_scaling_config was documented as read-only after construction but
nothing prevented accidental mutation. Move its initialization from
the constructor body into the member initializer list (using a
ternary to handle the nullptr case) and mark it const.
_autoscale_enabled cannot be made const because the constructor
clears it if the monitor thread fails to spawn.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
These fields are written once in the constructor before any worker or
monitor threads are spawned, and are read-only thereafter. Add an
explicit comment noting the write-once-then-read-only invariant so
future maintainers know that making them mutable would require
synchronization.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
The CAS loop on _remove_count uses memory_order_relaxed for both
success and failure, which is correct but non-obvious. Add a comment
explaining that _remove_self acquires _threads_mutex immediately after
the successful CAS, and that mutex acquisition provides the necessary
happens-before edge between the scaling-side increment and the
worker-side erase/detach.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
When pthread_create fails (e.g., EAGAIN from OS resource limits),
the monitor and fast-path pressure grow would retry on the next cycle
with no delay, potentially flooding syslog with identical warnings
every 100ms.
Fix: stamp _last_scale_time_usecs on creation failure so the existing
cooldown mechanism (200ms default) suppresses immediate retries. This
reuses the scaling cooldown infrastructure rather than adding a
separate backoff timer.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
When throughput dropped to zero, the monitor reset direction and
decline_streak but not warmup_samples. On resumption, the first
meaningful throughput sample was compared against a stale pre-idle
EMA value, which could cause the hill-climber to make a scaling
decision based on a meaningless delta (new throughput vs. zero-decayed
EMA from potentially minutes ago).
Reset warmup_samples to 0 in the idle branch so the EMA is re-seeded
from two fresh samples before the hill-climber acts. This ensures
the first scaling decision after an idle period is based on current
throughput trends rather than stale history.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
The monitor thread used nanosleep for its sample interval, which meant
the destructor had to block for up to one full sample period (100ms by
default) waiting for the monitor to wake up and notice _stop.
Replace nanosleep with pthread_cond_timedwait on a dedicated condvar
(_monitor_cond) using CLOCK_MONOTONIC. The destructor now signals
the condvar immediately after setting _stop, so the monitor wakes up
and exits without waiting for the interval to expire.
This also eliminates the EINTR retry loop — pthread_cond_timedwait
handles signals internally and returns when either the timeout expires
or the condvar is signaled.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
set_threads computed its delta from _threads.size(), which does not
reflect _remove_count signals already in flight. If called twice
before workers drained the first batch of removals, the second call
would see the original thread count and issue a full set of redundant
removal signals, potentially shrinking the pool well below the
requested target.
Fix: compute the effective thread count as _threads.size() minus
pending _remove_count, and use that as the baseline for the grow/shrink
delta. This makes set_threads idempotent regardless of drain timing.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
The monitor thread checked _last_scale_time_usecs without holding
_scaling_mutex, then called add_thread()/remove_thread() which each
acquired the mutex independently. The fast-path _maybe_grow_on_pressure
could scale between the monitor's cooldown check and its scaling action,
allowing two scaling events closer together than cooldown_usecs.
Fix: hold _scaling_mutex across both the cooldown check and the scaling
action in the monitor loop, using the _locked variants directly. This
ensures the monitor and fast-path pressure grow are mutually exclusive
for the entire check-and-act sequence.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
enqueue_work_autoscale relies on BoundedQueue::try_enqueue leaving the
callable untouched when the semaphore check fails. This is correct
today because tryWait() is evaluated before std::forward moves the
item, but the coupling is implicit — a future BoundedQueue change
that reorders these operations would silently introduce a use-after-
move bug on the blocking fallback path.
Add a debug assert after the failed try_enqueue to verify the Func is
still valid before using it on the blocking path. Strengthen the
comment to explicitly name the invariant being relied upon.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
_remove_thread_scaling_locked increments _remove_count to signal a
worker to self-exit, but this counter is only checked after a worker
completes a real task. Workers blocked on wait_dequeue (standard loop)
or try_wait_dequeue_for (autoscale loop) never see the removal request,
so shrinking an idle pool via remove_thread() or set_threads() stalls
indefinitely.
Fix: enqueue a wakeup poison pill alongside each _remove_count
increment so a blocked worker is guaranteed to wake up.
To prevent double-removal (one worker exits via the task-completion CAS
loop consuming _remove_count, another exits via the wakeup pill), the
poison pill handler now attempts to consume a _remove_count before
self-removing. If the count is already zero (claimed by a worker that
finished a real task), the pill is treated as stale and the worker
continues processing.
Shutdown path is unaffected: the destructor sets _stop before enqueueing
its own pills, and the pill handler checks _stop first, returning BREAK
unconditionally during shutdown regardless of _remove_count.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
The previous ordering (enqueued first, then completed) meant a
task completing between the two loads would inflate the pending
count. For scaling heuristics, over-reporting pressure is worse
than under-reporting: it can trigger unnecessary thread creation.
Swap the load order so completed is read first. A completion
between the two loads now reduces the delta toward zero, which
is the safer direction for autoscale decisions.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
If remove_thread() was called but no worker claimed the decrement
before shutdown, _remove_count remains positive. Workers exiting
via poison pills would enter the _remove_count CAS loop in
_process_func, but since _stop is set, _remove_self is a no-op
anyway. The stale counter is harmless at runtime but confusing
when debugging — it suggests pending work that will never complete.
Zero the counter after _stop is set and before enqueuing poison
pills so the shutdown path is clean and unambiguous.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
The _stop check in _try_remove_self_if_above was outside the
_threads_mutex lock, creating a window where a worker thread could
read _stop==false, then the destructor could set _stop and snapshot
_threads (including that worker), then the worker could acquire the
lock, detach itself, and exit. The destructor would then call
pthread_join on a detached thread — undefined behavior.
Move the _stop check inside the _threads_mutex lock so it is
serialized with the destructor's snapshot. Now either the worker
holds the lock first (detaches and erases itself before the
snapshot) or the destructor sets _stop first (worker sees it and
bails out, remaining joinable in the snapshot).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
<chrono> was used by the std::chrono::seconds(5) future wait in the
old promise-based _remove_thread_scaling_locked, which was replaced
by the atomic _remove_count counter. <memory> was used for
std::make_shared in the same code path. Neither header is used
anywhere else in thread_pool.hpp.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
The pre-lock cooldown check used a stale _last_scale_time_usecs read.
If another thread (monitor hill-climb, concurrent enqueue) updated
the timestamp between the pre-lock check and the lock acquisition,
the function would proceed to acquire _scaling_mutex and contend
unnecessarily. The CAS that followed would correctly reject the
stale value, so this was not a correctness bug, but it caused
avoidable lock contention on the hot enqueue path.
Add an authoritative cooldown re-check after acquiring _scaling_mutex
with a fresh timestamp read. Replace the CAS with a plain store
since the _scaling_mutex already serializes writers. Keep the
pre-lock speculative check as a fast-path filter.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
The hill-climb monitor samples throughput, perturbs thread count by
+/-1, and checks whether throughput improved. But between samples,
other mechanisms (fast-path pressure grow, idle self-exit) can also
change the thread count. The monitor had no way to distinguish its
own perturbation's effect from these external changes, which could
cause it to reinforce or reverse direction based on unrelated scaling.
Track the expected thread count after each perturbation. At the next
sample, compare the actual count to the expected one. If they differ,
external scaling occurred — skip the throughput attribution, reset the
decline streak, and re-baseline from the current count.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
The monitor thread read the thread count before calling add_thread or
remove_thread, then logged count+1 or count-1 as the new size. If
concurrent scaling (fast-path grow, idle self-exit) changed the count
between the read and the action, the logged value was wrong.
Read thread_count() after the scaling action for the log message so
it reflects reality. The pre-action read is kept for the bounds
check (direction guard) since that only needs to be approximately
correct.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
set_threads did not enforce ScalingConfig::min_threads / max_threads,
so a caller could push the pool outside the range that the monitor
thread and idle self-exit logic expect. For example, set_threads(1)
with min_threads=2 would be rejected by _remove_thread_scaling_locked
partway through, leaving the pool in a partially-shrunk state.
Growing above max_threads had no guard at all.
Clamp the requested count to [min_threads, max_threads] when
autoscaling is enabled, mirroring the constructor's clamping logic.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
_remove_thread_scaling_locked previously enqueued a heap-allocated
lambda (capturing a shared_ptr<promise>) to signal a specific worker
to exit. This had two problems:
1. Bug: if the 5-second future::wait_for timed out, the function
returned -EBUSY but the lambda remained in the queue. When a
worker eventually executed it, it would unconditionally set
_tl_should_exit and call _remove_self, silently killing an
arbitrary thread with no compensating add.
2. Held _scaling_mutex for up to 5 seconds while blocking on the
future, serializing all concurrent scaling operations (add_thread,
remove_thread, _maybe_grow_on_pressure, monitor hill-climb).
Replace the entire mechanism with an atomic _remove_count counter.
_remove_thread_scaling_locked simply increments the counter and
returns immediately. After each task, _process_func does a
CAS-decrement; the winning worker detaches itself via _remove_self
and exits.
This eliminates:
- All heap allocations (shared_ptr, promise, lambda captures)
- The thread-local _tl_should_exit flag
- The 5-second blocking window under _scaling_mutex
- The stale-lambda bug entirely (no lambda to go stale)
- The <memory> include (no longer needed)
During shutdown, _remove_self is a no-op (_stop is set), so a worker
that wins the CAS stays in _threads and is joined normally by the
destructor. Any outstanding _remove_count credits are harmlessly
abandoned.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
start_routine and start_routine_autoscale shared ~30 lines of identical
post-dequeue logic: poison-pill detection, exception-guarded execution,
func destruction, tasks_completed bookkeeping, and the _tl_should_exit
exit protocol. Any bug fix to this logic had to be applied in both
places.
Extract the shared code into _process_func which returns a WorkerAction
enum (CONTINUE / BREAK / EXIT). Both worker loops now call this helper,
reducing the maintenance surface and eliminating the duplication.
Also document the BoundedQueue::try_enqueue no-move-on-failure contract
in enqueue_work_autoscale, since the correctness of the try-then-block
pattern depends on it.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
tasks_pending() reads _tasks_enqueued and _tasks_completed in two
separate relaxed loads. Between the loads, additional tasks can
complete, making the result transiently inconsistent (the (e > c)
guard prevents underflow but not staleness).
Add a comment clarifying that this is intentionally approximate and
should not be used for synchronization decisions.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
After _try_remove_self_if_above detaches and erases the thread, the
destructor's snapshot will not include it — so pthread_join is never
called on it, and the ThreadPool object can be destroyed while this
thread is still returning.
This is safe because the thread only accesses local variables after
detach (name is a std::string copy made at function entry), but the
reasoning is subtle. Add an explicit safety comment explaining why
there is no use-after-free.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
The _tl_should_exit mechanism in start_routine (and
start_routine_autoscale) is correct but the control flow is non-obvious:
_remove_thread_scaling_locked enqueues a lambda that sets the
thread-local flag and calls _remove_self (detach + erase), then the
worker checks the flag after running the func and exits.
Replace the terse one-line comments with an explanation of the full
protocol so future readers don't mistake it for dead code.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
When throughput dropped below 0.5 (idle), decline_streak was reset but
direction was preserved. When work resumed, the monitor continued in
whatever direction it held before the idle period — often a shrink
direction that was correct for the previous workload but wrong for the
new burst. This could cause the pool to shrink immediately when it
should be growing.
Reset direction to +1 (grow) when entering the idle branch so the pool
starts the next active period biased toward growth, which is the safer
default under new load.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
nanosleep returns -1 with errno EINTR when interrupted by a signal.
The remaining time was discarded (second arg was NULL), causing the
monitor to wake early and sample a shortened interval. This produced
artificially low throughput numbers that corrupted the EMA and could
trigger unnecessary scale-down decisions.
Loop on EINTR using the remaining time output parameter so the full
sample interval is always honored. Also check _stop on each retry
so the monitor exits promptly during shutdown.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
The CAS on _last_scale_time_usecs was performed before checking whether
the pool was already at max_threads. When the pool was at capacity, the
cooldown window was consumed but no thread was added. This suppressed
legitimate scaling attempts from the monitor thread for the entire
cooldown period.
Move the CAS after the max-threads check and the _scaling_mutex
acquisition so the cooldown is only claimed when a thread will actually
be added. The mutex serializes concurrent callers, and the CAS still
deduplicates concurrent pressure-grow attempts that pass the check
simultaneously.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
When auto-scaling is enabled, the constructor did not enforce that the
initial thread_count falls within [min_threads, max_threads]. Starting
with fewer threads than min_threads meant the monitor had to
immediately grow the pool, and starting above max_threads created
threads that the monitor would then immediately try to shrink.
Clamp the count before spawning so the pool starts within its
configured operating range.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
usleep is POSIX-obsolescent and its behavior on signal interruption is
under-specified. Replace with nanosleep which is the current POSIX
standard and has well-defined EINTR semantics. Pre-compute the
timespec once outside the loop since the interval is constant.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
The _locked suffix conventionally means the caller already holds the
lock being named. These functions expect _scaling_mutex to be held,
but the old name was ambiguous — it looked like _threads_mutex.
Rename to _add_thread_scaling_locked and _remove_thread_scaling_locked
to make the precondition explicit.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
_remove_thread_locked enqueues a self-exit task and blocks on the
future while holding _scaling_mutex. If all workers are busy with
long-running tasks, this blocks indefinitely — starving the monitor
thread (and any concurrent add_thread/set_threads) since they all
contend on _scaling_mutex.
Replace the unbounded wait with a 5-second timed wait. On timeout,
return -EBUSY so callers can retry or report the condition. The
enqueued self-exit task will still be picked up eventually and the
thread will remove itself when a worker becomes free.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
The old macro expanded to two statements (mutex_lock + DEFER unlock).
In an unbraced if/else only the lock would be conditional while the
DEFER always executed — a latent double-unlock / unbalanced-unlock bug.
Add ScopedMutexLock to both debug and ndebug headers:
- debug: stores __FILE__/__func__/__LINE__ from the call site and
passes them to _mutex_lock/_mutex_unlock for diagnostics
- ndebug: lightweight wrapper around mutex_lock/mutex_unlock
Redefine mutex_lockguard to create a single uniquely-named
ScopedMutexLock variable (via __COUNTER__), making it safe in any
statement context. Remove the now-unused scope_guard include.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
cooldown_usecs was only checked in _maybe_grow_on_pressure (the
fast-path enqueue grow). The monitor thread scaled every
sample_interval_usecs unconditionally, so a recent fast-path grow
could be immediately followed by a monitor grow, defeating the
cooldown.
Check _last_scale_time_usecs against cooldown_usecs before applying
any monitor scaling decision, making the cooldown global across both
scaling paths.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
A single noisy throughput sample would immediately flip the scaling
direction, causing the monitor to oscillate under bursty workloads —
continuously adding and removing threads every sample interval.
Replace raw throughput comparison with an exponential moving average
(alpha=0.3) and require two consecutive declining samples before
reversing direction. This filters out transient dips and keeps the
hill-climber from thrashing.
Also use a 2-sample warmup period to seed the EMA before making any
scaling decisions.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
set_threads silently ignored failures from _add_thread_locked (e.g.
pthread_create failure) and _remove_thread_locked (e.g. at min_threads).
The caller had no way to know the pool was undersized or that shrinking
stopped early.
Stop on the first error and return it so callers can detect partial
completion.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Multiple concurrent enqueue_work_autoscale callers could all pass the
cooldown check and each add a thread, overshooting max_threads and
bypassing the intended rate limit.
Use compare_exchange_strong on _last_scale_time_usecs so only one
caller wins the cooldown window. Then hold _scaling_mutex for the
thread-count check and _add_thread_locked call so the grow is atomic
with respect to other scaling operations (monitor, set_threads).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
After _try_remove_self_if_above detaches the thread, the destructor
will not join it. If the destructor runs before the detached thread
reaches the syslog call, btp->_name is a dangling reference.
Copy _name into a local string at thread start so the syslog after
self-removal uses stack-local data that outlives the detach.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
The file-scope function _tp_now_usecs had a leading underscore
followed by a lowercase letter, which is reserved at file scope
by the C++ standard. Move it to a private static member function
ThreadPool::_now_usecs where leading underscores on member names
are permitted.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
ScalingConfig was value-initialized to all zeros when no config
is provided. While the non-autoscale path gates all access, a
zero idle_threshold_usecs would cause try_wait_dequeue_for to
return immediately if the autoscale path were ever reached by
mistake.
Add default member initializers: min_threads=2, max_threads=64,
sample_interval=100ms, cooldown=200ms, idle_threshold=5s. These
match the values recommended in the scaling policy design and
provide safe behavior even if a caller constructs ScalingConfig
without setting all fields.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
The counter was incremented before the enqueue call, which can
block indefinitely for bounded queues. If tasks_pending() is used
as a drain-complete signal (pending == 0), the pre-increment
would overcount while a producer is blocked waiting for a slot.
Move the increment to after the enqueue returns in all paths:
enqueue_work, enqueue_work_autoscale, and enqueue_task. The
try_enqueue variants already incremented after success.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
The hill-climb monitor and idle self-exit paths could both shrink
the pool past min_threads. The monitor read thread count, released
the lock, then called remove_thread - but between those steps an
idle worker could also self-exit, dropping below the threshold.
Three changes:
- _remove_thread_locked now checks min_threads (from ScalingConfig)
under the same lock as the size check, refusing removal if at or
below the bound.
- Idle self-exit replaced with _try_remove_self_if_above which
atomically checks thread count and erases under a single lock
acquisition, closing the TOCTOU window.
- The previous _remove_self is retained for the non-scaling paths
(poison pill exit during set_threads/remove_thread).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
enqueue_unbounded bypasses the semaphore so control messages
(poison pills, thread removal tasks) can never block on a full
queue. But wait_dequeue always signals a slot after consuming,
so each unbounded item that gets dequeued inflated the semaphore
by +1, permanently raising the effective max queue depth.
Fix by calling tryWait() in enqueue_unbounded before enqueueing.
When the queue has room (common case), this consumes a slot and
the dequeue-side signal restores balance perfectly. When the queue
is genuinely full (tryWait fails), the item still enqueues but
temporarily exceeds max_depth by one until consumed - a transient
overshoot bounded by control messages in flight.
A CAS-based approach that absorbed extra signals on the dequeue
side was also considered but rejected: it can starve blocked
bounded producers by swallowing the signal they are waiting for.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
The try-enqueue and fallback block-enqueue both called
std::forward<FuncType>(func_) on the same parameter. If
try_enqueue's internals ever change to move from the argument
before failing, the second forward would operate on a moved-from
object.
Fix by materializing into a Func up front and using std::move
for both attempts. Also extract the duplicated scaling logic into
_maybe_grow_on_pressure(), eliminating the copy-paste between the
PToken and non-PToken overloads.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
The destructor and autoscale idle-exit path could race: a worker
calling _remove_self would erase itself from _threads and detach
while the destructor was iterating _threads to enqueue poison pills
and join. This could cause too few poison pills (deadlocking
remaining workers) or pthread_join on a detached thread (UB).
Two changes:
- _remove_self now checks _stop and returns immediately during
shutdown, ensuring no thread detaches after destruction begins.
- Destructor snapshots _threads under the mutex before enqueueing
poison pills and joining, so it operates on a stable copy.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
