MDEV-24270: Collect multiple completed events at a time

tpool::aio::N_PENDING: Replaces OS_AIO_N_PENDING_IOS_PER_THREAD.
This limits two similar things: the number of outstanding requests
that a thread may io_submit(), and the number of completed requests
collected at a time by io_getevents().

storage/innobase/buf/buf0buf.cc

@@ -1467,7 +1467,7 @@ bool buf_pool_t::create()
   ut_d(lru_scan_itr.m_mutex= &mutex);
 
   io_buf.create((srv_n_read_io_threads + srv_n_write_io_threads) *
-                OS_AIO_N_PENDING_IOS_PER_THREAD);
+                tpool::aio::N_PENDING);
 
   /* FIXME: remove some of these variables */
   srv_buf_pool_curr_size= curr_pool_size;

storage/innobase/include/os0file.h

@@ -265,9 +265,6 @@ struct os_file_size_t {
 	os_offset_t	m_alloc_size;
 };
 
-/** Win NT does not allow more than 64 */
-static const ulint OS_AIO_N_PENDING_IOS_PER_THREAD = 256;
-
 extern ulint	os_n_file_reads;
 extern ulint	os_n_file_writes;
 extern ulint	os_n_fsyncs;

storage/innobase/os/os0file.cc

@@ -4027,8 +4027,8 @@ static bool is_linux_native_aio_supported()
 
 bool os_aio_init(ulint n_reader_threads, ulint n_writer_threads, ulint)
 {
-  int max_write_events= int(n_writer_threads * OS_AIO_N_PENDING_IOS_PER_THREAD);
-  int max_read_events= int(n_reader_threads * OS_AIO_N_PENDING_IOS_PER_THREAD);
+  int max_write_events= int(n_writer_threads * tpool::aio::N_PENDING);
+  int max_read_events= int(n_reader_threads * tpool::aio::N_PENDING);
   int max_events = max_read_events + max_write_events;
 	int ret;
 

tpool/aio_linux.cc

@@ -60,27 +60,26 @@ class aio_linux final : public aio
 
   static void getevent_thread_routine(aio_linux *aio)
   {
-    io_event events[1];
+    io_event events[N_PENDING];
     for (;;)
     {
-      switch (int ret= my_getevents(aio->m_io_ctx, 1, 1, events)) {
+      switch (int ret= my_getevents(aio->m_io_ctx, 1, N_PENDING, events)) {
       case -EINTR:
-      case 0:
         continue;
       case -EINVAL:
         if (shutdown_in_progress)
           return;
         /* fall through */
       default:
-        if (ret != 1)
+        if (ret < 0)
         {
           fprintf(stderr, "io_getevents returned %d\n", ret);
           abort();
           return;
         }
-        else
+        for (int i= 0; i < ret; i++)
         {
-          const io_event &event= events[0];
+          const io_event &event= events[i];
           aiocb *iocb= static_cast<aiocb*>(event.obj);
           if (static_cast<int>(event.res) < 0)
           {

tpool/tpool.h

@@ -155,6 +155,8 @@ struct aiocb
 class aio
 {
 public:
+  /** Maximum number of pending requests per thread */
+  static constexpr unsigned N_PENDING= 256;
   /**
     Submit asyncronous IO.
     On completion, cb->m_callback is executed.