libblock: Add read-ahead task

Read-ahead requests were previously executed in the context of the
reading task.  This blocks the reading task until the complete read
with read-ahead transfer is finished.  A read-ahead task is introduced
to off-load the read-ahead transfer.  This allows the reading task to
work with the requested block more quickly.  The read-ahead is triggered
after two misses of ascending consecutive blocks or a read hit of a
block read by the most-recent read-ahead transfer.  The read-ahead
feature is configurable and can be disabled.

cpukit/libblock/include/rtems/bdbuf.h

@@ -46,7 +46,7 @@ extern "C" {
  * @ingroup rtems_libblock
  *
  * The Block Device Buffer Management implements a cache between the disk
- * devices and file systems.  The code provides read ahead and write queuing to
+ * devices and file systems.  The code provides read-ahead and write queuing to
  * the drivers and fast cache look-up using an AVL tree.
  *
  * The block size used by a file system can be set at runtime and must be a
@@ -152,10 +152,13 @@ extern "C" {
  * written.  This would be a performance problem.
  *
  * The code performs multiple block reads and writes.  Multiple block reads or
- * read ahead increases performance with hardware that supports it.  It also
+ * read-ahead increases performance with hardware that supports it.  It also
  * helps with a large cache as the disk head movement is reduced.  It however
  * is a speculative operation so excessive use can remove valuable and needed
- * blocks from the cache.
+ * blocks from the cache.  The read-ahead is triggered after two misses of
+ * ascending consecutive blocks or a read hit of a block read by the
+ * most-resent read-ahead transfer.  The read-ahead works per disk, but all
+ * transfers are issued by the read-ahead task.
  *
  * The cache has the following lists of buffers:
  *  - LRU: Accessed or transfered buffers released in least recently used
@@ -379,6 +382,8 @@ typedef struct rtems_bdbuf_config {
   uint32_t            buffer_max;              /**< Maximum buffer size
                                                 * supported. It is also the
                                                 * allocation size. */
+  rtems_task_priority read_ahead_priority;     /**< Priority of the read-ahead
+                                                * task. */
 } rtems_bdbuf_config;
 
 /**
@@ -389,8 +394,8 @@ typedef struct rtems_bdbuf_config {
 extern const rtems_bdbuf_config rtems_bdbuf_configuration;
 
 /**
- * The max_read_ahead_blocks value is altered if there are fewer buffers
- * than this defined max. This stops thrashing in the cache.
+ * The default value for the maximum read-ahead blocks disables the read-ahead
+ * feature.
  */
 #define RTEMS_BDBUF_MAX_READ_AHEAD_BLOCKS_DEFAULT    0
 
@@ -425,6 +430,12 @@ extern const rtems_bdbuf_config rtems_bdbuf_configuration;
 #define RTEMS_BDBUF_SWAPOUT_WORKER_TASK_PRIORITY_DEFAULT \
                              RTEMS_BDBUF_SWAPOUT_TASK_PRIORITY_DEFAULT
 
+/**
+ * Default read-ahead task priority.  The same as the swap-out task.
+ */
+#define RTEMS_BDBUF_READ_AHEAD_TASK_PRIORITY_DEFAULT \
+  RTEMS_BDBUF_SWAPOUT_TASK_PRIORITY_DEFAULT
+
 /**
  * Default task stack size for swap-out and worker tasks.
  */
@@ -621,7 +632,7 @@ rtems_bdbuf_syncdev (rtems_disk_device *dd);
 /**
  * @brief Purges all buffers corresponding to the disk device @a dd.
  *
- * This may result in loss of data.
+ * This may result in loss of data.  The read-ahead state of this device is reset.
  *
  * Before you can use this function, the rtems_bdbuf_init() routine must be
  * called at least once to initialize the cache, otherwise a fatal error will
@@ -635,7 +646,8 @@ rtems_bdbuf_purge_dev (rtems_disk_device *dd);
 /**
  * @brief Sets the block size of a disk device.
  *
- * This will set the block size derived fields of the disk device.
+ * This will set the block size derived fields of the disk device.  The
+ * read-ahead state of this device is reset.
  *
  * Before you can use this function, the rtems_bdbuf_init() routine must be
  * called at least once to initialize the cache, otherwise a fatal error will

cpukit/libblock/include/rtems/diskdevs.h

@@ -16,6 +16,7 @@
 
 #include <rtems.h>
 #include <rtems/libio.h>
+#include <rtems/chain.h>
 #include <stdlib.h>
 
 #ifdef __cplusplus
@@ -54,6 +55,37 @@ typedef int (*rtems_block_device_ioctl)(
   void *argp
 );
 
+/**
+ * @brief Trigger value to disable further read-ahead requests.
+ */
+#define RTEMS_DISK_READ_AHEAD_NO_TRIGGER ((rtems_blkdev_bnum) -1)
+
+/**
+ * @brief Read-ahead control.
+ */
+typedef struct {
+  /**
+   * @brief Chain node for the read-ahead request queue of the read-ahead task.
+   */
+  rtems_chain_node node;
+
+  /**
+   * @brief Block value to trigger the read-ahead request.
+   *
+   * A value of @ref RTEMS_DISK_READ_AHEAD_NO_TRIGGER will disable further
+   * read-ahead requests since no valid block can have this value.
+   */
+  rtems_blkdev_bnum trigger;
+
+  /**
+   * @brief Start block for the next read-ahead request.
+   *
+   * In case the trigger value is out of range of valid blocks, this value my
+   * be arbitrary.
+   */
+  rtems_blkdev_bnum next;
+} rtems_disk_read_ahread;
+
 /**
  * @brief Description of a disk device (logical and physical disks).
  *
@@ -168,6 +200,11 @@ struct rtems_disk_device {
    * releases this disk.
    */
   bool deleted;
+
+  /**
+   * @brief Read-ahead control for this disk.
+   */
+  rtems_disk_read_ahread read_ahead;
 };
 
 /**

cpukit/libblock/src/diskdevs-init.c

@@ -36,6 +36,7 @@ rtems_status_code rtems_disk_init_phys(
   dd->media_block_size = block_size;
   dd->ioctl = handler;
   dd->driver_data = driver_data;
+  dd->read_ahead.trigger = RTEMS_DISK_READ_AHEAD_NO_TRIGGER;
 
   if (block_count > 0) {
     if ((*handler)(dd, RTEMS_BLKIO_CAPABILITIES, &dd->capabilities) != 0) {
@@ -67,6 +68,7 @@ rtems_status_code rtems_disk_init_log(
   dd->media_block_size = phys_dd->media_block_size;
   dd->ioctl = phys_dd->ioctl;
   dd->driver_data = phys_dd->driver_data;
+  dd->read_ahead.trigger = RTEMS_DISK_READ_AHEAD_NO_TRIGGER;
 
   if (phys_dd->phys_dev == phys_dd) {
     rtems_blkdev_bnum phys_block_count = phys_dd->size;

cpukit/sapi/include/confdefs.h

@@ -1241,6 +1241,10 @@ rtems_fs_init_functions_t    rtems_fs_init_helper =
     #define CONFIGURE_BDBUF_BUFFER_MAX_SIZE \
                               RTEMS_BDBUF_BUFFER_MAX_SIZE_DEFAULT
   #endif
+  #ifndef CONFIGURE_BDBUF_READ_AHEAD_TASK_PRIORITY
+    #define CONFIGURE_BDBUF_READ_AHEAD_TASK_PRIORITY \
+                              RTEMS_BDBUF_READ_AHEAD_TASK_PRIORITY_DEFAULT
+  #endif
   #ifdef CONFIGURE_INIT
     const rtems_bdbuf_config rtems_bdbuf_configuration = {
       CONFIGURE_BDBUF_MAX_READ_AHEAD_BLOCKS,
@@ -1253,11 +1257,14 @@ rtems_fs_init_functions_t    rtems_fs_init_helper =
       CONFIGURE_BDBUF_TASK_STACK_SIZE,
       CONFIGURE_BDBUF_CACHE_MEMORY_SIZE,
       CONFIGURE_BDBUF_BUFFER_MIN_SIZE,
-      CONFIGURE_BDBUF_BUFFER_MAX_SIZE
+      CONFIGURE_BDBUF_BUFFER_MAX_SIZE,
+      CONFIGURE_BDBUF_READ_AHEAD_TASK_PRIORITY
     };
   #endif
 
-  #define CONFIGURE_LIBBLOCK_TASKS (1 + CONFIGURE_SWAPOUT_WORKER_TASKS)
+  #define CONFIGURE_LIBBLOCK_TASKS \
+    (1 + CONFIGURE_SWAPOUT_WORKER_TASKS + \
+    (CONFIGURE_BDBUF_MAX_READ_AHEAD_BLOCKS != 0))
 
   #define CONFIGURE_LIBBLOCK_TASK_EXTRA_STACKS \
     (CONFIGURE_LIBBLOCK_TASKS * \

testsuites/libtests/Makefile.am

@@ -1,6 +1,7 @@
 ACLOCAL_AMFLAGS = -I ../aclocal
 
 SUBDIRS = POSIX
+SUBDIRS += block13
 SUBDIRS += rbheap01
 SUBDIRS += flashdisk01
 

testsuites/libtests/block13/Makefile.am

@@ -0,0 +1,19 @@
+rtems_tests_PROGRAMS = block13
+block13_SOURCES = init.c
+
+dist_rtems_tests_DATA = block13.scn block13.doc
+
+include $(RTEMS_ROOT)/make/custom/@RTEMS_BSP@.cfg
+include $(top_srcdir)/../automake/compile.am
+include $(top_srcdir)/../automake/leaf.am
+
+AM_CPPFLAGS += -I$(top_srcdir)/../support/include
+
+LINK_OBJS = $(block13_OBJECTS)
+LINK_LIBS = $(block13_LDLIBS)
+
+block13$(EXEEXT): $(block13_OBJECTS) $(block13_DEPENDENCIES)
+	@rm -f block13$(EXEEXT)
+	$(make-exe)
+
+include $(top_srcdir)/../automake/local.am

testsuites/libtests/block13/block13.doc

@@ -0,0 +1,11 @@
+This file describes the directives and concepts tested by this test set.
+
+test set name: block13
+
+directives:
+
+  TBD
+
+concepts:
+
+  Tests the read-ahead feature of bdbuf.

testsuites/libtests/block13/block13.scn

@@ -0,0 +1,11 @@
+*** TEST BLOCK 13 ***
+0 2 3 4 5 6 7 8 
+reset
+8 
+reset
+7 
+reset
+6 7 
+reset
+5 6 8 
+*** END OF TEST BLOCK 13 ***

testsuites/libtests/configure.ac

@@ -41,6 +41,7 @@ AM_CONDITIONAL(NETTESTS,test "$rtems_cv_RTEMS_NETWORKING" = "yes")
 
 # Explicitly list all Makefiles here
 AC_CONFIG_FILES([Makefile
+block13/Makefile
 rbheap01/Makefile
 syscall01/Makefile
 flashdisk01/Makefile