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SubjectRe: SSD and IO schedulers
From
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
--------------------------------------------------------------------------
The self-confidence of a warrior is not the self-confidence of the average
man. The average man seeks certainty in the eyes of the onlooker and calls
that self-confidence. The warrior seeks impeccability in his own eyes and
calls that humbleness.
Tales of Power - C. Castaneda
/* -*- 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");
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