Linux Kernel Architecture

Chapter 6: Device Drivers

RequestQueues

Read and write requests to block devices are placed on a queue known as arequest queue.Thegendisk

structure includes a pointer to the device-specific queue, which is represented by the following data type.

<blkdev.h>

struct request_queue

{

/*

* Together with queue_head for cacheline sharing

*/

struct list_head queue_head;

struct list_head *last_merge;

elevator_t elevator;

struct request_list rq; /* Queue request freelists */

request_fn_proc *request_fn;

make_request_fn *make_request_fn;

prep_rq_fn *prep_rq_fn;

unplug_fn *unplug_fn;

merge_bvec_fn *merge_bvec_fn;

prepare_flush_fn *prepare_flush_fn;

softirq_done_fn *softirq_done_fn;

...

/*

* Auto-unplugging state

*/

struct timer_list unplug_timer;

int unplug_thresh; /* After this many requests */

unsigned long unplug_delay; /* After this many jiffies */

struct work_struct unplug_work;

struct backing_dev_info backing_dev_info;

...

/* queue needs bounce pages for pages above this limit */

unsigned long bounce_pfn;

int bounce_gfp;

unsigned long queue_flags;

/* queue settings */

unsigned long nr_requests; /* Max # of requests */

unsigned int nr_congestion_on;

unsigned int nr_congestion_off;

unsigned int nr_batching;

unsigned short max_sectors;

unsigned short max_hw_sectors;

unsigned short max_phys_segments;

unsigned short max_hw_segments;

unsigned short hardsect_size;

unsigned int max_segment_size;

};