/** * @file * * @ingroup rtems_bdbuf * * Block device buffer management. */ /* * Copyright (C) 2001 OKTET Ltd., St.-Petersburg, Russia * Author: Victor V. Vengerov * * Copyright (C) 2008,2009 Chris Johns * Rewritten to remove score mutex access. Fixes many performance * issues. Change to support demand driven variable buffer sizes. * * @(#) bdbuf.h,v 1.9 2005/02/02 00:06:18 joel Exp */ #ifndef _RTEMS_BDBUF_H #define _RTEMS_BDBUF_H #include #include #include #include #include #ifdef __cplusplus extern "C" { #endif /** * @defgroup rtems_libblock Block Device Library * * Block device modules. */ /** * @defgroup rtems_bdbuf Block Device Buffer Management * * @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 * 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 * multiple of the disk device block size. The disk device's physical block * size is called the media block size. The file system can set the block size * it uses to a larger multiple of the media block size. The driver must be * able to handle buffers sizes larger than one media block. * * The user configures the amount of memory to be used as buffers in the cache, * and the minimum and maximum buffer size. The cache will allocate additional * memory for the buffer descriptors and groups. There are enough buffer * descriptors allocated so all the buffer memory can be used as minimum sized * buffers. * * The cache is a single pool of buffers. The buffer memory is divided into * groups where the size of buffer memory allocated to a group is the maximum * buffer size. A group's memory can be divided down into small buffer sizes * that are a multiple of 2 of the minimum buffer size. A group is the minumum * allocation unit for buffers of a specific size. If a buffer of maximum size * is request the group will have a single buffer. If a buffer of minium size * is requested the group is divided into minimum sized buffers and the * remaining buffers are held ready for use. A group keeps track of which * buffers are with a file system or driver and groups who have buffer in use * cannot be realloced. Groups with no buffers in use can be taken and * realloced to a new size. This is how buffers of different sizes move around * the cache. * The buffers are held in various lists in the cache. All buffers follow this * state machine: * * @dot * digraph g { * ready [label="Ready\nRead Ahead"]; * transfer [label="Transfer"]; * accessed [label="Accessed\nAccessed Modified"]; * modified [label="Modified\nSynchronized"]; * cached [label="Cached"]; * ready -> transfer [label="Read\nRead Ahead"]; * transfer -> ready [label="Read Ahead Complete"]; * ready -> accessed [label="Get"]; * transfer -> accessed [label="Read or Write\nComplete"]; * transfer -> cached [label="Read or Write\nComplete"]; * accessed -> cached [label="Release"]; * cached -> accessed [label="Get"]; * modified -> accessed [label="Get"]; * accessed -> modified [label="Modified"]; * accessed -> transfer [label="Swap"]; * } * @enddot * * Empty buffers are added to the ready list and removed from this queue when a * caller requests a buffer. This is referred to as getting a buffer in the * code and the event get in the state diagram. The buffer is assigned to a * block and inserted to the AVL based on the block/device key. If the block is * to be read by the user and not in the cache (ready) it is transfered from * the disk into memory. If no ready buffers exist the buffer is taken from the * LRU list. If no buffers are on the LRU list the modified list is check. If * no buffers are on the modified list the request blocks. If buffers are on * the modified list the buffers hold timer is expired and the swap out task * woken. * * A block being accessed is given to the file system layer and not accessable * to another requester until released back to the cache. The same goes to a * buffer in the transfer state. The transfer state means being read or * written. If the file system has modifed the block and releases it as * modified it placed on the cache's modified list and a hold timer * initialised. The buffer is held for the hold time before being written to * disk. Buffers are held for a configurable period of time on the modified * list as a write sets the state to transfer and this locks the buffer out * from the file system until the write completes. Buffers are often accessed * and modified in a series of small updates so if sent to the disk when * released as modified the user would have to block waiting until it had been * written. This would be a performance problem. * * The code performs mulitple block reads and writes. Multiple block reads or * read ahead increases performance with hardware that supports it. It also * helps with a large cache as the disk head movement is reduced. It how-ever * is a speculative operation so excessive use can remove valuable and needed * blocks from the cache. The get call knows if a read is a for the file system * or if it is a read ahead get. If the get is for a read ahead block and the * block is already in the cache or no ready buffers are available the read * ahead is stopped. The transfer occurs with the blocks so far. If a buffer is * in the read ahead state and release it is placed on the ready list rather * than the LRU list. This means these buffers are used before buffers used by * the file system. * * The cache has the following lists of buffers: * - @c ready: Empty buffers created when the pool is initialised. * - @c modified: Buffers waiting to be written to disk. * - @c sync: Buffers to be synced to disk. * - @c lru: Accessed buffers released in least recently used order. * * The cache scans the ready list then the LRU list for a suitable buffer in * this order. A suitable buffer is one that matches the same allocation size * as the device the buffer is for. The a buffer's group has no buffers in use * with the file system or driver the group is reallocated. This means the * buffers in the group are invalidated, resized and placed on the ready queue. * There is a performance issue with this design. The reallocation of a group * may forced recently accessed buffers out of the cache when they should * not. The design should be change to have groups on a LRU list if they have * no buffers in use. * * @{ */ /** * State of a buffer in the cache. */ typedef enum { RTEMS_BDBUF_STATE_EMPTY = 0, /**< Not in use. */ RTEMS_BDBUF_STATE_READ_AHEAD = 1, /**< Holds read ahead data only */ RTEMS_BDBUF_STATE_CACHED = 2, /**< In the cache and available */ RTEMS_BDBUF_STATE_ACCESS = 3, /**< The user has the buffer */ RTEMS_BDBUF_STATE_MODIFIED = 4, /**< In the cache but modified */ RTEMS_BDBUF_STATE_ACCESS_MODIFIED = 5, /**< With the user but modified */ RTEMS_BDBUF_STATE_SYNC = 6, /**< Requested to be sync'ed */ RTEMS_BDBUF_STATE_TRANSFER = 7 /**< Being transferred to or from disk */ } rtems_bdbuf_buf_state; /** * Forward reference to the block. */ struct rtems_bdbuf_group; typedef struct rtems_bdbuf_group rtems_bdbuf_group; /** * To manage buffers we using buffer descriptors (BD). A BD holds a buffer plus * a range of other information related to managing the buffer in the cache. To * speed-up buffer lookup descriptors are organized in AVL-Tree. The fields * 'dev' and 'block' are search keys. */ typedef struct rtems_bdbuf_buffer { rtems_chain_node link; /**< Link the BD onto a number of lists. */ struct rtems_bdbuf_avl_node { signed char cache; /**< Cache */ struct rtems_bdbuf_buffer* left; /**< Left Child */ struct rtems_bdbuf_buffer* right; /**< Right Child */ signed char bal; /**< The balance of the sub-tree */ } avl; dev_t dev; /**< device number */ rtems_blkdev_bnum block; /**< block number on the device */ unsigned char* buffer; /**< Pointer to the buffer memory area */ int error; /**< If not 0 indicate an error value (errno) * which can be used by user later */ volatile rtems_bdbuf_buf_state state; /**< State of the buffer. */ volatile uint32_t waiters; /**< The number of threads waiting on this * buffer. */ rtems_bdbuf_group* group; /**< Pointer to the group of BDs this BD is * part of. */ volatile uint32_t hold_timer; /**< Timer to indicate how long a buffer * has been held in the cache modified. */ int references; /**< Allow reference counting by owner. */ void* user; /**< User data. */ } rtems_bdbuf_buffer; /** * A group is a continuous block of buffer descriptors. A group covers the * maximum configured buffer size and is the allocation size for the buffers to * a specific buffer size. If you allocate a buffer to be a specific size, all * buffers in the group, if there are more than 1 will also be that size. The * number of buffers in a group is a multiple of 2, ie 1, 2, 4, 8, etc. */ struct rtems_bdbuf_group { rtems_chain_node link; /**< Link the groups on a LRU list if they * have no buffers in use. */ size_t bds_per_group; /**< The number of BD allocated to this * group. This value must be a multiple of * 2. */ uint32_t users; /**< How many users the block has. */ rtems_bdbuf_buffer* bdbuf; /**< First BD this block covers. */ }; /** * Buffering configuration definition. See confdefs.h for support on using this * structure. */ typedef struct rtems_bdbuf_config { uint32_t max_read_ahead_blocks; /**< Number of blocks to read * ahead. */ uint32_t max_write_blocks; /**< Number of blocks to write * at once. */ rtems_task_priority swapout_priority; /**< Priority of the swap out * task. */ uint32_t swapout_period; /**< Period swapout checks buf * timers. */ uint32_t swap_block_hold; /**< Period a buffer is held. */ size_t swapout_workers; /**< The number of worker * threads for the swapout * task. */ rtems_task_priority swapout_worker_priority; /**< Priority of the swap out * task. */ size_t size; /**< Size of memory in the * cache */ uint32_t buffer_min; /**< Minimum buffer size. */ uint32_t buffer_max; /**< Maximum buffer size * supported. It is also the * allocation size. */ } rtems_bdbuf_config; /** * External reference to the configuration. * * The configuration is provided by the application. */ 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. */ #define RTEMS_BDBUF_MAX_READ_AHEAD_BLOCKS_DEFAULT 32 /** * Default maximum number of blocks to write at once. */ #define RTEMS_BDBUF_MAX_WRITE_BLOCKS_DEFAULT 16 /** * Default swap-out task priority. */ #define RTEMS_BDBUF_SWAPOUT_TASK_PRIORITY_DEFAULT 15 /** * Default swap-out task swap period in milli seconds. */ #define RTEMS_BDBUF_SWAPOUT_TASK_SWAP_PERIOD_DEFAULT 250 /** * Default swap-out task block hold time in milli seconds. */ #define RTEMS_BDBUF_SWAPOUT_TASK_BLOCK_HOLD_DEFAULT 1000 /** * Default swap-out worker tasks. Currently disabled. */ #define RTEMS_BDBUF_SWAPOUT_WORKER_TASKS_DEFAULT 0 /** * Default swap-out worker task priority. The same as the swapout task. */ #define RTEMS_BDBUF_SWAPOUT_WORKER_TASK_PRIORITY_DEFAULT \ RTEMS_BDBUF_SWAPOUT_TASK_PRIORITY_DEFAULT /** * Default size of memory allocated to the cache. */ #define RTEMS_BDBUF_CACHE_MEMORY_SIZE_DEFAULT (64 * 512) /** * Default minimum size of buffers. */ #define RTEMS_BDBUF_BUFFER_MIN_SIZE_DEFAULT (512) /** * Default maximum size of buffers. */ #define RTEMS_BDBUF_BUFFER_MAX_SIZE_DEFAULT (4096) /** * Prepare buffering layer to work - initialize buffer descritors and (if it is * neccessary) buffers. After initialization all blocks is placed into the * ready state. * * @return RTEMS status code (RTEMS_SUCCESSFUL if operation completed * successfully or error code if error is occured) */ rtems_status_code rtems_bdbuf_init (void); /** * Get block buffer for data to be written into. The buffers is set to the * access or modifed access state. If the buffer is in the cache and modified * the state is access modified else the state is access. This buffer contents * are not initialised if the buffer is not already in the cache. If the block * is already resident in memory it is returned how-ever if not in memory the * buffer is not read from disk. This call is used when writing the whole block * on a disk rather than just changing a part of it. If there is no buffers * available this call will block. A buffer obtained with this call will not be * involved in a transfer request and will not be returned to another user * until released. If the buffer is already with a user when this call is made * the call is blocked until the buffer is returned. The highest priority * waiter will obtain the buffer first. * * The block number is the linear block number. This is relative to the start * of the partition on the media. * * @param device Device number (constructed of major and minor device number) * @param block Linear media block number * @param bd Reference to the buffer descriptor pointer. * * @return RTEMS status code (RTEMS_SUCCESSFUL if operation completed * successfully or error code if error is occured) */ rtems_status_code rtems_bdbuf_get (dev_t device, rtems_blkdev_bnum block, rtems_bdbuf_buffer** bd); /** * Get the block buffer and if not already in the cache read from the disk. If * specified block already cached return. The buffer is set to the access or * modifed access state. If the buffer is in the cache and modified the state * is access modified else the state is access. If block is already being read * from disk for being written to disk this call blocks. If the buffer is * waiting to be written it is removed from modified queue and returned to the * user. If the buffer is not in the cache a new buffer is obtained and the * data read from disk. The call may block until these operations complete. A * buffer obtained with this call will not be involved in a transfer request * and will not be returned to another user until released. If the buffer is * already with a user when this call is made the call is blocked until the * buffer is returned. The highest priority waiter will obtain the buffer * first. * * @param device Device number (constructed of major and minor device number) * @param block Linear media block number * @param bd Reference to the buffer descriptor pointer. * * @return RTEMS status code (RTEMS_SUCCESSFUL if operation completed * successfully or error code if error is occured) */ rtems_status_code rtems_bdbuf_read (dev_t device, rtems_blkdev_bnum block, rtems_bdbuf_buffer** bd); /** * Release the buffer obtained by a read call back to the cache. If the buffer * was obtained by a get call and was not already in the cache the release * modified call should be used. A buffer released with this call obtained by a * get call may not be in sync with the contents on disk. If the buffer was in * the cache and modified before this call it will be returned to the modified * queue. The buffers is returned to the end of the LRU list. * * @param bd Reference to the buffer descriptor. * * @return RTEMS status code (RTEMS_SUCCESSFUL if operation completed * successfully or error code if error is occured) */ rtems_status_code rtems_bdbuf_release (rtems_bdbuf_buffer* bd); /** * Release the buffer allocated with a get or read call placing it on the * modidied list. If the buffer was not released modified before the hold * timer is set to the configuration value. If the buffer had been released * modified before but not written to disk the hold timer is not updated. The * buffer will be written to disk when the hold timer has expired, there are * not more buffers available in the cache and a get or read buffer needs one * or a sync call has been made. If the buffer is obtained with a get or read * before the hold timer has expired the buffer will be returned to the user. * * @param bd Reference to the buffer descriptor. * * @return RTEMS status code (RTEMS_SUCCESSFUL if operation completed * successfully or error code if error is occured) */ rtems_status_code rtems_bdbuf_release_modified (rtems_bdbuf_buffer* bd); /** * Release the buffer as modified and wait until it has been synchronized with * the disk by writing it. This buffer will be the first to be transfer to disk * and other buffers may also be written if the maximum number of blocks in a * requests allows it. * * @note This code does not lock the sync mutex and stop additions to the * modified queue. * @param bd Reference to the buffer descriptor. * * @return RTEMS status code (RTEMS_SUCCESSFUL if operation completed * successfully or error code if error is occured) */ rtems_status_code rtems_bdbuf_sync (rtems_bdbuf_buffer* bd); /** * Synchronize all modified buffers for this device with the disk and wait * until the transfers have completed. The sync mutex for the cache is locked * stopping the addition of any further modifed buffers. It is only the * currently modified buffers that are written. * * @note Nesting calls to sync multiple devices will be handled sequentially. A * nested call will be blocked until the first sync request has complete. * * @param dev Block device number * * @return RTEMS status code (RTEMS_SUCCESSFUL if operation completed * successfully or error code if error is occured) */ rtems_status_code rtems_bdbuf_syncdev (dev_t dev); /** @} */ #ifdef __cplusplus } #endif #endif