Skip to content

Commit

Permalink
readahead: sanify file_ra_state names
Browse files Browse the repository at this point in the history 
Rename some file_ra_state variables and remove some accessors.

It results in much simpler code.
Kudos to Rusty!

Signed-off-by: Fengguang Wu <[email protected]>
Cc: Rusty Russell <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
  • Loading branch information
Fengguang Wu authored and Linus Torvalds committed Jul 19, 2007
1 parent cf914a7 commit f9acc8c
Show file tree
Hide file tree
Showing 2 changed files with 31 additions and 98 deletions.
61 changes: 6 additions & 55 deletions include/linux/fs.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -695,16 +695,12 @@ struct fown_struct {

/*
* Track a single file's readahead state
*
* ================#============|==================#==================|
* ^ ^ ^ ^
* file_ra_state.la_index .ra_index .lookahead_index .readahead_index
*/
struct file_ra_state {
pgoff_t la_index; /* enqueue time */
pgoff_t ra_index; /* begin offset */
pgoff_t lookahead_index; /* time to do next readahead */
pgoff_t readahead_index; /* end offset */
pgoff_t start; /* where readahead started */
unsigned long size; /* # of readahead pages */
unsigned long async_size; /* do asynchronous readahead when
there are only # of pages ahead */

unsigned long ra_pages; /* Maximum readahead window */
unsigned long mmap_hit; /* Cache hit stat for mmap accesses */
Expand All @@ -713,60 +709,15 @@ struct file_ra_state {
unsigned int prev_offset; /* Offset where last read() ended in a page */
};

/*
* Measuring read-ahead sizes.
*
* |----------- readahead size ------------>|
* ===#============|==================#=====================|
* |------- invoke interval ------>|-- lookahead size -->|
*/
static inline unsigned long ra_readahead_size(struct file_ra_state *ra)
{
return ra->readahead_index - ra->ra_index;
}

static inline unsigned long ra_lookahead_size(struct file_ra_state *ra)
{
return ra->readahead_index - ra->lookahead_index;
}

static inline unsigned long ra_invoke_interval(struct file_ra_state *ra)
{
return ra->lookahead_index - ra->la_index;
}

/*
* Check if @index falls in the readahead windows.
*/
static inline int ra_has_index(struct file_ra_state *ra, pgoff_t index)
{
return (index >= ra->la_index &&
index < ra->readahead_index);
}

/*
* Where is the old read-ahead and look-ahead?
*/
static inline void ra_set_index(struct file_ra_state *ra,
pgoff_t la_index, pgoff_t ra_index)
{
ra->la_index = la_index;
ra->ra_index = ra_index;
return (index >= ra->start &&
index < ra->start + ra->size);
}

/*
* Where is the new read-ahead and look-ahead?
*/
static inline void ra_set_size(struct file_ra_state *ra,
unsigned long ra_size, unsigned long la_size)
{
ra->readahead_index = ra->ra_index + ra_size;
ra->lookahead_index = ra->ra_index + ra_size - la_size;
}

unsigned long ra_submit(struct file_ra_state *ra,
struct address_space *mapping, struct file *filp);

struct file {
/*
* fu_list becomes invalid after file_free is called and queued via
Expand Down
68 changes: 25 additions & 43 deletions mm/readahead.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -253,21 +253,16 @@ unsigned long max_sane_readahead(unsigned long nr)
/*
* Submit IO for the read-ahead request in file_ra_state.
*/
unsigned long ra_submit(struct file_ra_state *ra,
static unsigned long ra_submit(struct file_ra_state *ra,
struct address_space *mapping, struct file *filp)
{
unsigned long ra_size;
unsigned long la_size;
int actual;

ra_size = ra_readahead_size(ra);
la_size = ra_lookahead_size(ra);
actual = __do_page_cache_readahead(mapping, filp,
ra->ra_index, ra_size, la_size);
ra->start, ra->size, ra->async_size);

return actual;
}
EXPORT_SYMBOL_GPL(ra_submit);

/*
* Set the initial window size, round to next power of 2 and square
Expand Down Expand Up @@ -296,7 +291,7 @@ static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
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 cur = ra->size;
unsigned long newsize;

if (cur < max / 16)
Expand All @@ -313,28 +308,21 @@ static unsigned long get_next_ra_size(struct file_ra_state *ra,
* The fields in struct file_ra_state represent the most-recently-executed
* readahead attempt:
*
* |-------- last readahead window -------->|
* |-- application walking here -->|
* ======#============|==================#=====================|
* ^la_index ^ra_index ^lookahead_index ^readahead_index
*
* [ra_index, readahead_index) represents the last readahead window.
*
* [la_index, lookahead_index] is where the application would be walking(in
* the common case of cache-cold sequential reads): the last window was
* established when the application was at la_index, and the next window will
* be bring in when the application reaches lookahead_index.
* |<----- async_size ---------|
* |------------------- size -------------------->|
* |==================#===========================|
* ^start ^page marked with PG_readahead
*
* To overlap application thinking time and disk I/O time, we do
* `readahead pipelining': Do not wait until the application consumed all
* readahead pages and stalled on the missing page at readahead_index;
* Instead, submit an asynchronous readahead I/O as early as the application
* reads on the page at lookahead_index. Normally lookahead_index will be
* equal to ra_index, for maximum pipelining.
* Instead, submit an asynchronous readahead I/O as soon as there are
* only async_size pages left in the readahead window. Normally async_size
* will be equal to size, for maximum pipelining.
*
* In interleaved sequential reads, concurrent streams on the same fd can
* be invalidating each other's readahead state. So we flag the new readahead
* page at lookahead_index with PG_readahead, and use it as readahead
* page at (start+size-async_size) with PG_readahead, and use it as readahead
* indicator. The flag won't be set on already cached pages, to avoid the
* readahead-for-nothing fuss, saving pointless page cache lookups.
*
Expand Down Expand Up @@ -363,24 +351,21 @@ ondemand_readahead(struct address_space *mapping,
unsigned long req_size)
{
unsigned long max; /* max readahead pages */
pgoff_t ra_index; /* readahead index */
unsigned long ra_size; /* readahead size */
unsigned long la_size; /* lookahead size */
int sequential;

max = ra->ra_pages;
sequential = (offset - ra->prev_index <= 1UL) || (req_size > max);

/*
* Lookahead/readahead hit, assume sequential access.
* It's the expected callback offset, assume sequential access.
* Ramp up sizes, and push forward the readahead window.
*/
if (offset && (offset == ra->lookahead_index ||
offset == ra->readahead_index)) {
ra_index = ra->readahead_index;
ra_size = get_next_ra_size(ra, max);
la_size = ra_size;
goto fill_ra;
if (offset && (offset == (ra->start + ra->size - ra->async_size) ||
offset == (ra->start + ra->size))) {
ra->start += ra->size;
ra->size = get_next_ra_size(ra, max);
ra->async_size = ra->size;
goto readit;
}

/*
Expand All @@ -399,24 +384,21 @@ ondemand_readahead(struct address_space *mapping,
* - oversize random read
* Start readahead for it.
*/
ra_index = offset;
ra_size = get_init_ra_size(req_size, max);
la_size = ra_size > req_size ? ra_size - req_size : ra_size;
ra->start = offset;
ra->size = get_init_ra_size(req_size, max);
ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;

/*
* Hit on a lookahead page without valid readahead state.
* Hit on a marked page without valid readahead state.
* E.g. interleaved reads.
* Not knowing its readahead pos/size, bet on the minimal possible one.
*/
if (hit_readahead_marker) {
ra_index++;
ra_size = min(4 * ra_size, max);
ra->start++;
ra->size = get_next_ra_size(ra, max);
}

fill_ra:
ra_set_index(ra, offset, ra_index);
ra_set_size(ra, ra_size, la_size);

readit:
return ra_submit(ra, mapping, filp);
}

Expand Down

0 comments on commit f9acc8c

Please sign in to comment.