediction is valid. Cache the last FrameInfo tuple per code object/instruction offset, reuse cached thread id objects, and append cached parent frames directly on full frame-cache hits. This cuts Python allocation churn in the steady-state profiler path.
1 parent 06a2830 commit 661df25

12 files changed

Lines changed: 738 additions & 126 deletions

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‎Lib/profiling/sampling/sample.py‎

Lines changed: 27 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -327,6 +327,33 @@ def _print_unwinder_stats(self):
327327
print(f" Hits: {code_hits:n} ({ANSIColors.GREEN}{fmt(code_hits_pct)}%{ANSIColors.RESET})")
328328
print(f" Misses: {code_misses:n} ({ANSIColors.RED}{fmt(code_misses_pct)}%{ANSIColors.RESET})")
329329

330+
batched_attempts = stats.get('batched_read_attempts', 0)
331+
batched_successes = stats.get('batched_read_successes', 0)
332+
batched_misses = stats.get('batched_read_misses', 0)
333+
segments_requested = stats.get('batched_read_segments_requested', 0)
334+
segments_completed = stats.get('batched_read_segments_completed', 0)
335+
if batched_attempts > 0:
336+
batched_success_rate = stats.get('batched_read_success_rate', 0.0)
337+
batched_miss_rate = 100.0 - batched_success_rate
338+
segment_completion_rate = stats.get(
339+
'batched_read_segment_completion_rate', 0.0
340+
)
341+
342+
print(f" {ANSIColors.CYAN}Batched Reads:{ANSIColors.RESET}")
343+
print(f" Attempts: {batched_attempts:n}")
344+
print(
345+
f" Successes: {batched_successes:n} "
346+
f"({ANSIColors.GREEN}{fmt(batched_success_rate)}%{ANSIColors.RESET})"
347+
)
348+
print(
349+
f" Misses: {batched_misses:n} "
350+
f"({ANSIColors.RED}{fmt(batched_miss_rate)}%{ANSIColors.RESET})"
351+
)
352+
print(
353+
f" Segments read: {segments_completed:n}/{segments_requested:n} "
354+
f"({ANSIColors.GREEN}{fmt(segment_completion_rate)}%{ANSIColors.RESET})"
355+
)
356+
330357
# Memory operations
331358
memory_reads = stats.get('memory_reads', 0)
332359
memory_bytes = stats.get('memory_bytes_read', 0)

‎Lib/test/test_external_inspection.py‎

Lines changed: 7 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -3767,6 +3767,13 @@ def test_get_stats(self):
37673767
"frames_read_from_cache",
37683768
"frames_read_from_memory",
37693769
"frame_cache_hit_rate",
3770+
"batched_read_attempts",
3771+
"batched_read_successes",
3772+
"batched_read_misses",
3773+
"batched_read_segments_requested",
3774+
"batched_read_segments_completed",
3775+
"batched_read_success_rate",
3776+
"batched_read_segment_completion_rate",
37703777
]
37713778
for key in expected_keys:
37723779
self.assertIn(key, stats)

‎Misc/NEWS.d/next/Library/2026-05-10-19-26-50.gh-issue-149584.x7Qm9A.rst‎

Lines changed: 4 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,4 @@
1+
Fix excessive overhead in the Tachyon profiler when inspecting a remote
2+
process by avoiding repeated remote page-cache scans, batching predicted
3+
remote reads, and reusing cached profiler result objects. Patch by Pablo
4+
Galindo and Maurycy Pawłowski-Wieroński.

‎Modules/_remote_debugging/_remote_debugging.h‎

Lines changed: 79 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -30,6 +30,7 @@ extern "C" {
3030
#include "internal/pycore_llist.h" // struct llist_node
3131
#include "internal/pycore_long.h" // _PyLong_GetZero
3232
#include "internal/pycore_pyerrors.h" // _PyErr_FormatFromCause
33+
#include "internal/pycore_pyhash.h" // _Py_HashPointerRaw
3334
#include "internal/pycore_stackref.h" // Py_TAG_BITS
3435
#include "../../Python/remote_debug.h"
3536

@@ -215,6 +216,8 @@ typedef struct {
215216
PyObject *file_name;
216217
int first_lineno;
217218
PyObject *linetable; // bytes
219+
PyObject *last_frame_info;
220+
ptrdiff_t last_addrq;
218221
uintptr_t addr_code_adaptive;
219222
} CachedCodeMetadata;
220223

@@ -224,11 +227,41 @@ typedef struct {
224227

225228
typedef struct {
226229
uint64_t thread_id; // 0 = empty slot
230+
uintptr_t thread_state_addr;
227231
uintptr_t addrs[FRAME_CACHE_MAX_FRAMES];
228232
Py_ssize_t num_addrs;
233+
PyObject *thread_id_obj; // owned reference, NULL if empty
229234
PyObject *frame_list; // owned reference, NULL if empty
230235
} FrameCacheEntry;
231236

237+
#define INTERPRETER_THREAD_CACHE_SIZE 32
238+
#if (INTERPRETER_THREAD_CACHE_SIZE & (INTERPRETER_THREAD_CACHE_SIZE - 1)) != 0
239+
# error "INTERPRETER_THREAD_CACHE_SIZE must be a power of two"
240+
#endif
241+
242+
// The two per-interpreter L2 caches below are split into per-field tables so
243+
// that a writer rebinding one slot cannot leave stale data in a field owned by
244+
// the other when the slot is reused across interpreters.
245+
typedef struct {
246+
uintptr_t interpreter_addr;
247+
uintptr_t thread_state_addr;
248+
} InterpreterTstateCacheEntry;
249+
typedef struct {
250+
uintptr_t interpreter_addr;
251+
uint64_t code_object_generation;
252+
} InterpreterGenerationCacheEntry;
253+
254+
// Carries already-read thread state and/or frame buffers across helpers so the
255+
// downstream callee can skip a remote read. Address fields are caller-supplied
256+
// inputs; buffer pointers (tstate, frame) are NULL unless a prior batched read
257+
// successfully populated them.
258+
typedef struct {
259+
const char *tstate;
260+
uintptr_t tstate_addr;
261+
const char *frame;
262+
uintptr_t frame_addr;
263+
} RemoteReadPrefetch;
264+
232265
/* Statistics for profiling performance analysis */
233266
typedef struct {
234267
uint64_t total_samples; // Total number of get_stack_trace calls
@@ -242,14 +275,44 @@ typedef struct {
242275
uint64_t code_object_cache_hits; // Code object cache hits
243276
uint64_t code_object_cache_misses; // Code object cache misses
244277
uint64_t stale_cache_invalidations; // Times stale entries were cleared
278+
uint64_t batched_read_attempts; // Batched remote-read attempts
279+
uint64_t batched_read_successes; // Attempts that read all requested segments
280+
uint64_t batched_read_misses; // Attempts that fell back or partially read
281+
uint64_t batched_read_segments_requested; // Segments requested by batched reads
282+
uint64_t batched_read_segments_completed; // Segments completed by batched reads
245283
} UnwinderStats;
246284

285+
#if defined(__GNUC__) || defined(__clang__)
286+
# define REMOTE_DEBUG_UNLIKELY(value) __builtin_expect(!!(value), 0)
287+
#else
288+
# define REMOTE_DEBUG_UNLIKELY(value) (value)
289+
#endif
290+
247291
/* Stats tracking macros - no-op when stats collection is disabled */
248292
#define STATS_INC(unwinder, field) \
249-
do { if ((unwinder)->collect_stats) (unwinder)->stats.field++; } while(0)
293+
do { if (REMOTE_DEBUG_UNLIKELY((unwinder)->collect_stats)) (unwinder)->stats.field++; } while(0)
250294

251295
#define STATS_ADD(unwinder, field, val) \
252-
do { if ((unwinder)->collect_stats) (unwinder)->stats.field += (val); } while(0)
296+
do { if (REMOTE_DEBUG_UNLIKELY((unwinder)->collect_stats)) (unwinder)->stats.field += (val); } while(0)
297+
298+
#if HAVE_PROCESS_VM_READV
299+
# define STATS_BATCHED_READ(unwinder, requested, completed) \
300+
do { \
301+
if (REMOTE_DEBUG_UNLIKELY((unwinder)->collect_stats)) { \
302+
(unwinder)->stats.batched_read_attempts++; \
303+
(unwinder)->stats.batched_read_segments_requested += (uint64_t)(requested); \
304+
(unwinder)->stats.batched_read_segments_completed += (uint64_t)(completed); \
305+
if ((completed) == (requested)) { \
306+
(unwinder)->stats.batched_read_successes++; \
307+
} \
308+
else { \
309+
(unwinder)->stats.batched_read_misses++; \
310+
} \
311+
} \
312+
} while(0)
313+
#else
314+
# define STATS_BATCHED_READ(unwinder, requested, completed) ((void)0)
315+
#endif
253316

254317
typedef struct {
255318
PyTypeObject *RemoteDebugging_Type;
@@ -290,7 +353,6 @@ typedef struct {
290353
struct _Py_AsyncioModuleDebugOffsets async_debug_offsets;
291354
uintptr_t interpreter_addr;
292355
uintptr_t tstate_addr;
293-
uint64_t code_object_generation;
294356
_Py_hashtable_t *code_object_cache;
295357
int debug;
296358
int only_active_thread;
@@ -302,9 +364,17 @@ typedef struct {
302364
int cache_frames;
303365
int collect_stats; // whether to collect statistics
304366
uint32_t stale_invalidation_counter; // counter for throttling frame_cache_invalidate_stale
367+
// L1 single-entry shortcut over cached_tstates[]: most workloads sample one
368+
// interpreter, so check these pairs before hashing into the table below.
369+
uintptr_t cached_tstate_interpreter_addr;
370+
uintptr_t cached_tstate_addr;
371+
uintptr_t cached_generation_interpreter_addr;
372+
uint64_t cached_code_object_generation;
305373
RemoteDebuggingState *cached_state;
306374
FrameCacheEntry *frame_cache; // preallocated array of FRAME_CACHE_MAX_THREADS entries
307375
UnwinderStats stats; // statistics for performance analysis
376+
InterpreterTstateCacheEntry cached_tstates[INTERPRETER_THREAD_CACHE_SIZE];
377+
InterpreterGenerationCacheEntry cached_generations[INTERPRETER_THREAD_CACHE_SIZE];
308378
#ifdef Py_GIL_DISABLED
309379
uint32_t tlbc_generation;
310380
_Py_hashtable_t *tlbc_cache;
@@ -361,11 +431,13 @@ typedef struct {
361431
typedef struct {
362432
/* Inputs */
363433
uintptr_t frame_addr; // Starting frame address
434+
uintptr_t thread_state_addr; // Owning thread state address
364435
uintptr_t base_frame_addr; // Sentinel at bottom (for validation)
365436
uintptr_t gc_frame; // GC frame address (0 if not tracking)
366437
uintptr_t last_profiled_frame; // Last cached frame (0 if no cache)
367438
StackChunkList *chunks; // Pre-copied stack chunks
368439
int skip_first_frame; // Skip frame_addr itself (continue from its caller)
440+
RemoteReadPrefetch prefetch; // Optional already-read thread/frame buffers
369441

370442
/* Outputs */
371443
PyObject *frame_info; // List to append FrameInfo objects
@@ -548,6 +620,7 @@ extern int process_frame_chain(
548620
extern int frame_cache_init(RemoteUnwinderObject *unwinder);
549621
extern void frame_cache_cleanup(RemoteUnwinderObject *unwinder);
550622
extern FrameCacheEntry *frame_cache_find(RemoteUnwinderObject *unwinder, uint64_t thread_id);
623+
extern FrameCacheEntry *frame_cache_find_by_tstate(RemoteUnwinderObject *unwinder, uintptr_t tstate_addr);
551624
extern int clear_last_profiled_frames(RemoteUnwinderObject *unwinder);
552625
extern void frame_cache_invalidate_stale(RemoteUnwinderObject *unwinder, PyObject *result);
553626
extern int frame_cache_lookup_and_extend(
@@ -566,6 +639,7 @@ extern int frame_cache_store(
566639
PyObject *frame_list,
567640
const uintptr_t *addrs,
568641
Py_ssize_t num_addrs,
642+
uintptr_t thread_state_addr,
569643
uintptr_t base_frame_addr,
570644
uintptr_t last_frame_visited);
571645

@@ -605,7 +679,8 @@ extern PyObject* unwind_stack_for_thread(
605679
uintptr_t *current_tstate,
606680
uintptr_t gil_holder_tstate,
607681
uintptr_t gc_frame,
608-
uintptr_t main_thread_tstate
682+
uintptr_t main_thread_tstate,
683+
const RemoteReadPrefetch *prefetch
609684
);
610685

611686
/* Thread stopping functions (for blocking mode) */

‎Modules/_remote_debugging/clinic/module.c.h‎

Lines changed: 8 additions & 1 deletion
Some generated files are not rendered by default. Learn more about customizing how changed files appear on GitHub.

‎Modules/_remote_debugging/code_objects.c‎

Lines changed: 13 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -405,6 +405,8 @@ parse_code_object(RemoteUnwinderObject *unwinder,
405405
meta->func_name = func;
406406
meta->file_name = file;
407407
meta->linetable = linetable;
408+
meta->last_frame_info = NULL;
409+
meta->last_addrq = -1;
408410
meta->first_lineno = GET_MEMBER(int, code_object, unwinder->debug_offsets.code_object.firstlineno);
409411
meta->addr_code_adaptive = real_address + (uintptr_t)unwinder->debug_offsets.code_object.co_code_adaptive;
410412

@@ -482,6 +484,12 @@ parse_code_object(RemoteUnwinderObject *unwinder,
482484
addrq = (uint16_t *)ip - (uint16_t *)meta->addr_code_adaptive;
483485
#endif
484486
; // Empty statement to avoid C23 extension warning
487+
488+
if (!unwinder->opcodes && meta->last_frame_info != NULL && meta->last_addrq == addrq) {
489+
*result = Py_NewRef(meta->last_frame_info);
490+
return 0;
491+
}
492+
485493
LocationInfo info = {0};
486494
bool ok = parse_linetable(addrq, PyBytes_AS_STRING(meta->linetable),
487495
PyBytes_GET_SIZE(meta->linetable),
@@ -529,6 +537,11 @@ parse_code_object(RemoteUnwinderObject *unwinder,
529537
goto error;
530538
}
531539

540+
if (!unwinder->opcodes) {
541+
Py_XSETREF(meta->last_frame_info, Py_NewRef(tuple));
542+
meta->last_addrq = addrq;
543+
}
544+
532545
*result = tuple;
533546
return 0;
534547

‎Modules/_remote_debugging/frame_cache.c‎

Lines changed: 26 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -30,6 +30,7 @@ frame_cache_cleanup(RemoteUnwinderObject *unwinder)
3030
return;
3131
}
3232
for (int i = 0; i < FRAME_CACHE_MAX_THREADS; i++) {
33+
Py_CLEAR(unwinder->frame_cache[i].thread_id_obj);
3334
Py_CLEAR(unwinder->frame_cache[i].frame_list);
3435
}
3536
PyMem_Free(unwinder->frame_cache);
@@ -53,6 +54,21 @@ frame_cache_find(RemoteUnwinderObject *unwinder, uint64_t thread_id)
5354
return NULL;
5455
}
5556

57+
FrameCacheEntry *
58+
frame_cache_find_by_tstate(RemoteUnwinderObject *unwinder, uintptr_t tstate_addr)
59+
{
60+
if (!unwinder->frame_cache || tstate_addr == 0) {
61+
return NULL;
62+
}
63+
for (int i = 0; i < FRAME_CACHE_MAX_THREADS; i++) {
64+
if (unwinder->frame_cache[i].thread_state_addr == tstate_addr) {
65+
assert(unwinder->frame_cache[i].num_addrs <= FRAME_CACHE_MAX_FRAMES);
66+
return &unwinder->frame_cache[i];
67+
}
68+
}
69+
return NULL;
70+
}
71+
5672
// Allocate a cache slot for a thread
5773
// Returns NULL if cache is full (graceful degradation)
5874
static FrameCacheEntry *
@@ -127,8 +143,10 @@ frame_cache_invalidate_stale(RemoteUnwinderObject *unwinder, PyObject *result)
127143
}
128144
if (!found) {
129145
// Clear this entry
146+
Py_CLEAR(unwinder->frame_cache[i].thread_id_obj);
130147
Py_CLEAR(unwinder->frame_cache[i].frame_list);
131148
unwinder->frame_cache[i].thread_id = 0;
149+
unwinder->frame_cache[i].thread_state_addr = 0;
132150
unwinder->frame_cache[i].num_addrs = 0;
133151
STATS_INC(unwinder, stale_cache_invalidations);
134152
}
@@ -216,6 +234,7 @@ frame_cache_store(
216234
PyObject *frame_list,
217235
const uintptr_t *addrs,
218236
Py_ssize_t num_addrs,
237+
uintptr_t thread_state_addr,
219238
uintptr_t base_frame_addr,
220239
uintptr_t last_frame_visited)
221240
{
@@ -257,6 +276,13 @@ frame_cache_store(
257276
return -1;
258277
}
259278
entry->thread_id = thread_id;
279+
entry->thread_state_addr = thread_state_addr;
280+
if (entry->thread_id_obj == NULL) {
281+
entry->thread_id_obj = PyLong_FromUnsignedLongLong(thread_id);
282+
if (entry->thread_id_obj == NULL) {
283+
return -1;
284+
}
285+
}
260286
memcpy(entry->addrs, addrs, num_addrs * sizeof(uintptr_t));
261287
entry->num_addrs = num_addrs;
262288
assert(entry->num_addrs == num_addrs);

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