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]>
---
include/linux/fs.h | 61 +++-----------------------------------
mm/readahead.c | 68 +++++++++++++++----------------------------
2 files changed, 31 insertions(+), 98 deletions(-)
--- linux-2.6.22-rc4-mm2.orig/include/linux/fs.h
+++ linux-2.6.22-rc4-mm2/include/linux/fs.h
@@ -768,16 +768,12 @@ 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 */
@@ -787,59 +783,14 @@ struct file_ra_state {
};
/*
- * 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
--- linux-2.6.22-rc4-mm2.orig/mm/readahead.c
+++ linux-2.6.22-rc4-mm2/mm/readahead.c
@@ -245,21 +245,16 @@ unsigned long max_sane_readahead(unsigne
/*
* 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
@@ -288,7 +283,7 @@ static unsigned long get_init_ra_size(un
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)
@@ -305,28 +300,21 @@ static unsigned long get_next_ra_size(st
* 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.
*
@@ -355,24 +343,21 @@ ondemand_readahead(struct address_space
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;
}
/*
@@ -391,24 +376,21 @@ ondemand_readahead(struct address_space
* - 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);
}
--
On Thu, 2007-06-14 at 14:21 +0800, Fengguang Wu wrote:
> plain text document attachment (readahead-rename.patch)
> 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]>
Needless to say, I'm now very satisfied with the new readahead code.
That's not to say there won't be corner cases where the old code did
better, but given the replacement's simplicity and Fengguang's positive
benchmark results I really like it.
Thanks for Fengguang for coding and pushing this!
Rusty.