Re: SSD and IO schedulers

[Corrado Zoccolo <czoccolo@gmail.com>]

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Hi,
I found that elevator=deadline performs much better than noop for
writes, and almost as well for reads, and started to wonder how a
combination of noop (for read) + deadline (for write) would work in
practice (since deadline still doesn't support the automatic ssd
detection).
I developed a simple hybrid scheduler (see attached), that implements
this idea, and it saved 1 s of my boot time compared to all other
schedulers (cfq, deadline and noop, each loses on some part of the
workload), that is a mixed read write workload, with writes that go
mainly to a very slow device (SDHC card, where I mount /var).

This proof of concept still doesn't support priorities, but I'm
willing to add them at least for read, in which the latency for
sync-reads could be improved.

Corrado

On Sat, Feb 7, 2009 at 6:58 PM, Jan Knutar <jk-lkml@sci.fi> wrote:
> On Wednesday 04 February 2009, J.A. Magallón wrote:
>
>> Perhaps the reason is that, as the SSD is not so good, it behaves
>> more like a rotational drive ;).
>
> Do any other SSDs except Intel's exist that DON'T behave more like a
> rotational drive? I am guessing using something like LogFS would give
> the biggest boost on cheap SSDs and all memory cards.
> --
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-- 
__________________________________________________________________________

dott. Corrado Zoccolo                          mailto:czoccolo@gmail.com
PhD - Department of Computer Science - University of Pisa, Italy
--------------------------------------------------------------------------
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                               Tales of Power - C. Castaneda

[-- Attachment #2: ssd-iosched.c --]
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/* -*- mode:C; tab-width:8; c-basic-offset:8 -*-
 *  Ssd i/o scheduler.
 *
 *  Copyright (C) 2002 Jens Axboe <axboe@kernel.dk>
 */
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/blkdev.h>
#include <linux/elevator.h>
#include <linux/bio.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/compiler.h>
#include <linux/rbtree.h>

/*
 * See Documentation/block/ssd-iosched.txt
 */
static const int write_expire = 5 * HZ; /* ditto for writes, these limits are SOFT! */
static const int writes_starved = 2;    /* max times reads can starve a write */
static const int fifo_batch = 16;       /* # of sequential requests treated as one
				     by the above parameters. For throughput. */

struct ssd_data {
	/*
	 * run time data
	 */

	/*
	 * requests (ssd_rq s) are present on both sort_list and fifo_list
	 */
	/* for read */
        struct list_head read_queue;

	/* for write */
	struct rb_root sort_list_write;	
	struct list_head fifo_list_write;

	/*
	 * next in sort order. read, write or both are NULL
	 */
	struct request *next_rq_write;

	/* stats */
	unsigned int batching;		/* number of sequential requests made */
	sector_t last_sector;		/* head position */
	unsigned int starved;		/* times reads have starved writes */

	/*
	 * settings that change how the i/o scheduler behaves
	 */
	int fifo_expire_write;
	int fifo_batch;
	int writes_starved;
};

static void ssd_move_write_request(struct ssd_data *, struct request *);

/*
 * get the request after `rq' in sector-sorted order
 */
static inline struct request *
ssd_latter_write_request(struct request *rq)
{
	struct rb_node *node = rb_next(&rq->rb_node);

	if (node)
		return rb_entry_rq(node);

	return NULL;
}

static void
ssd_add_write_rq_rb(struct ssd_data *dd, struct request *rq)
{
	struct rb_root *root = &dd->sort_list_write;
	struct request *__alias;

	while (unlikely(__alias = elv_rb_add(root, rq)))
		ssd_move_write_request(dd, __alias);
}

static inline void
ssd_del_write_rq_rb(struct ssd_data *dd, struct request *rq)
{
	if (dd->next_rq_write == rq)
		dd->next_rq_write = ssd_latter_write_request(rq);

	elv_rb_del(&dd->sort_list_write, rq);
}

/*
 * add rq to rbtree and fifo
 */
static void
ssd_add_request(struct request_queue *q, struct request *rq)
{
	struct ssd_data *dd = q->elevator->elevator_data;
	const int data_dir = rq_data_dir(rq);
	if(data_dir==READ) {
		list_add_tail(&rq->queuelist, &dd->read_queue);
	} else {
		ssd_add_write_rq_rb(dd, rq);

		/*
		 * set expire time and add to fifo list
		 */
		rq_set_fifo_time(rq, jiffies + dd->fifo_expire_write);
		list_add_tail(&rq->queuelist, &dd->fifo_list_write);
	}
}

/*
 * remove rq from rbtree and fifo.
 */
static void ssd_remove_write_request(struct request_queue *q, struct request *rq)
{
	struct ssd_data *dd = q->elevator->elevator_data;

	rq_fifo_clear(rq);
	ssd_del_write_rq_rb(dd, rq);
}

static void ssd_merged_request(struct request_queue *q,
				    struct request *req, int type)
{
	struct ssd_data *dd = q->elevator->elevator_data;

	/*
	 * if the merge was a front merge, we need to reposition request
	 */
	if (type == ELEVATOR_FRONT_MERGE && rq_data_dir(req)==WRITE) {
		elv_rb_del(&dd->sort_list_write, req);
		ssd_add_write_rq_rb(dd, req);
	}
}

static void
ssd_merged_requests(struct request_queue *q, struct request *req,
			 struct request *next)
{
	if(rq_data_dir(req)==READ) {
		list_del_init(&next->queuelist);
	} else {
		/*
		 * if next expires before rq, assign its expire time to rq
		 * and move into next position (next will be deleted) in fifo
		 */
		if (!list_empty(&req->queuelist) && !list_empty(&next->queuelist)) {
			if (time_before(rq_fifo_time(next), rq_fifo_time(req))) {
				list_move(&req->queuelist, &next->queuelist);
				rq_set_fifo_time(req, rq_fifo_time(next));
			}
		}

		/*
		 * kill knowledge of next, this one is a goner
		 */
		ssd_remove_write_request(q, next);
	}
}

/*
 * move request from sort list to dispatch queue.
 */
static inline void
ssd_move_write_to_dispatch(struct ssd_data *dd, struct request *rq)
{
	struct request_queue *q = rq->q;

	ssd_remove_write_request(q, rq);
	elv_dispatch_add_tail(q, rq);
}

/*
 * move an entry to dispatch queue
 */
static void
ssd_move_write_request(struct ssd_data *dd, struct request *rq)
{
	dd->next_rq_write = ssd_latter_write_request(rq);
	dd->last_sector = rq_end_sector(rq);

	/*
	 * take it off the sort and fifo list, move
	 * to dispatch queue
	 */
	ssd_move_write_to_dispatch(dd, rq);
}

/*
 * ssd_check_fifo_write returns 0 if there are no expired requests on the fifo,
 * 1 otherwise. Requires !list_empty(&dd->fifo_list[data_dir])
 */
static inline int ssd_check_fifo_write(struct ssd_data *dd)
{
	struct request *rq = rq_entry_fifo(dd->fifo_list_write.next);

	/*
	 * rq is expired!
	 */
	if (time_after(jiffies, rq_fifo_time(rq)))
		return 1;

	return 0;
}

/*
 * ssd_dispatch_requests selects the best request according to
 * read/write expire, fifo_batch, etc
 */
static int ssd_dispatch_requests(struct request_queue *q, int force)
{
	struct ssd_data *dd = q->elevator->elevator_data;
	const int reads = !list_empty(&dd->read_queue);
	const int writes = !list_empty(&dd->fifo_list_write);
	struct request *rq;

	if (reads) {
		if (writes && (dd->starved++ >= dd->writes_starved))
			goto dispatch_writes;

		rq = list_entry(dd->read_queue.next, struct request, queuelist);
		list_del_init(&rq->queuelist);
		elv_dispatch_sort(q, rq);
		return 1;
	}

	/*
	 * batches are currently reads XOR writes
	 */
	if (dd->next_rq_write)
		rq = dd->next_rq_write;

	if (rq && dd->batching < dd->fifo_batch)
		/* we have a next request are still entitled to batch */
		goto dispatch_request;

	/*
	 * there are either no reads or writes have been starved
	 */

	if (writes) {
dispatch_writes:
		BUG_ON(RB_EMPTY_ROOT(&dd->sort_list_write));
		dd->starved = 0;
		goto dispatch_find_request;
	}
	return 0;

dispatch_find_request:
	/*
	 * we are not running a batch, find best request for WRITE
	 */
	if (ssd_check_fifo_write(dd) || !dd->next_rq_write) {
		/*
		 * A deadline has expired, the last request was in the other
		 * direction, or we have run out of higher-sectored requests.
		 * Start again from the request with the earliest expiry time.
		 */
		rq = rq_entry_fifo(dd->fifo_list_write.next);
	} else {
		/*
		 * The last req was the same dir and we have a next request in
		 * sort order. No expired requests so continue on from here.
		 */
		rq = dd->next_rq_write;
	}

	dd->batching = 0;

dispatch_request:
	/*
	 * rq is the selected appropriate request.
	 */
	dd->batching++;
	ssd_move_write_request(dd, rq);

	return 1;
}

static int ssd_queue_empty(struct request_queue *q)
{
	struct ssd_data *dd = q->elevator->elevator_data;

	return list_empty(&dd->fifo_list_write)
		&& list_empty(&dd->read_queue);
}

static struct request * ssd_former_request(struct request_queue *q, struct request *rq)
{
	struct ssd_data *nd = q->elevator->elevator_data;
	const int data_dir = rq_data_dir(rq);
	if(data_dir==READ) {
		if (rq->queuelist.prev == &nd->read_queue)
			return NULL;
		return list_entry(rq->queuelist.prev, struct request, queuelist);
	} else {
		return elv_rb_former_request(q,rq);
	}
}

static struct request * ssd_latter_request(struct request_queue *q, struct request *rq)
{
	struct ssd_data *dd = q->elevator->elevator_data;
	const int data_dir = rq_data_dir(rq);
	if(data_dir==READ) {
		if (rq->queuelist.next == &dd->read_queue)
			return NULL;
		return list_entry(rq->queuelist.next, struct request, queuelist);
	} else {
		return elv_rb_latter_request(q,rq);
	}
}

static void ssd_exit_queue(struct elevator_queue *e)
{
	struct ssd_data *dd = e->elevator_data;

	BUG_ON(!list_empty(&dd->read_queue));
	BUG_ON(!list_empty(&dd->fifo_list_write));

	kfree(dd);
}

/*
 * initialize elevator private data (ssd_data).
 */
static void *ssd_init_queue(struct request_queue *q)
{
	struct ssd_data *dd;

	dd = kmalloc_node(sizeof(*dd), GFP_KERNEL | __GFP_ZERO, q->node);
	if (!dd)
		return NULL;

	INIT_LIST_HEAD(&dd->read_queue);
	INIT_LIST_HEAD(&dd->fifo_list_write);
	dd->sort_list_write = RB_ROOT;
	dd->fifo_expire_write = write_expire;
	dd->writes_starved = writes_starved;
	dd->fifo_batch = fifo_batch;
	return dd;
}

/*
 * sysfs parts below
 */

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

static ssize_t
ssd_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 ssd_data *dd = e->elevator_data;			\
	int __data = __VAR;						\
	if (__CONV)							\
		__data = jiffies_to_msecs(__data);			\
	return ssd_var_show(__data, (page));			\
}
SHOW_FUNCTION(ssd_write_expire_show, dd->fifo_expire_write, 1);
SHOW_FUNCTION(ssd_writes_starved_show, dd->writes_starved, 0);
SHOW_FUNCTION(ssd_fifo_batch_show, dd->fifo_batch, 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 ssd_data *dd = e->elevator_data;			\
	int __data;							\
	int ret = ssd_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(ssd_write_expire_store, &dd->fifo_expire_write, 0, INT_MAX, 1);
STORE_FUNCTION(ssd_writes_starved_store, &dd->writes_starved, INT_MIN, INT_MAX, 0);
STORE_FUNCTION(ssd_fifo_batch_store, &dd->fifo_batch, 0, INT_MAX, 0);
#undef STORE_FUNCTION

#define DD_ATTR(name) \
	__ATTR(name, S_IRUGO|S_IWUSR, ssd_##name##_show, \
				      ssd_##name##_store)

static struct elv_fs_entry ssd_attrs[] = {
	DD_ATTR(write_expire),
	DD_ATTR(writes_starved),
	DD_ATTR(fifo_batch),
	__ATTR_NULL
};

static struct elevator_type iosched_ssd = {
	.ops = {
		.elevator_merged_fn =		ssd_merged_request,
		.elevator_merge_req_fn =	ssd_merged_requests,
		.elevator_dispatch_fn =		ssd_dispatch_requests,
		.elevator_add_req_fn =		ssd_add_request,
		.elevator_queue_empty_fn =	ssd_queue_empty,
		.elevator_former_req_fn	=       ssd_former_request,
		.elevator_latter_req_fn	=       ssd_latter_request,
		.elevator_init_fn =		ssd_init_queue,
		.elevator_exit_fn =		ssd_exit_queue,
	},

	.elevator_attrs = ssd_attrs,
	.elevator_name = "ssd",
	.elevator_owner = THIS_MODULE,
};

static int __init ssd_init(void)
{
	elv_register(&iosched_ssd);

	return 0;
}

static void __exit ssd_exit(void)
{
	elv_unregister(&iosched_ssd);
}

module_init(ssd_init);
module_exit(ssd_exit);

MODULE_AUTHOR("Corrado Zoccolo");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("ssd IO scheduler");