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Subject[PATCH 19/19] io: avoid too much latency from read-ahead
Since the VM_MAX_READAHEAD is greatly enlarged and the algorithm become more
complex, it may be necessary to insert some cond_resched() calls in the
read-ahead path.

----------------------------------------------------------------------------
If you feel more latency with the new read-ahead code, the cause can
either be the complex read-ahead code, or some low level routines not ready
to support the larger default 1M read-ahead size. It can be cleared out with
the following commands:

dd if=/dev/zero of=sparse bs=1M seek=10000 count=1
cp sparse /dev/null

If the above copy does not lead to audio jitters, then it's an low level
issue. It's hard to find them out for now, though there's a workaround:

blockdev --setra 256 /dev/hda # or whatever device you are using

Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
---


This is recommended by Con Kolivas to improve respond time for desktop.

fs/mpage.c | 4 +++-
mm/readahead.c | 16 +++++++++++++++-
2 files changed, 18 insertions(+), 2 deletions(-)

--- linux-2.6.15-rc2-mm1.orig/mm/readahead.c
+++ linux-2.6.15-rc2-mm1/mm/readahead.c
@@ -368,6 +368,7 @@ static void collect_aging_info(void)
page_aging = aging_total();
smooth_aging = node_readahead_aging();

+ cond_resched();
spin_lock_irq(&aging_info_lock);

for (i = AGING_INFO_SHIFTS - 1; i >= 0; i--) {
@@ -625,8 +626,10 @@ static int read_pages(struct address_spa
page->index, GFP_KERNEL)) {
ret = mapping->a_ops->readpage(filp, page);
if (ret != AOP_TRUNCATED_PAGE) {
- if (!pagevec_add(&lru_pvec, page))
+ if (!pagevec_add(&lru_pvec, page)) {
__pagevec_lru_add(&lru_pvec);
+ cond_resched();
+ }
continue;
} /* else fall through to release */
}
@@ -746,6 +749,7 @@ __do_page_cache_readahead(struct address
}

read_unlock_irq(&mapping->tree_lock);
+ cond_resched();
page = page_cache_alloc_cold(mapping);
read_lock_irq(&mapping->tree_lock);
if (!page)
@@ -1145,6 +1149,7 @@ static int rescue_pages(struct page *pag

spin_unlock_irq(&zone->lru_lock);

+ cond_resched();
page = find_page(mapping, index);
if (!page)
goto out;
@@ -1219,6 +1224,7 @@ static unsigned long node_readahead_agin
unsigned long sum = 0;
cpumask_t mask = node_to_cpumask(numa_node_id());

+ cond_resched();
for_each_cpu_mask(cpu, mask)
sum += per_cpu(readahead_aging, cpu);

@@ -1349,6 +1355,7 @@ static int ra_dispatch(struct file_ra_st
int actual;
enum ra_class ra_class;

+ cond_resched();
ra_class = (ra->flags & RA_CLASS_MASK);
BUG_ON(ra_class == 0 || ra_class > RA_CLASS_END);

@@ -1371,6 +1378,7 @@ static int ra_dispatch(struct file_ra_st

actual = __do_page_cache_readahead(mapping, filp,
ra->ra_index, ra_size, la_size);
+ cond_resched();

#ifdef READAHEAD_STREAMING
if (actual < ra_size) {
@@ -1609,6 +1617,7 @@ static int count_cache_hit(struct addres
int count = 0;
int i;

+ cond_resched();
read_lock_irq(&mapping->tree_lock);

/*
@@ -1646,6 +1655,7 @@ static int query_page_cache(struct addre
* Scan backward and check the near @ra_max pages.
* The count here determines ra_size.
*/
+ cond_resched();
read_lock_irq(&mapping->tree_lock);
index = radix_tree_lookup_head(&mapping->page_tree, offset, ra_max);
read_unlock_irq(&mapping->tree_lock);
@@ -1691,6 +1701,7 @@ static int query_page_cache(struct addre
if (nr_lookback > offset)
nr_lookback = offset;

+ cond_resched();
radix_tree_cache_init(&cache);
read_lock_irq(&mapping->tree_lock);
for (count += ra_max; count < nr_lookback; count += ra_max) {
@@ -1737,6 +1748,7 @@ static inline pgoff_t first_absent_page_
if (max_scan > index)
max_scan = index;

+ cond_resched();
radix_tree_cache_init(&cache);
read_lock_irq(&mapping->tree_lock);
for (; origin - index <= max_scan;) {
@@ -1760,6 +1772,7 @@ static inline pgoff_t first_absent_page(
{
pgoff_t ra_index;

+ cond_resched();
read_lock_irq(&mapping->tree_lock);
ra_index = radix_tree_lookup_tail(&mapping->page_tree,
index + 1, max_scan);
@@ -2469,6 +2482,7 @@ save_chunk:
n <= 3)
ret += save_chunk(chunk_head, live_head, page, save_list);

+ cond_resched();
if (&page->lru != page_list)
goto next_chunk;

--- linux-2.6.15-rc2-mm1.orig/fs/mpage.c
+++ linux-2.6.15-rc2-mm1/fs/mpage.c
@@ -343,8 +343,10 @@ mpage_readpages(struct address_space *ma
bio = do_mpage_readpage(bio, page,
nr_pages - page_idx,
&last_block_in_bio, get_block);
- if (!pagevec_add(&lru_pvec, page))
+ if (!pagevec_add(&lru_pvec, page)) {
__pagevec_lru_add(&lru_pvec);
+ cond_resched();
+ }
} else {
page_cache_release(page);
}
--
-
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