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readahead: remove the old algorithm 

Remove the old readahead algorithm.

Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
Cc: Steven Pratt <slpratt@austin.ibm.com>
Cc: Ram Pai <linuxram@us.ibm.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
hifive-unleashed-5.1
Fengguang Wu 2007-07-19 01:48:04 -07:00 committed by Linus Torvalds
parent dc7868fcb9
commit c743d96b6d
3 changed files with 26 additions and 365 deletions
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11
include/linux/fs.h
View File

@ -701,14 +701,6 @@ struct fown_struct {
* file_ra_state.la_index .ra_index .lookahead_index .readahead_index
*/
struct file_ra_state {
unsigned long start; /* Current window */
unsigned long size;
unsigned long flags; /* ra flags RA_FLAG_xxx*/
unsigned long cache_hit; /* cache hit count*/
unsigned long prev_index; /* Cache last read() position */
unsigned long ahead_start; /* Ahead window */
unsigned long ahead_size;
pgoff_t la_index; /* enqueue time */
pgoff_t ra_index; /* begin offset */
pgoff_t lookahead_index; /* time to do next readahead */
@ -717,10 +709,9 @@ struct file_ra_state {
unsigned long ra_pages; /* Maximum readahead window */
unsigned long mmap_hit; /* Cache hit stat for mmap accesses */
unsigned long mmap_miss; /* Cache miss stat for mmap accesses */
unsigned long prev_index; /* Cache last read() position */
unsigned int prev_offset; /* Offset where last read() ended in a page */
};
#define RA_FLAG_MISS 0x01 /* a cache miss occured against this file */
#define RA_FLAG_INCACHE 0x02 /* file is already in cache */
/*
* Measuring read-ahead sizes.

7
include/linux/mm.h
View File

@ -1144,13 +1144,6 @@ unsigned long page_cache_readahead_ondemand(struct address_space *mapping,
struct page *page,
pgoff_t offset,
unsigned long size);
unsigned long page_cache_readahead(struct address_space *mapping,
struct file_ra_state *ra,
struct file *filp,
pgoff_t offset,
unsigned long size);
void handle_ra_miss(struct address_space *mapping,
struct file_ra_state *ra, pgoff_t offset);
unsigned long max_sane_readahead(unsigned long nr);
/* Do stack extension */

373
mm/readahead.c
View File

@ -49,82 +49,6 @@ file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
}
EXPORT_SYMBOL_GPL(file_ra_state_init);
/*
* Return max readahead size for this inode in number-of-pages.
*/
static inline unsigned long get_max_readahead(struct file_ra_state *ra)
{
return ra->ra_pages;
}
static inline unsigned long get_min_readahead(struct file_ra_state *ra)
{
return MIN_RA_PAGES;
}
static inline void reset_ahead_window(struct file_ra_state *ra)
{
/*
* ... but preserve ahead_start + ahead_size value,
* see 'recheck:' label in page_cache_readahead().
* Note: We never use ->ahead_size as rvalue without
* checking ->ahead_start != 0 first.
*/
ra->ahead_size += ra->ahead_start;
ra->ahead_start = 0;
}
static inline void ra_off(struct file_ra_state *ra)
{
ra->start = 0;
ra->flags = 0;
ra->size = 0;
reset_ahead_window(ra);
return;
}
/*
* Set the initial window size, round to next power of 2 and square
* for small size, x 4 for medium, and x 2 for large
* for 128k (32 page) max ra
* 1-8 page = 32k initial, > 8 page = 128k initial
*/
static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
{
unsigned long newsize = roundup_pow_of_two(size);
if (newsize <= max / 32)
newsize = newsize * 4;
else if (newsize <= max / 4)
newsize = newsize * 2;
else
newsize = max;
return newsize;
}
/*
* Set the new window size, this is called only when I/O is to be submitted,
* not for each call to readahead. If a cache miss occured, reduce next I/O
* size, else increase depending on how close to max we are.
*/
static inline unsigned long get_next_ra_size(struct file_ra_state *ra)
{
unsigned long max = get_max_readahead(ra);
unsigned long min = get_min_readahead(ra);
unsigned long cur = ra->size;
unsigned long newsize;
if (ra->flags & RA_FLAG_MISS) {
ra->flags &= ~RA_FLAG_MISS;
newsize = max((cur - 2), min);
} else if (cur < max / 16) {
newsize = 4 * cur;
} else {
newsize = 2 * cur;
}
return min(newsize, max);
}
#define list_to_page(head) (list_entry((head)->prev, struct page, lru))
/**
@ -200,66 +124,6 @@ out:
return ret;
}
/*
* Readahead design.
*
* The fields in struct file_ra_state represent the most-recently-executed
* readahead attempt:
*
* start: Page index at which we started the readahead
* size: Number of pages in that read
* Together, these form the "current window".
* Together, start and size represent the `readahead window'.
* prev_index: The page which the readahead algorithm most-recently inspected.
* It is mainly used to detect sequential file reading.
* If page_cache_readahead sees that it is again being called for
* a page which it just looked at, it can return immediately without
* making any state changes.
* offset: Offset in the prev_index where the last read ended - used for
* detection of sequential file reading.
* ahead_start,
* ahead_size: Together, these form the "ahead window".
* ra_pages: The externally controlled max readahead for this fd.
*
* When readahead is in the off state (size == 0), readahead is disabled.
* In this state, prev_index is used to detect the resumption of sequential I/O.
*
* The readahead code manages two windows - the "current" and the "ahead"
* windows. The intent is that while the application is walking the pages
* in the current window, I/O is underway on the ahead window. When the
* current window is fully traversed, it is replaced by the ahead window
* and the ahead window is invalidated. When this copying happens, the
* new current window's pages are probably still locked. So
* we submit a new batch of I/O immediately, creating a new ahead window.
*
* So:
*
* ----|----------------|----------------|-----
* ^start ^start+size
* ^ahead_start ^ahead_start+ahead_size
*
* ^ When this page is read, we submit I/O for the
* ahead window.
*
* A `readahead hit' occurs when a read request is made against a page which is
* the next sequential page. Ahead window calculations are done only when it
* is time to submit a new IO. The code ramps up the size agressively at first,
* but slow down as it approaches max_readhead.
*
* Any seek/ramdom IO will result in readahead being turned off. It will resume
* at the first sequential access.
*
* There is a special-case: if the first page which the application tries to
* read happens to be the first page of the file, it is assumed that a linear
* read is about to happen and the window is immediately set to the initial size
* based on I/O request size and the max_readahead.
*
* This function is to be called for every read request, rather than when
* it is time to perform readahead. It is called only once for the entire I/O
* regardless of size unless readahead is unable to start enough I/O to satisfy
* the request (I/O request > max_readahead).
*/
/*
* do_page_cache_readahead actually reads a chunk of disk. It allocates all
* the pages first, then submits them all for I/O. This avoids the very bad
@ -295,7 +159,7 @@ __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
read_lock_irq(&mapping->tree_lock);
for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
pgoff_t page_offset = offset + page_idx;
if (page_offset > end_index)
break;
@ -360,28 +224,6 @@ int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
return ret;
}
/*
* Check how effective readahead is being. If the amount of started IO is
* less than expected then the file is partly or fully in pagecache and
* readahead isn't helping.
*
*/
static inline int check_ra_success(struct file_ra_state *ra,
unsigned long nr_to_read, unsigned long actual)
{
if (actual == 0) {
ra->cache_hit += nr_to_read;
if (ra->cache_hit >= VM_MAX_CACHE_HIT) {
ra_off(ra);
ra->flags |= RA_FLAG_INCACHE;
return 0;
}
} else {
ra->cache_hit=0;
}
return 1;
}
/*
* This version skips the IO if the queue is read-congested, and will tell the
* block layer to abandon the readahead if request allocation would block.
@ -398,191 +240,6 @@ int do_page_cache_readahead(struct address_space *mapping, struct file *filp,
return __do_page_cache_readahead(mapping, filp, offset, nr_to_read, 0);
}
/*
* Read 'nr_to_read' pages starting at page 'offset'. If the flag 'block'
* is set wait till the read completes. Otherwise attempt to read without
* blocking.
* Returns 1 meaning 'success' if read is successful without switching off
* readahead mode. Otherwise return failure.
*/
static int
blockable_page_cache_readahead(struct address_space *mapping, struct file *filp,
pgoff_t offset, unsigned long nr_to_read,
struct file_ra_state *ra, int block)
{
int actual;
if (!block && bdi_read_congested(mapping->backing_dev_info))
return 0;
actual = __do_page_cache_readahead(mapping, filp, offset, nr_to_read, 0);
return check_ra_success(ra, nr_to_read, actual);
}
static int make_ahead_window(struct address_space *mapping, struct file *filp,
struct file_ra_state *ra, int force)
{
int block, ret;
ra->ahead_size = get_next_ra_size(ra);
ra->ahead_start = ra->start + ra->size;
block = force || (ra->prev_index >= ra->ahead_start);
ret = blockable_page_cache_readahead(mapping, filp,
ra->ahead_start, ra->ahead_size, ra, block);
if (!ret && !force) {
/* A read failure in blocking mode, implies pages are
* all cached. So we can safely assume we have taken
* care of all the pages requested in this call.
* A read failure in non-blocking mode, implies we are
* reading more pages than requested in this call. So
* we safely assume we have taken care of all the pages
* requested in this call.
*
* Just reset the ahead window in case we failed due to
* congestion. The ahead window will any way be closed
* in case we failed due to excessive page cache hits.
*/
reset_ahead_window(ra);
}
return ret;
}
/**
* page_cache_readahead - generic adaptive readahead
* @mapping: address_space which holds the pagecache and I/O vectors
* @ra: file_ra_state which holds the readahead state
* @filp: passed on to ->readpage() and ->readpages()
* @offset: start offset into @mapping, in PAGE_CACHE_SIZE units
* @req_size: hint: total size of the read which the caller is performing in
* PAGE_CACHE_SIZE units
*
* page_cache_readahead() is the main function. It performs the adaptive
* readahead window size management and submits the readahead I/O.
*
* Note that @filp is purely used for passing on to the ->readpage[s]()
* handler: it may refer to a different file from @mapping (so we may not use
* @filp->f_mapping or @filp->f_path.dentry->d_inode here).
* Also, @ra may not be equal to &@filp->f_ra.
*
*/
unsigned long
page_cache_readahead(struct address_space *mapping, struct file_ra_state *ra,
struct file *filp, pgoff_t offset, unsigned long req_size)
{
unsigned long max, newsize;
int sequential;
/*
* We avoid doing extra work and bogusly perturbing the readahead
* window expansion logic.
*/
if (offset == ra->prev_index && --req_size)
++offset;
/* Note that prev_index == -1 if it is a first read */
sequential = (offset == ra->prev_index + 1);
ra->prev_index = offset;
ra->prev_offset = 0;
max = get_max_readahead(ra);
newsize = min(req_size, max);
/* No readahead or sub-page sized read or file already in cache */
if (newsize == 0 || (ra->flags & RA_FLAG_INCACHE))
goto out;
ra->prev_index += newsize - 1;
/*
* Special case - first read at start of file. We'll assume it's
* a whole-file read and grow the window fast. Or detect first
* sequential access
*/
if (sequential && ra->size == 0) {
ra->size = get_init_ra_size(newsize, max);
ra->start = offset;
if (!blockable_page_cache_readahead(mapping, filp, offset,
ra->size, ra, 1))
goto out;
/*
* If the request size is larger than our max readahead, we
* at least want to be sure that we get 2 IOs in flight and
* we know that we will definitly need the new I/O.
* once we do this, subsequent calls should be able to overlap
* IOs,* thus preventing stalls. so issue the ahead window
* immediately.
*/
if (req_size >= max)
make_ahead_window(mapping, filp, ra, 1);
goto out;
}
/*
* Now handle the random case:
* partial page reads and first access were handled above,
* so this must be the next page otherwise it is random
*/
if (!sequential) {
ra_off(ra);
blockable_page_cache_readahead(mapping, filp, offset,
newsize, ra, 1);
goto out;
}
/*
* If we get here we are doing sequential IO and this was not the first
* occurence (ie we have an existing window)
*/
if (ra->ahead_start == 0) { /* no ahead window yet */
if (!make_ahead_window(mapping, filp, ra, 0))
goto recheck;
}
/*
* Already have an ahead window, check if we crossed into it.
* If so, shift windows and issue a new ahead window.
* Only return the #pages that are in the current window, so that
* we get called back on the first page of the ahead window which
* will allow us to submit more IO.
*/
if (ra->prev_index >= ra->ahead_start) {
ra->start = ra->ahead_start;
ra->size = ra->ahead_size;
make_ahead_window(mapping, filp, ra, 0);
recheck:
/* prev_index shouldn't overrun the ahead window */
ra->prev_index = min(ra->prev_index,
ra->ahead_start + ra->ahead_size - 1);
}
out:
return ra->prev_index + 1;
}
EXPORT_SYMBOL_GPL(page_cache_readahead);
/*
* handle_ra_miss() is called when it is known that a page which should have
* been present in the pagecache (we just did some readahead there) was in fact
* not found. This will happen if it was evicted by the VM (readahead
* thrashing)
*
* Turn on the cache miss flag in the RA struct, this will cause the RA code
* to reduce the RA size on the next read.
*/
void handle_ra_miss(struct address_space *mapping,
struct file_ra_state *ra, pgoff_t offset)
{
ra->flags |= RA_FLAG_MISS;
ra->flags &= ~RA_FLAG_INCACHE;
ra->cache_hit = 0;
}
/*
* Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
* sensible upper limit.
@ -612,20 +269,40 @@ unsigned long ra_submit(struct file_ra_state *ra,
}
EXPORT_SYMBOL_GPL(ra_submit);
/*
* Set the initial window size, round to next power of 2 and square
* for small size, x 4 for medium, and x 2 for large
* for 128k (32 page) max ra
* 1-8 page = 32k initial, > 8 page = 128k initial
*/
static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
{
unsigned long newsize = roundup_pow_of_two(size);
if (newsize <= max / 32)
newsize = newsize * 4;
else if (newsize <= max / 4)
newsize = newsize * 2;
else
newsize = max;
return newsize;
}
/*
* Get the previous window size, ramp it up, and
* return it as the new window size.
*/
static unsigned long get_next_ra_size2(struct file_ra_state *ra,
static unsigned long get_next_ra_size(struct file_ra_state *ra,
unsigned long max)
{
unsigned long cur = ra->readahead_index - ra->ra_index;
unsigned long newsize;
if (cur < max / 16)
newsize = cur * 4;
newsize = 4 * cur;
else
newsize = cur * 2;
newsize = 2 * cur;
return min(newsize, max);
}
@ -701,7 +378,7 @@ ondemand_readahead(struct address_space *mapping,
if (offset && (offset == ra->lookahead_index ||
offset == ra->readahead_index)) {
ra_index = ra->readahead_index;
ra_size = get_next_ra_size2(ra, max);
ra_size = get_next_ra_size(ra, max);
la_size = ra_size;
goto fill_ra;
}