数据库内核月报 - 2017 / 07

MySQL · 源码分析 · InnoDB 异步IO工作流程

之前的一篇内核月报InnoDB IO子系统 中介绍了InnoDB IO子系统中包含的同步IO以及异步IO。本篇文章将从源码层面剖析一下InnoDB IO子系统中,数据页的同步IO以及异步IO请求的具体实现过程。

在MySQL5.6中,InnoDB的异步IO主要是用来处理预读以及对数据文件的写请求的。而对于正常的页面数据读取则是通过同步IO进行的。到底二者在代码层面上的实现过程有什么样的区别? 接下来我们将以Linux native io的执行过程为主线,对IO请求的执行过程进行梳理。

重点数据结构

  • os_aio_array_t
/** 用来记录某一类(ibuf,log,read,write)异步IO(aio)请求的数组类型。每一个异步IO请求都会在类型对应的数组中注册一个innodb
  aio对象。*/

os_aio_array_t  {
	
  os_ib_mutex_t mutex;  // 主要用来控制异步read/write线程的并发操作。对于ibuf,log类型,由于只有一个线程,所以不存在并发操作问题
  os_event_t  not_full; // 一个条件变量event,用来通知等待获取slot的线程是否os_aio_array_t数组有空闲的slot供aio请求

  os_event_t  is_empty; // 条件变量event,用来通知IO线程os_aio_array_t数组是否有pening的IO请求。
  
  ulint   n_slots; // 数组容纳的IO请求数。= 线程数 * 每个segment允许pending的请求数(256)
  
  ulint   n_segments; // 允许独立wait的segment数,即某种类型的IO的允许最大线程数

  ulint   cur_seg; /* IO请求会按照round robin的方式分配到不同的segment中,该变量指示下一个IO请求可以分配的segment */
 ulint   n_reserved; // 已经Pending的IO请求数

  os_aio_slot_t*  slots; // 用来记录具体的每个IO请求对象的数组,也即n_segments 个线程共用n_slots个槽位来存放pending io请求  

  \#ifdef __WIN__

  HANDLE*   handles;
        /*!< Pointer to an array of OS native
        event handles where we copied the
        handles from slots, in the same
        order. This can be used in
        WaitForMultipleObjects; used only in
		Windows */

 \#endif __WIN__

 \#if defined(LINUX_NATIVE_AIO)

  io_context_t*   aio_ctx; // aio上下文的数组,每个segment拥有独立的一个aio上下文数组,用来记录以及完成的IO请求上下文

  struct io_event*  aio_events; // 该数组用来记录已经完成的IO请求事件。异步IO通过设置事件通知IO线程处理完成的IO请求

  struct iocb**  pending; // 用来记录pending的aio请求

  ulint*         count; // 该数组记录了每个segment对应的pending aio请求数量

 \#endif /* LINUX_NATIV_AIO */

 }
  • os_aio_slot_t
// os_aio_array_t数组中用来记录一个异步IO(aio)请求的对象
 os_aio_slot_t {

  ibool   is_read;  /*!< TRUE if a read operation */

  ulint   pos;    // os_aio_array_t数组中所在的位置 

  ibool   reserved; // TRUE表示该Slot已经被别的IO请求占用了

  time_t    reservation_time; // 占用的时间

  ulint   len;    // io请求的长度

  byte*   buf;    // 数据读取或者需要写入的buffer,通常指向buffer pool的一个页面,压缩页面有特殊处理

  ulint   type;   /* 请求类型,即读还是写IO请求 */ 

  os_offset_t offset;   /*!< file offset in bytes */

  os_file_t file;   /*!< file where to read or write */

  const char* name;   /*!< 需要读取的文件及路径信息 */

  ibool   io_already_done; /* TRUE表示IO已经完成了

  fil_node_t* message1; /* 该aio操作的innodb文件描述符(f_node_t)*/

  void*   message2; /* 用来记录完成IO请求所对应的具体buffer pool bpage页 */

 \#ifdef WIN_ASYNC_IO

  HANDLE    handle;   /*!< handle object we need in the
          OVERLAPPED struct */

  OVERLAPPED  control;  /*!< Windows control block for the
          aio request */

  \#elif defined(LINUX_NATIVE_AIO)

  struct iocb control;  /* 该slot使用的aio请求控制块iocb */

  int   n_bytes;  /* 读写bytes */

  int   ret;    /* AIO return code */

  \#endif /* WIN_ASYNC_IO */

}

流程图

flow-aio.png

源码分析

  • 物理数据页操作入口函数os_aio_func
ibool
os_aio_func(
/*========*/
  ulint   type, /* IO类型,READ还是WRITE IO */
  ulint   mode, /* 这里表示是否使用SIMULATED aio执行异步IO请求 */
  const char* name, /* IO需要打开的tablespace路径+名称 */
  os_file_t file, /* IO操作的文件 */
  void*   buf,  // 数据读取或者需要写入的buffer,通常指向buffer pool的一个页面,压缩页面有特殊处理
  os_offset_t offset, /*!< in: file offset where to read or write */
  ulint   n,  /* 读取或写入字节数 */
  fil_node_t* message1, /* 该aio操作的innodb文件描述符(f_node_t),只对异步IO起作用 */ 
  void*   message2, /* 用来记录完成IO请求所对应的具体buffer pool bpage页,只对异步IO起作用 */
  ibool   should_buffer, // 是否需要缓存aio请求,该变量主要对预读起作用
  ibool   page_encrypt,
        /*!< in: Whether to encrypt */
  ulint   page_size)
       /*!< in: Page size */
{
...

  wake_later = mode & OS_AIO_SIMULATED_WAKE_LATER;
  mode = mode & (~OS_AIO_SIMULATED_WAKE_LATER);

  if (mode == OS_AIO_SYNC
#ifdef WIN_ASYNC_IO
      && !srv_use_native_aio
#endif /* WIN_ASYNC_IO */
      ) {
    /* This is actually an ordinary synchronous read or write:
    no need to use an i/o-handler thread. NOTE that if we use
    Windows async i/o, Windows does not allow us to use
    ordinary synchronous os_file_read etc. on the same file,
    therefore we have built a special mechanism for synchronous
    wait in the Windows case.
    Also note that the Performance Schema instrumentation has
    been performed by current os_aio_func()'s wrapper function
    pfs_os_aio_func(). So we would no longer need to call
    Performance Schema instrumented os_file_read() and
    os_file_write(). Instead, we should use os_file_read_func()
    and os_file_write_func() */

	/* 这里如果是同步IO,并且native io没有开启的情况下,直接使用os_file_read/write函数进行读取,
       不需要经过IO线程进行处理 */

    if (type == OS_FILE_READ) {
      if (page_encrypt) {
        return(os_file_read_decrypt_page(file, buf, offset, n, page_size));
      } else {
        return(os_file_read_func(file, buf, offset, n));
      }
    }
    ut_ad(!srv_read_only_mode);
    ut_a(type == OS_FILE_WRITE);
    if (page_encrypt) {
      return(os_file_write_encrypt_page(name, file, buf, offset, n, page_size));
    } else {
      return(os_file_write_func(name, file, buf, offset, n));
    }
  }
try_again:
  switch (mode) {
	// 根据访问类型,定位IO请求数组
  case OS_AIO_NORMAL:
    if (type == OS_FILE_READ) {
      array = os_aio_read_array;
    } else {
      ut_ad(!srv_read_only_mode);
      array = os_aio_write_array;
    }
    break;
  case OS_AIO_IBUF:
    ut_ad(type == OS_FILE_READ);
    /* Reduce probability of deadlock bugs in connection with ibuf:
    do not let the ibuf i/o handler sleep */

    wake_later = FALSE;

    if (srv_read_only_mode) {
      array = os_aio_read_array;
   }
    break;
  case OS_AIO_LOG:
    if (srv_read_only_mode) {
      array = os_aio_read_array;
    } else {
      array = os_aio_log_array;
    }
    break;
  case OS_AIO_SYNC:
    array = os_aio_sync_array;
#if defined(LINUX_NATIVE_AIO)
    /* In Linux native AIO we don't use sync IO array. */
    ut_a(!srv_use_native_aio);
#endif /* LINUX_NATIVE_AIO */
    break;
  default:
    ut_error;
    array = NULL; /* Eliminate compiler warning */
  }
  // 阻塞为当前IO请求申请一个用来执行异步IO的slot
  slot = os_aio_array_reserve_slot(type, array, message1, message2, file,
           name, buf, offset, n, page_encrypt, page_size);

        DBUG_EXECUTE_IF("simulate_slow_aio",
                        {
                          os_thread_sleep(1000000);
                        }
                        );
  if (type == OS_FILE_READ) {
    if (srv_use_native_aio) {
      os_n_file_reads++;
      os_bytes_read_since_printout += n;
#ifdef WIN_ASYNC_IO
	 // 这里是Windows用来处理异步IO读请求
      ret = ReadFile(file, buf, (DWORD) n, &len,
               &(slot->control));

#elif defined(LINUX_NATIVE_AIO)
	  // 这里是Linux来处理native io
      if (!os_aio_linux_dispatch(array, slot, should_buffer)) {
        goto err_exit;
#endif /* WIN_ASYNC_IO */
    } else {
      if (!wake_later) {
		// 唤醒simulated aio处理线程
        os_aio_simulated_wake_handler_thread(
          os_aio_get_segment_no_from_slot(
            array, slot));
      }
    }
  } else if (type == OS_FILE_WRITE) {
    ut_ad(!srv_read_only_mode);
    if (srv_use_native_aio) {
      os_n_file_writes++;
#ifdef WIN_ASYNC_IO
	  // 这里是Windows用来处理异步IO写请求
      ret = WriteFile(file, buf, (DWORD) n, &len,
          &(slot->control));

#elif defined(LINUX_NATIVE_AIO)
	  // 这里是Linux来处理native io
      if (!os_aio_linux_dispatch(array, slot, false)) {
        goto err_exit;
      }
#endif /* WIN_ASYNC_IO */
    } else {
      if (!wake_later) {
		// 唤醒simulated aio处理线程
        os_aio_simulated_wake_handler_thread(
          os_aio_get_segment_no_from_slot(
            array, slot));
      }
    }
  } else {
    ut_error;
  }

...
}

  • 负责通知Linux内核执行native IO请求的函数os_aio_linux_dispatch
static
ibool
os_aio_linux_dispatch(
/*==================*/
  os_aio_array_t* array,  /* IO请求函数 */
  os_aio_slot_t*  slot, /* 申请好的slot */
        ibool           should_buffer)  // 是否需要缓存aio 请求,该变量主要对预读起作用
{
	...

  /* Find out what we are going to work with.
  The iocb struct is directly in the slot.
  The io_context is one per segment. */

  // 每个segment包含的slot个数,Linux下每个segment包含256个slot
  slots_per_segment = array->n_slots / array->n_segments;
  iocb = &slot->control;
  io_ctx_index = slot->pos / slots_per_segment;
  if (should_buffer) {
  	/* 这里也可以看到aio请求缓存只对读请求起作用 */
  	ut_ad(array == os_aio_read_array);
  
    ulint n;
    ulint count;
    os_mutex_enter(array->mutex);
    /* There are array->n_slots elements in array->pending, which is divided into
     * array->n_segments area of equal size.  The iocb of each segment are 
     * buffered in its corresponding area in the pending array consecutively as
     * they come.  array->count[i] records the number of buffered aio requests in
     * the ith segment.*/
  	 n = io_ctx_index * slots_per_segment
      + array->count[io_ctx_index];
      array->pending[n] = iocb;
      array->count[io_ctx_index] ++; 
      count = array->count[io_ctx_index];
      os_mutex_exit(array->mutex);
	  // 如果当前segment的slot都已经被占用了,就需要提交一次异步aio请求
      if (count == slots_per_segment) {
            os_aio_linux_dispatch_read_array_submit(); //no cover line
      }   
	  // 否则就直接返回
  	  return (TRUE);                  
   } 
	// 直接提交IO请求到内核
  ret = io_submit(array->aio_ctx[io_ctx_index], 1, &iocb);
  ...
}
  • IO线程负责监控aio请求的主函数fil_aio_wait
void
fil_aio_wait(
/*=========*/
  ulint segment)  /*!< in: the number of the segment in the aio
        array to wait for */
{
  ibool   ret; 
  fil_node_t* fil_node;
  void*   message;
  ulint   type;

  ut_ad(fil_validate_skip());

  if (srv_use_native_aio) { // 使用native io
    srv_set_io_thread_op_info(segment, "native aio handle");
#ifdef WIN_ASYNC_IO
    ret = os_aio_windows_handle( // Window监控入口
      segment, 0, &fil_node, &message, &type);
#elif defined(LINUX_NATIVE_AIO)
    ret = os_aio_linux_handle( // Linux native io监控入口
	  segment, &fil_node, &message, &type);
#else
    ut_error;
    ret = 0; /* Eliminate compiler warning */
#endif /* WIN_ASYNC_IO */
  } else {
    srv_set_io_thread_op_info(segment, "simulated aio handle");

    ret = os_aio_simulated_handle( // Simulated aio监控入口
      segment, &fil_node, &message, &type);
  }

  ut_a(ret);
  if (fil_node == NULL) {
    ut_ad(srv_shutdown_state == SRV_SHUTDOWN_EXIT_THREADS);
    return;
  }
  srv_set_io_thread_op_info(segment, "complete io for fil node");
  mutex_enter(&fil_system->mutex);

  // 到这里表示至少有一个IO请求已经完成,该函数设置状态信息
  fil_node_complete_io(fil_node, fil_system, type);

  mutex_exit(&fil_system->mutex);

  ut_ad(fil_validate_skip());

  /* Do the i/o handling */
  /* IMPORTANT: since i/o handling for reads will read also the insert
  buffer in tablespace 0, you have to be very careful not to introduce
  deadlocks in the i/o system. We keep tablespace 0 data files always
  open, and use a special i/o thread to serve insert buffer requests. */

  if (fil_node->space->purpose == FIL_TABLESPACE) { // 数据文件读写IO
    srv_set_io_thread_op_info(segment, "complete io for buf page");
    // IO请求完成后,这里处理buffer pool对应的bpage相关的一些状态信息并根据checksum验证数据的正确性
    buf_page_io_complete(static_cast<buf_page_t*>(message));
  } else { // 日志文件的读写IO
    srv_set_io_thread_op_info(segment, "complete io for log");
    log_io_complete(static_cast<log_group_t*>(message));
  }
}
#endif /* UNIV_HOTBACKUP */
  • IO线程负责处理native IO请求的函数os_aio_linux_handle
ibool
os_aio_linux_handle(ulint	global_seg, // 属于哪个segment
					fil_node_t**message1, /* 该aio操作的innodb文件描述符(f_node_t)*/
					void**	message2, /* 用来记录完成IO请求所对应的具体buffer pool bpage页 */
					ulint*	type){ // 读or写IO
	// 根据global_seg获得该aio 的os_aio_array_t数组,并返回对应的segment
	segment = os_aio_get_array_and_local_segment(&array, global_seg); 
	n = array->n_slots / array->n_segments; //获得一个线程可监控的io event数
	/* Loop until we have found a completed request. */
	for (;;) {
		ibool	any_reserved = FALSE;
		os_mutex_enter(array->mutex);
		for (i = 0; i < n; ++i) {  // 遍历该线程所发起的所有aio请求
			slot = os_aio_array_get_nth_slot(
				array, i + segment * n); 
			if (!slot->reserved) {  // 该slot是否被占用
				continue;
			} else if (slot->io_already_done) {  // IO请求已经完成,可以通知主线程返回数据了
				/* Something for us to work on. */
				goto found;
			} else {
				any_reserved = TRUE;
			}
		}
		os_mutex_exit(array->mutex);
       // 到这里说明没有找到一个完成的io,则再去collect
		os_aio_linux_collect(array, segment, n); 
found:   // 找到一个完成的io,将内容返回
	*message1 = slot->message1;  
	*message2 = slot->message2; // 返回完成IO所对应的bpage页
	*type = slot->type;
	if (slot->ret == 0 && slot->n_bytes == (long) slot->len) {
		if (slot->page_encrypt
        && slot->type == OS_FILE_READ) {
      	os_decrypt_page(slot->buf, slot->len, slot->page_size, FALSE);
    }    

    ret = TRUE;
  } else {
    errno = -slot->ret;
    /* os_file_handle_error does tell us if we should retry
    this IO. As it stands now, we don't do this retry when
    reaping requests from a different context than
    the dispatcher. This non-retry logic is the same for
    windows and linux native AIO.
    We should probably look into this to transparently
    re-submit the IO. */
    os_file_handle_error(slot->name, "Linux aio");

    ret = FALSE;
  }

  os_mutex_exit(array->mutex);

  os_aio_array_free_slot(array, slot);
  return(ret);
}


  • 等待native IO请求完成os_aio_linux_collect
os_aio_linux_collect(os_aio_array_t* array,
 					ulint segment, 
					ulint seg_size){
	events = &array->aio_events[segment * seg_size]; // 定位segment所对应的io event的数组位置
	/* 获得该线程的aio上下文数组 */
	io_ctx = array->aio_ctx[segment];
	/* Starting point of the segment we will be working on. */
	start_pos = segment * seg_size;
	/* End point. */
	end_pos = start_pos + seg_size;


retry: 
	/* Initialize the events. The timeout value is arbitrary.
	  We probably need to experiment with it a little. */
	memset(events, 0, sizeof(*events) * seg_size);
	timeout.tv_sec = 0;
	timeout.tv_nsec = OS_AIO_REAP_TIMEOUT;

	ret = io_getevents(io_ctx, 1, seg_size, events, &timeout); // 阻塞等待该IO线程所监控的任一IO请求完成

	if (ret > 0) { // 有IO请求完成
		for (i = 0; i < ret; i++) {
       // 记录完成IO的请求信息到对应的os_aio_slot_t 对象
			os_aio_slot_t*	slot;
			struct iocb*	control;
			control = (struct iocb*) events[i].obj; // 获得完成的aio的iocb,即提交这个aio请求的iocb
			ut_a(control != NULL);
			slot = (os_aio_slot_t*) control->data; // 通过data获得这个aio iocb所对应的os_aio_slot_t
			/* Some sanity checks. */
			ut_a(slot != NULL);
			ut_a(slot->reserved);
			os_mutex_enter(array->mutex);
			slot->n_bytes = events[i].res; // 将该io执行的结果保存到slot里
			slot->ret = events[i].res2;
			slot->io_already_done = TRUE; // 标志该io已经完成了,这个标志也是外层判断的条件
			os_mutex_exit(array->mutex);
		}
		return;
	}
…
}

综上重点对InnoDB navtive IO读写数据文件从源码角度进行了分析,有兴趣的读者也可以继续了解InnoDB自带的simulated IO的实现过程,原理雷同native IO,只是在实现方式上自己进行了处理。本篇文章对InnoDB IO请求的执行流程进行了梳理,对重点数据结构以及函数进行了分析,希望对读者日后进行源码阅读及修改有所帮助。