method_worker.c

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/*-------------------------------------------------------------------------
 *
 * method_worker.c
 *    AIO - perform AIO using worker processes
 *
 * IO workers consume IOs from a shared memory submission queue, run
 * traditional synchronous system calls, and perform the shared completion
 * handling immediately.  Client code submits most requests by pushing IOs
 * into the submission queue, and waits (if necessary) using condition
 * variables.  Some IOs cannot be performed in another process due to lack of
 * infrastructure for reopening the file, and must processed synchronously by
 * the client code when submitted.
 *
 * The pool of workers tries to stabilize at a size that can handle recently
 * seen variation in demand, within the configured limits.
 *
 * This method of AIO is available in all builds on all operating systems, and
 * is the default.
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/storage/aio/method_worker.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include <limits.h>
 
#include "libpq/pqsignal.h"
#include "miscadmin.h"
#include "port/pg_bitutils.h"
#include "postmaster/auxprocess.h"
#include "postmaster/interrupt.h"
#include "storage/aio.h"
#include "storage/aio_internal.h"
#include "storage/aio_subsys.h"
#include "storage/io_worker.h"
#include "storage/ipc.h"
#include "storage/latch.h"
#include "storage/lwlock.h"
#include "storage/pmsignal.h"
#include "storage/proc.h"
#include "storage/shmem.h"
#include "tcop/tcopprot.h"
#include "utils/injection_point.h"
#include "utils/memdebug.h"
#include "utils/ps_status.h"
#include "utils/wait_event.h"
 
/*
 * Saturation for counters used to estimate wakeup:IO ratio.
 *
 * We maintain hist_wakeups for wakeups received and hist_ios for IOs
 * processed by each worker.  When either counter reaches this saturation
 * value, we divide both by two.  The result is an exponentially decaying
 * ratio of wakeups to IOs, with a very short memory.
 *
 * If a worker is itself experiencing useless wakeups, it assumes that
 * higher-numbered workers would experience even more, so it should end the
 * chain.
 */
#define PGAIO_WORKER_WAKEUP_RATIO_SATURATE 4
 
/* Debugging support: show current IO and wakeups:ios statistics in ps. */
/* #define PGAIO_WORKER_SHOW_PS_INFO */
 
typedef struct PgAioWorkerSubmissionQueue
{
    uint32      size;
    uint32      head;
    uint32      tail;
    int         sqes[FLEXIBLE_ARRAY_MEMBER];
} PgAioWorkerSubmissionQueue;
 
typedef struct PgAioWorkerSlot
{
    ProcNumber  proc_number;
} PgAioWorkerSlot;
 
/*
 * Sets of worker IDs are held in a simple bitmap, accessed through functions
 * that provide a more readable abstraction.  If we wanted to support more
 * workers than that, the contention on the single queue would surely get too
 * high, so we might want to consider multiple pools instead of widening this.
 */
typedef uint64 PgAioWorkerSet;
 
#define PGAIO_WORKERSET_BITS (sizeof(PgAioWorkerSet) * CHAR_BIT)
 
static_assert(PGAIO_WORKERSET_BITS >= MAX_IO_WORKERS, "too small");
 
typedef struct PgAioWorkerControl
{
    /* Seen by postmaster */
    bool        grow;
    bool        grow_signal_sent;
 
    /* Protected by AioWorkerSubmissionQueueLock. */
    PgAioWorkerSet idle_workerset;
 
    /* Protected by AioWorkerControlLock. */
    PgAioWorkerSet workerset;
    int         nworkers;
 
    /* Protected by AioWorkerControlLock. */
    PgAioWorkerSlot workers[FLEXIBLE_ARRAY_MEMBER];
} PgAioWorkerControl;
 
 
static void pgaio_worker_shmem_request(void *arg);
static void pgaio_worker_shmem_init(void *arg);
 
static bool pgaio_worker_needs_synchronous_execution(PgAioHandle *ioh);
static int  pgaio_worker_submit(uint16 num_staged_ios, PgAioHandle **staged_ios);
 
 
const IoMethodOps pgaio_worker_ops = {
    .shmem_callbacks.request_fn = pgaio_worker_shmem_request,
    .shmem_callbacks.init_fn = pgaio_worker_shmem_init,
 
    .needs_synchronous_execution = pgaio_worker_needs_synchronous_execution,
    .submit = pgaio_worker_submit,
};
 
 
/* GUCs */
int         io_min_workers = 2;
int         io_max_workers = 8;
int         io_worker_idle_timeout = 60000;
int         io_worker_launch_interval = 100;
 
 
static int  io_worker_queue_size = 64;
static int  MyIoWorkerId = -1;
static PgAioWorkerSubmissionQueue *io_worker_submission_queue;
static PgAioWorkerControl *io_worker_control;
 
 
static void
pgaio_workerset_initialize(PgAioWorkerSet *set)
{
    *set = 0;
}
 
static bool
pgaio_workerset_is_empty(PgAioWorkerSet *set)
{
    return *set == 0;
}
 
static PgAioWorkerSet
pgaio_workerset_singleton(int worker)
{
    Assert(worker >= 0 && worker < MAX_IO_WORKERS);
    return UINT64_C(1) << worker;
}
 
static void
pgaio_workerset_all(PgAioWorkerSet *set)
{
    *set = UINT64_MAX >> (PGAIO_WORKERSET_BITS - MAX_IO_WORKERS);
}
 
static void
pgaio_workerset_subtract(PgAioWorkerSet *set1, const PgAioWorkerSet *set2)
{
    *set1 &= ~*set2;
}
 
static void
pgaio_workerset_insert(PgAioWorkerSet *set, int worker)
{
    Assert(worker >= 0 && worker < MAX_IO_WORKERS);
    *set |= pgaio_workerset_singleton(worker);
}
 
static void
pgaio_workerset_remove(PgAioWorkerSet *set, int worker)
{
    Assert(worker >= 0 && worker < MAX_IO_WORKERS);
    *set &= ~pgaio_workerset_singleton(worker);
}
 
static void
pgaio_workerset_remove_lte(PgAioWorkerSet *set, int worker)
{
    Assert(worker >= 0 && worker < MAX_IO_WORKERS);
    *set &= (~(PgAioWorkerSet) 0) << (worker + 1);
}
 
static int
pgaio_workerset_get_highest(PgAioWorkerSet *set)
{
    Assert(!pgaio_workerset_is_empty(set));
    return pg_leftmost_one_pos64(*set);
}
 
static int
pgaio_workerset_get_lowest(PgAioWorkerSet *set)
{
    Assert(!pgaio_workerset_is_empty(set));
    return pg_rightmost_one_pos64(*set);
}
 
static int
pgaio_workerset_pop_lowest(PgAioWorkerSet *set)
{
    int         worker = pgaio_workerset_get_lowest(set);
 
    pgaio_workerset_remove(set, worker);
    return worker;
}
 
#ifdef USE_ASSERT_CHECKING
static bool
pgaio_workerset_contains(PgAioWorkerSet *set, int worker)
{
    Assert(worker >= 0 && worker < MAX_IO_WORKERS);
    return (*set & pgaio_workerset_singleton(worker)) != 0;
}
 
static int
pgaio_workerset_count(PgAioWorkerSet *set)
{
    return pg_popcount64(*set);
}
#endif
 
static void
pgaio_worker_shmem_request(void *arg)
{
    size_t      size;
    int         queue_size;
 
    /* Round size up to next power of two so we can make a mask. */
    queue_size = pg_nextpower2_32(io_worker_queue_size);
 
    size = offsetof(PgAioWorkerSubmissionQueue, sqes) + sizeof(int) * queue_size;
    ShmemRequestStruct(.name = "AioWorkerSubmissionQueue",
                       .size = size,
                       .ptr = (void **) &io_worker_submission_queue,
        );
 
    size = offsetof(PgAioWorkerControl, workers) + sizeof(PgAioWorkerSlot) * MAX_IO_WORKERS;
    ShmemRequestStruct(.name = "AioWorkerControl",
                       .size = size,
                       .ptr = (void **) &io_worker_control,
        );
}
 
static void
pgaio_worker_shmem_init(void *arg)
{
    int         queue_size;
 
    /* Round size up like in pgaio_worker_shmem_request() */
    queue_size = pg_nextpower2_32(io_worker_queue_size);
 
    io_worker_submission_queue->size = queue_size;
    io_worker_submission_queue->head = 0;
    io_worker_submission_queue->tail = 0;
    io_worker_control->grow = false;
    pgaio_workerset_initialize(&io_worker_control->workerset);
    pgaio_workerset_initialize(&io_worker_control->idle_workerset);
 
    for (int i = 0; i < MAX_IO_WORKERS; ++i)
        io_worker_control->workers[i].proc_number = INVALID_PROC_NUMBER;
}
 
/*
 * Tell postmaster that we think a new worker is needed.
 */
static void
pgaio_worker_request_grow(void)
{
    /*
     * Suppress useless signaling if we already know that we're at the
     * maximum.  This uses an unlocked read of nworkers, but that's OK for
     * this heuristic purpose.
     */
    if (io_worker_control->nworkers >= io_max_workers)
        return;
 
    /* Already requested? */
    if (io_worker_control->grow)
        return;
 
    io_worker_control->grow = true;
    pg_memory_barrier();
 
    /*
     * If the postmaster has already been signaled, don't do it again until
     * the postmaster clears this flag.  There is no point in repeated signals
     * if grow is being set and cleared repeatedly while the postmaster is
     * waiting for io_worker_launch_interval, which it applies even to
     * canceled requests.
     */
    if (io_worker_control->grow_signal_sent)
        return;
 
    io_worker_control->grow_signal_sent = true;
    pg_memory_barrier();
    SendPostmasterSignal(PMSIGNAL_IO_WORKER_GROW);
}
 
/*
 * Cancel any request for a new worker, after observing an empty queue.
 */
static void
pgaio_worker_cancel_grow(void)
{
    if (!io_worker_control->grow)
        return;
 
    io_worker_control->grow = false;
    pg_memory_barrier();
}
 
/*
 * Called by the postmaster to check if a new worker has been requested (but
 * possibly canceled since).
 */
bool
pgaio_worker_pm_test_grow_signal_sent(void)
{
    pg_memory_barrier();
    return io_worker_control && io_worker_control->grow_signal_sent;
}
 
/*
 * Called by the postmaster to check if a new worker has been requested and
 * not canceled since.
 */
bool
pgaio_worker_pm_test_grow(void)
{
    pg_memory_barrier();
    return io_worker_control && io_worker_control->grow;
}
 
/*
 * Called by the postmaster to clear the request for a new worker.
 */
void
pgaio_worker_pm_clear_grow_signal_sent(void)
{
    if (!io_worker_control)
        return;
 
    io_worker_control->grow = false;
    io_worker_control->grow_signal_sent = false;
    pg_memory_barrier();
}
 
static int
pgaio_worker_choose_idle(int only_workers_above)
{
    PgAioWorkerSet workerset;
    int         worker;
 
    Assert(LWLockHeldByMeInMode(AioWorkerSubmissionQueueLock, LW_EXCLUSIVE));
 
    workerset = io_worker_control->idle_workerset;
    if (only_workers_above >= 0)
        pgaio_workerset_remove_lte(&workerset, only_workers_above);
    if (pgaio_workerset_is_empty(&workerset))
        return -1;
 
    /* Find the lowest numbered idle worker and mark it not idle. */
    worker = pgaio_workerset_get_lowest(&workerset);
    pgaio_workerset_remove(&io_worker_control->idle_workerset, worker);
 
    return worker;
}
 
/*
 * Try to wake a worker by setting its latch, to tell it there are IOs to
 * process in the submission queue.
 */
static void
pgaio_worker_wake(int worker)
{
    ProcNumber  proc_number;
 
    /*
     * If the selected worker is concurrently exiting, then pgaio_worker_die()
     * had not yet removed it as of when we saw it in idle_workerset.  That's
     * OK, because it will wake all remaining workers to close wakeup-vs-exit
     * races: *someone* will see the queued IO.  If there are no workers
     * running, the postmaster will start a new one.
     */
    proc_number = io_worker_control->workers[worker].proc_number;
    if (proc_number != INVALID_PROC_NUMBER)
        SetLatch(&GetPGProcByNumber(proc_number)->procLatch);
}
 
/*
 * Try to wake a set of workers.  Used on pool change, to close races
 * described in the callers.
 */
static void
pgaio_workerset_wake(PgAioWorkerSet workerset)
{
    while (!pgaio_workerset_is_empty(&workerset))
        pgaio_worker_wake(pgaio_workerset_pop_lowest(&workerset));
}
 
static bool
pgaio_worker_submission_queue_insert(PgAioHandle *ioh)
{
    PgAioWorkerSubmissionQueue *queue;
    uint32      new_head;
 
    Assert(LWLockHeldByMeInMode(AioWorkerSubmissionQueueLock, LW_EXCLUSIVE));
 
    queue = io_worker_submission_queue;
    new_head = (queue->head + 1) & (queue->size - 1);
    if (new_head == queue->tail)
    {
        pgaio_debug(DEBUG3, "io queue is full, at %u elements",
                    io_worker_submission_queue->size);
        return false;           /* full */
    }
 
    queue->sqes[queue->head] = pgaio_io_get_id(ioh);
    queue->head = new_head;
 
    return true;
}
 
static int
pgaio_worker_submission_queue_consume(void)
{
    PgAioWorkerSubmissionQueue *queue;
    int         result;
 
    Assert(LWLockHeldByMeInMode(AioWorkerSubmissionQueueLock, LW_EXCLUSIVE));
 
    queue = io_worker_submission_queue;
    if (queue->tail == queue->head)
        return -1;              /* empty */
 
    result = queue->sqes[queue->tail];
    queue->tail = (queue->tail + 1) & (queue->size - 1);
 
    return result;
}
 
static uint32
pgaio_worker_submission_queue_depth(void)
{
    uint32      head;
    uint32      tail;
 
    Assert(LWLockHeldByMeInMode(AioWorkerSubmissionQueueLock, LW_EXCLUSIVE));
 
    head = io_worker_submission_queue->head;
    tail = io_worker_submission_queue->tail;
 
    if (tail > head)
        head += io_worker_submission_queue->size;
 
    Assert(head >= tail);
 
    return head - tail;
}
 
static bool
pgaio_worker_needs_synchronous_execution(PgAioHandle *ioh)
{
    return
        !IsUnderPostmaster
        || ioh->flags & PGAIO_HF_REFERENCES_LOCAL
        || !pgaio_io_can_reopen(ioh);
}
 
static int
pgaio_worker_submit(uint16 num_staged_ios, PgAioHandle **staged_ios)
{
    PgAioHandle **synchronous_ios = NULL;
    int         nsync = 0;
    int         worker = -1;
 
    Assert(num_staged_ios <= PGAIO_SUBMIT_BATCH_SIZE);
 
    for (int i = 0; i < num_staged_ios; i++)
        pgaio_io_prepare_submit(staged_ios[i]);
 
    if (LWLockConditionalAcquire(AioWorkerSubmissionQueueLock, LW_EXCLUSIVE))
    {
        for (int i = 0; i < num_staged_ios; ++i)
        {
            Assert(!pgaio_worker_needs_synchronous_execution(staged_ios[i]));
            if (!pgaio_worker_submission_queue_insert(staged_ios[i]))
            {
                /*
                 * Do the rest synchronously. If the queue is full, give up
                 * and do the rest synchronously. We're holding an exclusive
                 * lock on the queue so nothing can consume entries.
                 */
                synchronous_ios = &staged_ios[i];
                nsync = (num_staged_ios - i);
 
                break;
            }
        }
        /* Choose one worker to wake for this batch. */
        worker = pgaio_worker_choose_idle(-1);
        LWLockRelease(AioWorkerSubmissionQueueLock);
 
        /* Wake up chosen worker.  It will wake peers if necessary. */
        if (worker != -1)
            pgaio_worker_wake(worker);
    }
    else
    {
        /* do everything synchronously, no wakeup needed */
        synchronous_ios = staged_ios;
        nsync = num_staged_ios;
    }
 
    /* Run whatever is left synchronously. */
    if (nsync > 0)
    {
        for (int i = 0; i < nsync; ++i)
        {
            pgaio_io_perform_synchronously(synchronous_ios[i]);
        }
    }
 
    return num_staged_ios;
}
 
/*
 * on_shmem_exit() callback that releases the worker's slot in
 * io_worker_control.
 */
static void
pgaio_worker_die(int code, Datum arg)
{
    PgAioWorkerSet notify_set;
 
    LWLockAcquire(AioWorkerSubmissionQueueLock, LW_EXCLUSIVE);
    pgaio_workerset_remove(&io_worker_control->idle_workerset, MyIoWorkerId);
    LWLockRelease(AioWorkerSubmissionQueueLock);
 
    LWLockAcquire(AioWorkerControlLock, LW_EXCLUSIVE);
    Assert(io_worker_control->workers[MyIoWorkerId].proc_number == MyProcNumber);
    io_worker_control->workers[MyIoWorkerId].proc_number = INVALID_PROC_NUMBER;
    Assert(pgaio_workerset_contains(&io_worker_control->workerset, MyIoWorkerId));
    pgaio_workerset_remove(&io_worker_control->workerset, MyIoWorkerId);
    notify_set = io_worker_control->workerset;
    Assert(io_worker_control->nworkers > 0);
    io_worker_control->nworkers--;
    Assert(pgaio_workerset_count(&io_worker_control->workerset) ==
           io_worker_control->nworkers);
    LWLockRelease(AioWorkerControlLock);
 
    /*
     * Notify other workers on pool change.  This allows the new highest
     * worker to know that it is now the one that can time out, and closes a
     * wakeup-loss race described in pgaio_worker_wake().
     */
    pgaio_workerset_wake(notify_set);
}
 
/*
 * Register the worker in shared memory, assign MyIoWorkerId and register a
 * shutdown callback to release registration.
 */
static void
pgaio_worker_register(void)
{
    PgAioWorkerSet free_workerset;
    PgAioWorkerSet old_workerset;
 
    MyIoWorkerId = -1;
 
    LWLockAcquire(AioWorkerControlLock, LW_EXCLUSIVE);
    /* Find lowest unused worker ID. */
    pgaio_workerset_all(&free_workerset);
    pgaio_workerset_subtract(&free_workerset, &io_worker_control->workerset);
    if (!pgaio_workerset_is_empty(&free_workerset))
        MyIoWorkerId = pgaio_workerset_get_lowest(&free_workerset);
    if (MyIoWorkerId == -1)
        elog(ERROR, "couldn't find a free worker ID");
 
    Assert(io_worker_control->workers[MyIoWorkerId].proc_number ==
           INVALID_PROC_NUMBER);
    io_worker_control->workers[MyIoWorkerId].proc_number = MyProcNumber;
 
    old_workerset = io_worker_control->workerset;
    Assert(!pgaio_workerset_contains(&old_workerset, MyIoWorkerId));
    pgaio_workerset_insert(&io_worker_control->workerset, MyIoWorkerId);
    io_worker_control->nworkers++;
    Assert(io_worker_control->nworkers <= MAX_IO_WORKERS);
    Assert(pgaio_workerset_count(&io_worker_control->workerset) ==
           io_worker_control->nworkers);
    LWLockRelease(AioWorkerControlLock);
 
    /*
     * Notify other workers on pool change.  If we were the highest worker,
     * this allows the new highest worker to know that it can time out.
     */
    pgaio_workerset_wake(old_workerset);
 
    on_shmem_exit(pgaio_worker_die, 0);
}
 
static void
pgaio_worker_error_callback(void *arg)
{
    ProcNumber  owner;
    PGPROC     *owner_proc;
    int32       owner_pid;
    PgAioHandle *ioh = arg;
 
    if (!ioh)
        return;
 
    Assert(ioh->owner_procno != MyProcNumber);
    Assert(MyBackendType == B_IO_WORKER);
 
    owner = ioh->owner_procno;
    owner_proc = GetPGProcByNumber(owner);
    owner_pid = owner_proc->pid;
 
    errcontext("I/O worker executing I/O on behalf of process %d", owner_pid);
}
 
/*
 * Check if this backend is allowed to time out, and thus should use a
 * non-infinite sleep time.  Only the highest-numbered worker is allowed to
 * time out, and only if the pool is above io_min_workers.  Serializing
 * timeouts keeps IDs in a range 0..N without gaps, and avoids undershooting
 * io_min_workers.
 *
 * The result is only instantaneously true and may be temporarily inconsistent
 * in different workers around transitions, but all workers are woken up on
 * pool size or GUC changes making the result eventually consistent.
 */
static bool
pgaio_worker_can_timeout(void)
{
    PgAioWorkerSet workerset;
 
    if (MyIoWorkerId < io_min_workers)
        return false;
 
    /* Serialize against pool size changes. */
    LWLockAcquire(AioWorkerControlLock, LW_SHARED);
    workerset = io_worker_control->workerset;
    LWLockRelease(AioWorkerControlLock);
 
    if (MyIoWorkerId != pgaio_workerset_get_highest(&workerset))
        return false;
 
    return true;
}
 
void
IoWorkerMain(const void *startup_data, size_t startup_data_len)
{
    sigjmp_buf  local_sigjmp_buf;
    TimestampTz idle_timeout_abs = 0;
    int         timeout_guc_used = 0;
    PgAioHandle *volatile error_ioh = NULL;
    ErrorContextCallback errcallback = {0};
    volatile int error_errno = 0;
    char        cmd[128];
    int         hist_ios = 0;
    int         hist_wakeups = 0;
 
    AuxiliaryProcessMainCommon();
 
    pqsignal(SIGHUP, SignalHandlerForConfigReload);
    pqsignal(SIGINT, die);      /* to allow manually triggering worker restart */
 
    /*
     * Ignore SIGTERM, will get explicit shutdown via SIGUSR2 later in the
     * shutdown sequence, similar to checkpointer.
     */
    pqsignal(SIGTERM, SIG_IGN);
    /* SIGQUIT handler was already set up by InitPostmasterChild */
    pqsignal(SIGALRM, SIG_IGN);
    pqsignal(SIGPIPE, SIG_IGN);
    pqsignal(SIGUSR1, procsignal_sigusr1_handler);
    pqsignal(SIGUSR2, SignalHandlerForShutdownRequest);
 
    /* also registers a shutdown callback to unregister */
    pgaio_worker_register();
 
    sprintf(cmd, "%d", MyIoWorkerId);
    set_ps_display(cmd);
 
    errcallback.callback = pgaio_worker_error_callback;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* see PostgresMain() */
    if (sigsetjmp(local_sigjmp_buf, 1) != 0)
    {
        error_context_stack = NULL;
        HOLD_INTERRUPTS();
 
        EmitErrorReport();
 
        /*
         * In the - very unlikely - case that the IO failed in a way that
         * raises an error we need to mark the IO as failed.
         *
         * Need to do just enough error recovery so that we can mark the IO as
         * failed and then exit (postmaster will start a new worker).
         */
        LWLockReleaseAll();
 
        if (error_ioh != NULL)
        {
            /* should never fail without setting error_errno */
            Assert(error_errno != 0);
 
            errno = error_errno;
 
            START_CRIT_SECTION();
            pgaio_io_process_completion(error_ioh, -error_errno);
            END_CRIT_SECTION();
        }
 
        proc_exit(1);
    }
 
    /* We can now handle ereport(ERROR) */
    PG_exception_stack = &local_sigjmp_buf;
 
    sigprocmask(SIG_SETMASK, &UnBlockSig, NULL);
 
    while (!ShutdownRequestPending)
    {
        uint32      io_index;
        int         worker = -1;
        int         queue_depth = 0;
        bool        maybe_grow = false;
 
        /*
         * Try to get a job to do.
         *
         * The lwlock acquisition also provides the necessary memory barrier
         * to ensure that we don't see an outdated data in the handle.
         */
        LWLockAcquire(AioWorkerSubmissionQueueLock, LW_EXCLUSIVE);
        if ((io_index = pgaio_worker_submission_queue_consume()) == -1)
        {
            /* Nothing to do.  Mark self idle. */
            pgaio_workerset_insert(&io_worker_control->idle_workerset,
                                   MyIoWorkerId);
        }
        else
        {
            /* Got one.  Clear idle flag. */
            pgaio_workerset_remove(&io_worker_control->idle_workerset,
                                   MyIoWorkerId);
 
            /*
             * See if we should wake up a higher numbered peer.  Only do that
             * if this worker is not receiving spurious wakeups itself.  The
             * intention is create a frontier beyond which idle workers stay
             * asleep.
             *
             * This heuristic tries to discover the useful wakeup propagation
             * chain length when IOs are very fast and workers wake up to find
             * that all IOs have already been taken.
             *
             * If we chose not to wake a worker when we ideally should have,
             * then the ratio will soon change to correct that.
             */
            if (hist_wakeups <= hist_ios)
            {
                queue_depth = pgaio_worker_submission_queue_depth();
                if (queue_depth > 0)
                {
                    /* Choose a worker higher than me to wake. */
                    worker = pgaio_worker_choose_idle(MyIoWorkerId);
                    if (worker == -1)
                        maybe_grow = true;
                }
            }
        }
        LWLockRelease(AioWorkerSubmissionQueueLock);
 
        /* Propagate wakeups. */
        if (worker != -1)
        {
            pgaio_worker_wake(worker);
        }
        else if (maybe_grow)
        {
            /*
             * We know there was at least one more item in the queue, and we
             * failed to find a higher-numbered idle worker to wake.  Now we
             * decide if we should try to start one more worker.
             *
             * We do this with a simple heuristic: is the queue depth greater
             * than the current number of workers?
             *
             * Consider the following situations:
             *
             * 1. The queue depth is constantly increasing, because IOs are
             * arriving faster than they can possibly be serviced.  It doesn't
             * matter much which threshold we choose, as we will surely hit
             * it.  Crossing the current worker count is a useful signal
             * because it's clearly too deep to avoid queuing latency already,
             * but still leaves a small window of opportunity to improve the
             * situation before the queue oveflows.
             *
             * 2. The worker pool is keeping up, no latency is being
             * introduced and an extra worker would be a waste of resources.
             * Queue depth distributions tend to be heavily skewed, with long
             * tails of low probability spikes (due to submission clustering,
             * scheduling, jitter, stalls, noisy neighbors, etc).  We want a
             * number that is very unlikely to be triggered by an outlier, and
             * we bet that an exponential or similar distribution whose
             * outliers never reach this threshold must be almost entirely
             * concentrated at the low end.  If we do see a spike as big as
             * the worker count, we take it as a signal that the distribution
             * is surely too wide.
             *
             * On its own, this is an extremely crude signal.  When combined
             * with the wakeup propagation test that precedes it (but on its
             * own tends to overshoot) and io_worker_launch_delay, the result
             * is that we gradually test each pool size until we find one that
             * doesn't trigger further expansion, and then hold it for at
             * least io_worker_idle_timeout.
             *
             * XXX Perhaps ideas from queueing theory or control theory could
             * do a better job of this.
             */
 
            /* Read nworkers without lock for this heuristic purpose. */
            if (queue_depth > io_worker_control->nworkers)
                pgaio_worker_request_grow();
        }
 
        if (io_index != -1)
        {
            PgAioHandle *ioh = NULL;
 
            /* Cancel timeout and update wakeup:work ratio. */
            idle_timeout_abs = 0;
            if (++hist_ios == PGAIO_WORKER_WAKEUP_RATIO_SATURATE)
            {
                hist_wakeups /= 2;
                hist_ios /= 2;
            }
 
            ioh = &pgaio_ctl->io_handles[io_index];
            error_ioh = ioh;
            errcallback.arg = ioh;
 
            pgaio_debug_io(DEBUG4, ioh,
                           "worker %d processing IO",
                           MyIoWorkerId);
 
            /*
             * Prevent interrupts between pgaio_io_reopen() and
             * pgaio_io_perform_synchronously() that otherwise could lead to
             * the FD getting closed in that window.
             */
            HOLD_INTERRUPTS();
 
            /*
             * It's very unlikely, but possible, that reopen fails. E.g. due
             * to memory allocations failing or file permissions changing or
             * such.  In that case we need to fail the IO.
             *
             * There's not really a good errno we can report here.
             */
            error_errno = ENOENT;
            pgaio_io_reopen(ioh);
 
            /*
             * To be able to exercise the reopen-fails path, allow injection
             * points to trigger a failure at this point.
             */
            INJECTION_POINT("aio-worker-after-reopen", ioh);
 
            error_errno = 0;
            error_ioh = NULL;
 
            /*
             * As part of IO completion the buffer will be marked as NOACCESS,
             * until the buffer is pinned again - which never happens in io
             * workers. Therefore the next time there is IO for the same
             * buffer, the memory will be considered inaccessible. To avoid
             * that, explicitly allow access to the memory before reading data
             * into it.
             */
#ifdef USE_VALGRIND
            {
                struct iovec *iov;
                uint16      iov_length = pgaio_io_get_iovec_length(ioh, &iov);
 
                for (int i = 0; i < iov_length; i++)
                    VALGRIND_MAKE_MEM_UNDEFINED(iov[i].iov_base, iov[i].iov_len);
            }
#endif
 
#ifdef PGAIO_WORKER_SHOW_PS_INFO
            {
                char       *description = pgaio_io_get_target_description(ioh);
 
                sprintf(cmd, "%d: [%s] %s",
                        MyIoWorkerId,
                        pgaio_io_get_op_name(ioh),
                        description);
                pfree(description);
                set_ps_display(cmd);
            }
#endif
 
            /*
             * We don't expect this to ever fail with ERROR or FATAL, no need
             * to keep error_ioh set to the IO.
             * pgaio_io_perform_synchronously() contains a critical section to
             * ensure we don't accidentally fail.
             */
            pgaio_io_perform_synchronously(ioh);
 
            RESUME_INTERRUPTS();
            errcallback.arg = NULL;
        }
        else
        {
            int         timeout_ms;
 
            /* Cancel new worker request if pending. */
            pgaio_worker_cancel_grow();
 
            /* Compute the remaining allowed idle time. */
            if (io_worker_idle_timeout == -1)
            {
                /* Never time out. */
                timeout_ms = -1;
            }
            else
            {
                TimestampTz now = GetCurrentTimestamp();
 
                /* If the GUC changes, reset timer. */
                if (idle_timeout_abs != 0 &&
                    io_worker_idle_timeout != timeout_guc_used)
                    idle_timeout_abs = 0;
 
                /* Only the highest-numbered worker can time out. */
                if (pgaio_worker_can_timeout())
                {
                    if (idle_timeout_abs == 0)
                    {
                        /*
                         * I have just been promoted to the timeout worker, or
                         * the GUC changed.  Compute new absolute time from
                         * now.
                         */
                        idle_timeout_abs =
                            TimestampTzPlusMilliseconds(now,
                                                        io_worker_idle_timeout);
                        timeout_guc_used = io_worker_idle_timeout;
                    }
                    timeout_ms =
                        TimestampDifferenceMilliseconds(now, idle_timeout_abs);
                }
                else
                {
                    /* No timeout for me. */
                    idle_timeout_abs = 0;
                    timeout_ms = -1;
                }
            }
 
#ifdef PGAIO_WORKER_SHOW_PS_INFO
            sprintf(cmd, "%d: idle, wakeups:ios = %d:%d",
                    MyIoWorkerId, hist_wakeups, hist_ios);
            set_ps_display(cmd);
#endif
 
            if (WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH | WL_TIMEOUT,
                          timeout_ms,
                          WAIT_EVENT_IO_WORKER_MAIN) == WL_TIMEOUT)
            {
                /* WL_TIMEOUT */
                if (pgaio_worker_can_timeout())
                    if (GetCurrentTimestamp() >= idle_timeout_abs)
                        break;
            }
            else
            {
                /* WL_LATCH_SET */
                if (++hist_wakeups == PGAIO_WORKER_WAKEUP_RATIO_SATURATE)
                {
                    hist_wakeups /= 2;
                    hist_ios /= 2;
                }
            }
            ResetLatch(MyLatch);
        }
 
        CHECK_FOR_INTERRUPTS();
 
        if (ConfigReloadPending)
        {
            ConfigReloadPending = false;
            ProcessConfigFile(PGC_SIGHUP);
 
            /* If io_max_workers has been decreased, exit highest first. */
            if (MyIoWorkerId >= io_max_workers)
                break;
        }
    }
 
    error_context_stack = errcallback.previous;
    proc_exit(0);
}
 
bool
pgaio_workers_enabled(void)
{
    return io_method == IOMETHOD_WORKER;
}