Simple I/O scheduler(SIO scheduler) is desinged for Solid State Drive. This gist is modified to run in Linux Kernel4.x.

sio-iosched.c

/*
 * Simple IO scheduler
 * Based on Noop, Deadline and V(R) IO schedulers.
 * This algorithm does not do any kind of sorting, as it is aimed for
 * aleatory access devices, but it does some basic merging. We try to
 * keep minimum overhead to achieve low latency.
 *
 * Asynchronous and synchronous requests are not treated separately, but
 * we relay on deadlines to ensure fairness.
 *
 */
#include <linux/blkdev.h>
#include <linux/elevator.h>
#include <linux/bio.h>
#include <linux/module.h>
#include <linux/init.h>

enum { ASYNC, SYNC };

/* Tunables (sysfs parameters)*/
static const int sync_read_expire  = HZ / 2;	/* max time before a sync read is submitted. */
static const int sync_write_expire = 2 * HZ;	/* max time before a sync write is submitted. */

static const int async_read_expire  =  4 * HZ;	/* ditto for async, these limits are SOFT! */
static const int async_write_expire = 16 * HZ;	/* ditto for async, these limits are SOFT! */

static const int writes_starved = 2;		/* max times reads can starve a write */
static const int fifo_batch     = 8;		/* # of sequential requests treated as one by the above parameters. For throughput. */

/* Elevator data */
struct sio_data {
	/* Request queues */
	struct list_head fifo_list[2][2];	/* Attributes */
	unsigned int batched;
	unsigned int starved;

	/* Settings */
	int fifo_expire[2][2];
	int fifo_batch;
	int writes_starved;
};


/*
  elevator_merge_req_fn
  called when two requests get merged. the one which gets merged into the other one will be never seen by I/O scheduler again.
  IOW, after being merged, the request is gone.
 */
static void
sio_merged_requests(struct request_queue *q, struct request *rq, struct request *next)
{
	/*
	 * If next expires before rq, assign its expire time to rq
	 * and move into next position (next will be deleted) in fifo.
	 */
	printk(KERN_DEBUG "sio_merged_requests");
	if (!list_empty(&rq->queuelist) && !list_empty(&next->queuelist)) {
		if (time_before(next->fifo_time, rq->fifo_time)){
			list_move(&rq->queuelist, &next->queuelist);
			rq->fifo_time = next->fifo_time;
		}
	}

	/*
	 Delete next request.
	*/
	rq_fifo_clear(next); //	#define rq_fifo_clear(rq)       list_del_init(&(rq)->queuelist)
}

static void
sio_add_request(struct request_queue *q, struct request *rq)
{
	struct sio_data *sd = q->elevator->elevator_data;
	const int sync = rq_is_sync(rq);
	const int data_dir = rq_data_dir(rq);
	printk(KERN_DEBUG "sio_add_request");
	/*
	 * Add request to the proper fifo list and set its
	 * expire time.
	 */
	rq->fifo_time = jiffies + sd->fifo_expire[sync][data_dir];
	list_add_tail(&rq->queuelist, &sd->fifo_list[sync][data_dir]);
}

static struct request *
sio_expired_request(struct sio_data *sd, int sync, int data_dir)
{

	struct list_head *list = &sd->fifo_list[sync][data_dir];
	struct request *rq;

	printk(KERN_DEBUG "sio_expired_request");
	if (list_empty(list))
		return NULL;

	/* Retrieve request */
	rq = rq_entry_fifo(list->next); // #define rq_entry_fifo(ptr)	list_entry((ptr), struct request, queuelist)

	/* Request has expired */
	if (time_after(jiffies, rq->fifo_time)) // time_after(a,b) returns true if the time a is after time b.
		return rq;

	return NULL;
}

// get expires request
static struct request *
sio_choose_expired_request(struct sio_data *sd)
{
	struct request *rq;
	printk(KERN_DEBUG "sio_choose_expired_request");
	/*
	 * Check expired requests.
	 * Asynchronous requests have priority over synchronous.
	 * Write requests have priority over read.
	 */
	rq = sio_expired_request(sd, ASYNC, WRITE);
	if (rq)
		return rq;
	rq = sio_expired_request(sd, ASYNC, READ);
	if (rq)
		return rq;

	rq = sio_expired_request(sd, SYNC, WRITE);
	if (rq)
		return rq;
	rq = sio_expired_request(sd, SYNC, READ);
	if (rq)
		return rq;

	return NULL;
}

static struct request *
sio_choose_request(struct sio_data *sd, int data_dir)
{
	struct list_head *sync = sd->fifo_list[SYNC];
	struct list_head *async = sd->fifo_list[ASYNC];
	printk(KERN_DEBUG "sio_choose_request");
	/*
	 * Retrieve request from available fifo list.
	 * Synchronous requests have priority over asynchronous.
	 * Read requests have priority over write.
	 */
	if (!list_empty(&sync[data_dir])) // sync read
		return rq_entry_fifo(sync[data_dir].next);
	if (!list_empty(&async[data_dir])) // async read
		return rq_entry_fifo(async[data_dir].next);

	if (!list_empty(&sync[!data_dir])) // sync write
		return rq_entry_fifo(sync[!data_dir].next);
	if (!list_empty(&async[!data_dir])) // async write
		return rq_entry_fifo(async[!data_dir].next);

	return NULL;
}

static inline void
sio_dispatch_request(struct sio_data *sd, struct request *rq)
{
	/*
	 * Remove the request from the fifo list
	 * and dispatch it.
	 */
	printk(KERN_DEBUG "sio_dispatch_request");
	rq_fifo_clear(rq);
	elv_dispatch_add_tail(rq->q, rq);

	sd->batched++;

	if (rq_data_dir(rq)) // #define rq_data_dir(rq)         (((rq)->cmd_flags & 1) != 0)
		sd->starved = 0;
	else
		sd->starved++;
}

/*
  sio_dispatch_requests ---> sio_choose_expired_request -> sio_expired_request -|
                         |                                                      |
			  -> sio_shoose_request -> ------------------------------> sio_dispatch_request
 */
static int
sio_dispatch_requests(struct request_queue *q, int force)
{

	struct sio_data *sd = q->elevator->elevator_data;
	struct request *rq = NULL;
	int data_dir = READ;

	printk(KERN_DEBUG "sio_dispatch_requests");

	/*
	 * Retrieve any expired request after a batch of
	 * sequential requests.
	 */
	if (sd->batched > sd->fifo_batch) {
		sd->batched = 0;
		rq = sio_choose_expired_request(sd);
	}

	/* Retrieve request */
	if (!rq) {
		if (sd->starved > sd->writes_starved)
			data_dir = WRITE;

		rq = sio_choose_request(sd, data_dir);
		if (!rq)
			return 0;
	}

	/* Dispatch request */
	sio_dispatch_request(sd, rq);

	return 1;
}

static struct request *
sio_former_request(struct request_queue *q, struct request *rq)
{
	struct sio_data *sd = q->elevator->elevator_data;
	const int sync = rq_is_sync(rq);
	const int data_dir = rq_data_dir(rq);

	printk(KERN_DEBUG "sio_former_request");
	if (rq->queuelist.prev == &sd->fifo_list[sync][data_dir])
		return NULL;

	/* Return former request */
	return list_entry(rq->queuelist.prev, struct request, queuelist);
}

static struct request *
sio_latter_request(struct request_queue *q, struct request *rq)
{
	struct sio_data *sd = q->elevator->elevator_data;
	const int sync = rq_is_sync(rq);
	const int data_dir = rq_data_dir(rq);

	printk(KERN_DEBUG "sio_latter_request");
	if (rq->queuelist.next == &sd->fifo_list[sync][data_dir])
		return NULL;

	/* Return latter request */
	return list_entry(rq->queuelist.next, struct request, queuelist);
}


/*
  Initialization
 */
static int sio_init_queue(struct request_queue *q, struct elevator_type *e)
{
	struct sio_data *sd;
	struct elevator_queue *eq;
	printk(KERN_DEBUG "sio_init_queue()");
	
	eq = elevator_alloc(q, e);
	if (!eq)
		return -ENOMEM;

	sd = kmalloc_node(sizeof(*sd), GFP_KERNEL, q->node);
	if (!sd) {
		kobject_put(&eq->kobj);
		return -ENOMEM;
	}
	eq->elevator_data = sd;
	INIT_LIST_HEAD(&sd->fifo_list[SYNC][READ]);
	INIT_LIST_HEAD(&sd->fifo_list[SYNC][WRITE]);
	INIT_LIST_HEAD(&sd->fifo_list[ASYNC][READ]);
	INIT_LIST_HEAD(&sd->fifo_list[ASYNC][WRITE]);

	spin_lock_irq(q->queue_lock);
	/* Initialize data */
	q->elevator = eq;
	sd->batched = 0;
	sd->fifo_expire[SYNC][READ] = sync_read_expire;
	sd->fifo_expire[SYNC][WRITE] = sync_write_expire;
	sd->fifo_expire[ASYNC][READ] = async_read_expire;
	sd->fifo_expire[ASYNC][WRITE] = async_write_expire;
	sd->fifo_batch = fifo_batch;
	//q->elevator->elevator_data = sd;
	spin_unlock_irq(q->queue_lock);

	return 0;
}

/*
  Finishalize
 */
static void
sio_exit_queue(struct elevator_queue *e)
{
	struct sio_data *sd = e->elevator_data;
	printk(KERN_DEBUG "sio_exit_queue");
	BUG_ON(!list_empty(&sd->fifo_list[SYNC][READ]));
	BUG_ON(!list_empty(&sd->fifo_list[SYNC][WRITE]));
	BUG_ON(!list_empty(&sd->fifo_list[ASYNC][READ]));
	BUG_ON(!list_empty(&sd->fifo_list[ASYNC][WRITE]));

	/* Free structure */
	kfree(sd);
}

/*
 * for sysfs code
 */

static ssize_t
sio_var_show(int var, char *page)
{
	return sprintf(page, "%d\n", var);
}

static ssize_t
sio_var_store(int *var, const char *page, size_t count)
{
	char *p = (char *) page;

	*var = simple_strtol(p, &p, 10);
	return count;
}

#define SHOW_FUNCTION(__FUNC, __VAR, __CONV)				\
static ssize_t __FUNC(struct elevator_queue *e, char *page)		\
{									\
	struct sio_data *sd = e->elevator_data;			\
	int __data = __VAR;						\
	if (__CONV)							\
		__data = jiffies_to_msecs(__data);			\
	return sio_var_show(__data, (page));			\
}
SHOW_FUNCTION(sio_sync_read_expire_show, sd->fifo_expire[SYNC][READ], 1);
SHOW_FUNCTION(sio_sync_write_expire_show, sd->fifo_expire[SYNC][WRITE], 1);
SHOW_FUNCTION(sio_async_read_expire_show, sd->fifo_expire[ASYNC][READ], 1);
SHOW_FUNCTION(sio_async_write_expire_show, sd->fifo_expire[ASYNC][WRITE], 1);
SHOW_FUNCTION(sio_fifo_batch_show, sd->fifo_batch, 0);
SHOW_FUNCTION(sio_writes_starved_show, sd->writes_starved, 0);
#undef SHOW_FUNCTION

#define STORE_FUNCTION(__FUNC, __PTR, MIN, MAX, __CONV)			\
static ssize_t __FUNC(struct elevator_queue *e, const char *page, size_t count)	\
{									\
	struct sio_data *sd = e->elevator_data;			\
	int __data;							\
	int ret = sio_var_store(&__data, (page), count);		\
	if (__data < (MIN))						\
		__data = (MIN);						\
	else if (__data > (MAX))					\
		__data = (MAX);						\
	if (__CONV)							\
		*(__PTR) = msecs_to_jiffies(__data);			\
	else								\
		*(__PTR) = __data;					\
	return ret;							\
}
STORE_FUNCTION(sio_sync_read_expire_store, &sd->fifo_expire[SYNC][READ], 0, INT_MAX, 1);
STORE_FUNCTION(sio_sync_write_expire_store, &sd->fifo_expire[SYNC][WRITE], 0, INT_MAX, 1);
STORE_FUNCTION(sio_async_read_expire_store, &sd->fifo_expire[ASYNC][READ], 0, INT_MAX, 1);
STORE_FUNCTION(sio_async_write_expire_store, &sd->fifo_expire[ASYNC][WRITE], 0, INT_MAX, 1);
STORE_FUNCTION(sio_fifo_batch_store, &sd->fifo_batch, 0, INT_MAX, 0);
STORE_FUNCTION(sio_writes_starved_store, &sd->writes_starved, 0, INT_MAX, 0);
#undef STORE_FUNCTION
// S_IRUGIOS|S_IWUSR means sysfs be enable to writed and readed when running
#define DD_ATTR(name) \
	__ATTR(name, S_IRUGO|S_IWUSR, sio_##name##_show, \
				      sio_##name##_store)

static struct elv_fs_entry sio_attrs[] = {
	DD_ATTR(sync_read_expire),
	DD_ATTR(sync_write_expire),
	DD_ATTR(async_read_expire),
	DD_ATTR(async_write_expire),
	DD_ATTR(fifo_batch),
	DD_ATTR(writes_starved),
	__ATTR_NULL
};

static struct elevator_type iosched_sio = {
	.ops = {
		.elevator_merge_req_fn		= sio_merged_requests,
		.elevator_dispatch_fn		= sio_dispatch_requests,
		.elevator_add_req_fn		= sio_add_request,
		.elevator_former_req_fn		= sio_former_request,
		.elevator_latter_req_fn		= sio_latter_request,
		.elevator_init_fn		= sio_init_queue,
		.elevator_exit_fn		= sio_exit_queue,
	},

	.elevator_attrs = sio_attrs, // for sysfs
	.elevator_name = "sio",
	.elevator_owner = THIS_MODULE,
};

static int __init sio_init(void)
{
	/* Register elevator */
	printk(KERN_DEBUG "sio_init");
	elv_register(&iosched_sio);

	return 0;
}
static void __exit sio_exit(void)
{
	/* Unregister elevator */
	printk(KERN_DEBUG "sio_exit");
	elv_unregister(&iosched_sio);
}

module_init(sio_init);
module_exit(sio_exit);


MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Simple IO scheduler");