tests: the max_seconds timing tests flake under CPU contention, not on an idle machine #40

Closed
#106
Closed
tests: the max_seconds timing tests flake under CPU contention, not on an idle machine#40
#106

Description

@Shashankss1205
Shashankss1205
opened on Aug 2, 2026

What happens

The SIGALRM/max_seconds timing tests — three in tests/test_budget_enforcement.py, one in tests/test_cli.py, one in tests/test_cookbook_basics.py — fail intermittently when the machine is loaded, and pass reliably when it is quiet.

Evidence from one afternoon on a Linux 6.8 box:

  • Three full suites running in parallel (separate worktrees, separate venvs): one run reported exactly those five tests failing, identically on that worktree's untouched main; they passed when re-run individually.
  • The same three files run 16 consecutive times on the same machine while otherwise idle: 16 × exit 0.

So the failure mode is CPU-contention-dependent: when cores are oversubscribed, the deadline timer or the timed body overshoots its margin and the assertion on "raised in time" / "did not raise" goes the wrong way. A developer running one suite never sees it; CI on a busy runner, or anyone running suites in parallel, will.

Why it matters

A test that fails only under load is indistinguishable from a real regression in the deadline machinery — which this repo actually has strong claims about (the guard now refuses an overrun even when the timer never fired). Every false red spends someone's attention on the exact subsystem where a true red matters most.

What to consider

  • Widen the margins: the timed bodies sleep close to their deadlines; scale the sleep/deadline ratio so a 2-3× scheduling delay does not flip the outcome.
  • Or serialize: a timing marker plus -p no:randomly-style isolation (run those five tests in one process, not interleaved with the rest) would remove the contention within a single suite run, though not across parallel suites.
  • Or gate on load: skip-with-reason when the 1-minute loadavg exceeds the core count, so the signal stays honest instead of red.

Acceptance criteria

Five timing tests pass 20/20 while a parallel pytest suite runs on the same machine, and still fail when the guard's exit check is reverted.

Activity

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      Skip to content
      tests: the max_seconds timing tests flake under CPU contention, not on an idle machine #40
      Closed
      #106
      Closed
      tests: the max_seconds timing tests flake under CPU contention, not on an idle machine#40
      #106
      Description
      @Shashankss1205
      Shashankss1205
      opened on Aug 2, 2026
      What happens
      The SIGALRM/max_seconds timing tests — three in tests/test_budget_enforcement.py, one in tests/test_cli.py, one in tests/test_cookbook_basics.py — fail intermittently when the machine is loaded, and pass reliably when it is quiet.
      Evidence from one afternoon on a Linux 6.8 box:
      • Three full suites running in parallel (separate worktrees, separate venvs): one run reported exactly those five tests failing, identically on that worktree's untouched main; they passed when re-run individually.
      • The same three files run 16 consecutive times on the same machine while otherwise idle: 16 × exit 0.
      So the failure mode is CPU-contention-dependent: when cores are oversubscribed, the deadline timer or the timed body overshoots its margin and the assertion on "raised in time" / "did not raise" goes the wrong way. A developer running one suite never sees it; CI on a busy runner, or anyone running suites in parallel, will.
      Why it matters
      A test that fails only under load is indistinguishable from a real regression in the deadline machinery — which this repo actually has strong claims about (the guard now refuses an overrun even when the timer never fired). Every false red spends someone's attention on the exact subsystem where a true red matters most.
      What to consider
      • Widen the margins: the timed bodies sleep close to their deadlines; scale the sleep/deadline ratio so a 2-3× scheduling delay does not flip the outcome.
      • Or serialize: a timing marker plus -p no:randomly-style isolation (run those five tests in one process, not interleaved with the rest) would remove the contention within a single suite run, though not across parallel suites.
      • Or gate on load: skip-with-reason when the 1-minute loadavg exceeds the core count, so the signal stays honest instead of red.
      Acceptance criteria
      Five timing tests pass 20/20 while a parallel pytest suite runs on the same machine, and still fail when the guard's exit check is reverted.
      Activity
      Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
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(function(){
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var __m = "*";
var __re = new RegExp('^' + ".*" + '

          Skip to content
          tests: the max_seconds timing tests flake under CPU contention, not on an idle machine #40
          Closed
          #106
          Closed
          tests: the max_seconds timing tests flake under CPU contention, not on an idle machine#40
          #106
          Description
          @Shashankss1205
          Shashankss1205
          opened on Aug 2, 2026
          What happens
          The SIGALRM/max_seconds timing tests — three in tests/test_budget_enforcement.py, one in tests/test_cli.py, one in tests/test_cookbook_basics.py — fail intermittently when the machine is loaded, and pass reliably when it is quiet.
          Evidence from one afternoon on a Linux 6.8 box:
          • Three full suites running in parallel (separate worktrees, separate venvs): one run reported exactly those five tests failing, identically on that worktree's untouched main; they passed when re-run individually.
          • The same three files run 16 consecutive times on the same machine while otherwise idle: 16 × exit 0.
          So the failure mode is CPU-contention-dependent: when cores are oversubscribed, the deadline timer or the timed body overshoots its margin and the assertion on "raised in time" / "did not raise" goes the wrong way. A developer running one suite never sees it; CI on a busy runner, or anyone running suites in parallel, will.
          Why it matters
          A test that fails only under load is indistinguishable from a real regression in the deadline machinery — which this repo actually has strong claims about (the guard now refuses an overrun even when the timer never fired). Every false red spends someone's attention on the exact subsystem where a true red matters most.
          What to consider
          • Widen the margins: the timed bodies sleep close to their deadlines; scale the sleep/deadline ratio so a 2-3× scheduling delay does not flip the outcome.
          • Or serialize: a timing marker plus -p no:randomly-style isolation (run those five tests in one process, not interleaved with the rest) would remove the contention within a single suite run, though not across parallel suites.
          • Or gate on load: skip-with-reason when the 1-minute loadavg exceeds the core count, so the signal stays honest instead of red.
          Acceptance criteria
          Five timing tests pass 20/20 while a parallel pytest suite runs on the same machine, and still fail when the guard's exit check is reverted.
          Activity
          Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
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, 'i');
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(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

              Skip to content
              tests: the max_seconds timing tests flake under CPU contention, not on an idle machine #40
              Closed
              #106
              Closed
              tests: the max_seconds timing tests flake under CPU contention, not on an idle machine#40
              #106
              Description
              @Shashankss1205
              Shashankss1205
              opened on Aug 2, 2026
              What happens
              The SIGALRM/max_seconds timing tests — three in tests/test_budget_enforcement.py, one in tests/test_cli.py, one in tests/test_cookbook_basics.py — fail intermittently when the machine is loaded, and pass reliably when it is quiet.
              Evidence from one afternoon on a Linux 6.8 box:
              • Three full suites running in parallel (separate worktrees, separate venvs): one run reported exactly those five tests failing, identically on that worktree's untouched main; they passed when re-run individually.
              • The same three files run 16 consecutive times on the same machine while otherwise idle: 16 × exit 0.
              So the failure mode is CPU-contention-dependent: when cores are oversubscribed, the deadline timer or the timed body overshoots its margin and the assertion on "raised in time" / "did not raise" goes the wrong way. A developer running one suite never sees it; CI on a busy runner, or anyone running suites in parallel, will.
              Why it matters
              A test that fails only under load is indistinguishable from a real regression in the deadline machinery — which this repo actually has strong claims about (the guard now refuses an overrun even when the timer never fired). Every false red spends someone's attention on the exact subsystem where a true red matters most.
              What to consider
              • Widen the margins: the timed bodies sleep close to their deadlines; scale the sleep/deadline ratio so a 2-3× scheduling delay does not flip the outcome.
              • Or serialize: a timing marker plus -p no:randomly-style isolation (run those five tests in one process, not interleaved with the rest) would remove the contention within a single suite run, though not across parallel suites.
              • Or gate on load: skip-with-reason when the 1-minute loadavg exceeds the core count, so the signal stays honest instead of red.
              Acceptance criteria
              Five timing tests pass 20/20 while a parallel pytest suite runs on the same machine, and still fail when the guard's exit check is reverted.
              Activity
              Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
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, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Strip utm_, fbclid, gclid, etc. from all links on page\n(function() {\n var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',\n 'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',\n 'ref', 'ref_src', 'source', 'medium', 'campaign'];\n \n function cleanUrl(url) {\n try {\n var u = new URL(url, window.location.origin);\n var changed = false;\n trackingParams.forEach(function(p) {\n if (u.searchParams.has(p)) {\n u.searchParams.delete(p);\n changed = true;\n }\n });\n return changed ? u.toString() : url;\n } catch (e) {\n return url;\n }\n }\n \n function cleanLinks() {\n document.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n \n cleanLinks();\n \n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1) {\n if (node.tagName === 'A') cleanLinks();\n node.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Remove Tracking Parameters from Links");
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(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '

                  Skip to content
                  tests: the max_seconds timing tests flake under CPU contention, not on an idle machine #40
                  Closed
                  #106
                  Closed
                  tests: the max_seconds timing tests flake under CPU contention, not on an idle machine#40
                  #106
                  Description
                  @Shashankss1205
                  Shashankss1205
                  opened on Aug 2, 2026
                  What happens
                  The SIGALRM/max_seconds timing tests — three in tests/test_budget_enforcement.py, one in tests/test_cli.py, one in tests/test_cookbook_basics.py — fail intermittently when the machine is loaded, and pass reliably when it is quiet.
                  Evidence from one afternoon on a Linux 6.8 box:
                  • Three full suites running in parallel (separate worktrees, separate venvs): one run reported exactly those five tests failing, identically on that worktree's untouched main; they passed when re-run individually.
                  • The same three files run 16 consecutive times on the same machine while otherwise idle: 16 × exit 0.
                  So the failure mode is CPU-contention-dependent: when cores are oversubscribed, the deadline timer or the timed body overshoots its margin and the assertion on "raised in time" / "did not raise" goes the wrong way. A developer running one suite never sees it; CI on a busy runner, or anyone running suites in parallel, will.
                  Why it matters
                  A test that fails only under load is indistinguishable from a real regression in the deadline machinery — which this repo actually has strong claims about (the guard now refuses an overrun even when the timer never fired). Every false red spends someone's attention on the exact subsystem where a true red matters most.
                  What to consider
                  • Widen the margins: the timed bodies sleep close to their deadlines; scale the sleep/deadline ratio so a 2-3× scheduling delay does not flip the outcome.
                  • Or serialize: a timing marker plus -p no:randomly-style isolation (run those five tests in one process, not interleaved with the rest) would remove the contention within a single suite run, though not across parallel suites.
                  • Or gate on load: skip-with-reason when the 1-minute loadavg exceeds the core count, so the signal stays honest instead of red.
                  Acceptance criteria
                  Five timing tests pass 20/20 while a parallel pytest suite runs on the same machine, and still fail when the guard's exit check is reverted.
                  Activity
                  Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
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                      No branches or pull requests
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, 'i');
if (__m === '*' || __re.test(location.href)) {
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})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

                      Skip to content
                      tests: the max_seconds timing tests flake under CPU contention, not on an idle machine #40
                      Closed
                      #106
                      Closed
                      tests: the max_seconds timing tests flake under CPU contention, not on an idle machine#40
                      #106
                      Description
                      @Shashankss1205
                      Shashankss1205
                      opened on Aug 2, 2026
                      What happens
                      The SIGALRM/max_seconds timing tests — three in tests/test_budget_enforcement.py, one in tests/test_cli.py, one in tests/test_cookbook_basics.py — fail intermittently when the machine is loaded, and pass reliably when it is quiet.
                      Evidence from one afternoon on a Linux 6.8 box:
                      • Three full suites running in parallel (separate worktrees, separate venvs): one run reported exactly those five tests failing, identically on that worktree's untouched main; they passed when re-run individually.
                      • The same three files run 16 consecutive times on the same machine while otherwise idle: 16 × exit 0.
                      So the failure mode is CPU-contention-dependent: when cores are oversubscribed, the deadline timer or the timed body overshoots its margin and the assertion on "raised in time" / "did not raise" goes the wrong way. A developer running one suite never sees it; CI on a busy runner, or anyone running suites in parallel, will.
                      Why it matters
                      A test that fails only under load is indistinguishable from a real regression in the deadline machinery — which this repo actually has strong claims about (the guard now refuses an overrun even when the timer never fired). Every false red spends someone's attention on the exact subsystem where a true red matters most.
                      What to consider
                      • Widen the margins: the timed bodies sleep close to their deadlines; scale the sleep/deadline ratio so a 2-3× scheduling delay does not flip the outcome.
                      • Or serialize: a timing marker plus -p no:randomly-style isolation (run those five tests in one process, not interleaved with the rest) would remove the contention within a single suite run, though not across parallel suites.
                      • Or gate on load: skip-with-reason when the 1-minute loadavg exceeds the core count, so the signal stays honest instead of red.
                      Acceptance criteria
                      Five timing tests pass 20/20 while a parallel pytest suite runs on the same machine, and still fail when the guard's exit check is reverted.
                      Activity
                      Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
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                        Labels
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                          No branches or pull requests
                          Issue actions
                          
, 'i');
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(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

                          Skip to content
                          tests: the max_seconds timing tests flake under CPU contention, not on an idle machine #40
                          Closed
                          #106
                          Closed
                          tests: the max_seconds timing tests flake under CPU contention, not on an idle machine#40
                          #106
                          Description
                          @Shashankss1205
                          Shashankss1205
                          opened on Aug 2, 2026
                          What happens
                          The SIGALRM/max_seconds timing tests — three in tests/test_budget_enforcement.py, one in tests/test_cli.py, one in tests/test_cookbook_basics.py — fail intermittently when the machine is loaded, and pass reliably when it is quiet.
                          Evidence from one afternoon on a Linux 6.8 box:
                          • Three full suites running in parallel (separate worktrees, separate venvs): one run reported exactly those five tests failing, identically on that worktree's untouched main; they passed when re-run individually.
                          • The same three files run 16 consecutive times on the same machine while otherwise idle: 16 × exit 0.
                          So the failure mode is CPU-contention-dependent: when cores are oversubscribed, the deadline timer or the timed body overshoots its margin and the assertion on "raised in time" / "did not raise" goes the wrong way. A developer running one suite never sees it; CI on a busy runner, or anyone running suites in parallel, will.
                          Why it matters
                          A test that fails only under load is indistinguishable from a real regression in the deadline machinery — which this repo actually has strong claims about (the guard now refuses an overrun even when the timer never fired). Every false red spends someone's attention on the exact subsystem where a true red matters most.
                          What to consider
                          • Widen the margins: the timed bodies sleep close to their deadlines; scale the sleep/deadline ratio so a 2-3× scheduling delay does not flip the outcome.
                          • Or serialize: a timing marker plus -p no:randomly-style isolation (run those five tests in one process, not interleaved with the rest) would remove the contention within a single suite run, though not across parallel suites.
                          • Or gate on load: skip-with-reason when the 1-minute loadavg exceeds the core count, so the signal stays honest instead of red.
                          Acceptance criteria
                          Five timing tests pass 20/20 while a parallel pytest suite runs on the same machine, and still fail when the guard's exit check is reverted.
                          Activity
                          Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
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                              No branches or pull requests
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, 'i');
if (__m === '*' || __re.test(location.href)) {
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                              tests: the max_seconds timing tests flake under CPU contention, not on an idle machine #40
                              Closed
                              #106
                              Closed
                              tests: the max_seconds timing tests flake under CPU contention, not on an idle machine#40
                              #106
                              Description
                              @Shashankss1205
                              Shashankss1205
                              opened on Aug 2, 2026
                              What happens
                              The SIGALRM/max_seconds timing tests — three in tests/test_budget_enforcement.py, one in tests/test_cli.py, one in tests/test_cookbook_basics.py — fail intermittently when the machine is loaded, and pass reliably when it is quiet.
                              Evidence from one afternoon on a Linux 6.8 box:
                              • Three full suites running in parallel (separate worktrees, separate venvs): one run reported exactly those five tests failing, identically on that worktree's untouched main; they passed when re-run individually.
                              • The same three files run 16 consecutive times on the same machine while otherwise idle: 16 × exit 0.
                              So the failure mode is CPU-contention-dependent: when cores are oversubscribed, the deadline timer or the timed body overshoots its margin and the assertion on "raised in time" / "did not raise" goes the wrong way. A developer running one suite never sees it; CI on a busy runner, or anyone running suites in parallel, will.
                              Why it matters
                              A test that fails only under load is indistinguishable from a real regression in the deadline machinery — which this repo actually has strong claims about (the guard now refuses an overrun even when the timer never fired). Every false red spends someone's attention on the exact subsystem where a true red matters most.
                              What to consider
                              • Widen the margins: the timed bodies sleep close to their deadlines; scale the sleep/deadline ratio so a 2-3× scheduling delay does not flip the outcome.
                              • Or serialize: a timing marker plus -p no:randomly-style isolation (run those five tests in one process, not interleaved with the rest) would remove the contention within a single suite run, though not across parallel suites.
                              • Or gate on load: skip-with-reason when the 1-minute loadavg exceeds the core count, so the signal stays honest instead of red.
                              Acceptance criteria
                              Five timing tests pass 20/20 while a parallel pytest suite runs on the same machine, and still fail when the guard's exit check is reverted.
                              Activity
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