[PATCH 08/16] readahead: state based method

[Wu Fengguang <wfg@mail.ustc.edu.cn>]

[-- Attachment #1: readahead-method-stateful.patch --]
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This is the fast code path.

Major steps:
        - estimate a thrashing safe ra_size;
        - assemble the next read-ahead request in file_ra_state;
        - submit it.

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

 include/linux/fs.h |    8 +
 mm/readahead.c     |  340 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 mm/swap.c          |    3 
 mm/vmscan.c        |    5 
 4 files changed, 355 insertions(+), 1 deletion(-)

--- linux-2.6.14-mm1.orig/include/linux/fs.h
+++ linux-2.6.14-mm1/include/linux/fs.h
@@ -569,13 +569,19 @@ 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*/
+	uint64_t      cache_hit;	/* cache hit count*/
 	unsigned long prev_page;	/* Cache last read() position */
 	unsigned long ahead_start;	/* Ahead window */
 	unsigned long ahead_size;
 	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 age;
+	pgoff_t la_index;
+	pgoff_t ra_index;
+	pgoff_t lookahead_index;
+	pgoff_t readahead_index;
 };
 #define RA_FLAG_MISS 0x01	/* a cache miss occured against this file */
 #define RA_FLAG_INCACHE 0x02	/* file is already in cache */
--- linux-2.6.14-mm1.orig/mm/vmscan.c
+++ linux-2.6.14-mm1/mm/vmscan.c
@@ -406,6 +406,8 @@ cannot_free:
 	return 0;
 }
 
+DECLARE_PER_CPU(unsigned long, smooth_aging);
+
 /*
  * shrink_list adds the number of reclaimed pages to sc->nr_reclaimed
  */
@@ -453,6 +455,8 @@ static int shrink_list(struct list_head 
 		/* In active use or really unfreeable?  Activate it. */
 		if (referenced && page_mapping_inuse(page))
 			goto activate_locked;
+		if (!referenced)
+			__get_cpu_var(smooth_aging)++;
 
 #ifdef CONFIG_SWAP
 		/*
@@ -991,6 +995,7 @@ refill_inactive_zone(struct zone *zone, 
 				list_add(&page->lru, &l_active);
 				continue;
 			}
+			__get_cpu_var(smooth_aging)++;
 		}
 		list_add(&page->lru, &l_inactive);
 	}
--- linux-2.6.14-mm1.orig/mm/swap.c
+++ linux-2.6.14-mm1/mm/swap.c
@@ -112,6 +112,8 @@ void fastcall activate_page(struct page 
 	spin_unlock_irq(&zone->lru_lock);
 }
 
+DECLARE_PER_CPU(unsigned long, smooth_aging);
+
 /*
  * Mark a page as having seen activity.
  *
@@ -126,6 +128,7 @@ void fastcall mark_page_accessed(struct 
 		ClearPageReferenced(page);
 	} else if (!PageReferenced(page)) {
 		SetPageReferenced(page);
+		__get_cpu_var(smooth_aging)++;
 	}
 }
 
--- linux-2.6.14-mm1.orig/mm/readahead.c
+++ linux-2.6.14-mm1/mm/readahead.c
@@ -38,6 +38,12 @@ EXPORT_SYMBOL(readahead_hit_rate);
 int readahead_live_chunk = 2 * MAX_RA_PAGES;
 EXPORT_SYMBOL(readahead_live_chunk);
 
+/* Analog to zone->nr_page_aging.
+ * But mainly increased on fresh page references, so is much more smoother.
+ */
+DEFINE_PER_CPU(unsigned long, smooth_aging);
+EXPORT_PER_CPU_SYMBOL(smooth_aging);
+
 /* Detailed classification of read-ahead behaviors. */
 #define RA_CLASS_SHIFT 3
 #define RA_CLASS_MASK  ((1 << RA_CLASS_SHIFT) - 1)
@@ -789,6 +795,340 @@ out:
 }
 
 /*
+ * State based calculation of read-ahead request.
+ *
+ * This figure shows the meaning of file_ra_state members:
+ *
+ *             chunk A                            chunk B
+ *  +---------------------------+-------------------------------------------+
+ *  |             #             |                   #                       |
+ *  +---------------------------+-------------------------------------------+
+ *                ^             ^                   ^                       ^
+ *              la_index      ra_index     lookahead_index         readahead_index
+ */
+
+/*
+ * The global effective length of the inactive_list(s).
+ */
+static unsigned long nr_free_inactive(void)
+{
+	unsigned int i;
+	unsigned long sum = 0;
+	struct zone *zones = NODE_DATA(numa_node_id())->node_zones;
+
+	for (i = 0; i < MAX_NR_ZONES; i++)
+		sum += zones[i].nr_inactive +
+			zones[i].free_pages - zones[i].pages_low;
+
+	return sum;
+}
+
+/*
+ * A much smoother analog to nr_page_aging.
+ */
+static unsigned long nr_smooth_aging(void)
+{
+	unsigned long cpu;
+	unsigned long sum = 0;
+	cpumask_t mask = node_to_cpumask(numa_node_id());
+
+	for_each_cpu_mask(cpu, mask)
+		sum += per_cpu(smooth_aging, cpu);
+
+	return sum;
+}
+
+/*
+ * Set class of read-ahead
+ */
+static inline void set_ra_class(struct file_ra_state *ra,
+				enum ra_class ra_class)
+{
+	unsigned long FLAGS_MASK;
+	unsigned long flags;
+	unsigned long old_ra_class;
+
+	FLAGS_MASK = ~(RA_CLASS_MASK | (RA_CLASS_MASK << RA_CLASS_SHIFT));
+	flags = ra->flags & FLAGS_MASK;
+
+	old_ra_class = (ra->flags & RA_CLASS_MASK) << RA_CLASS_SHIFT;
+
+	ra->flags = flags | old_ra_class | ra_class;
+}
+
+/*
+ * The 64bit cache_hit stores three accumulated value and one counter value.
+ * MSB                                                                   LSB
+ * 3333333333333333 : 2222222222222222 : 1111111111111111 : 0000000000000000
+ */
+static inline int ra_cache_hit(struct file_ra_state *ra, int nr)
+{
+	return (ra->cache_hit >> (nr * 16)) & 0xFFFF;
+}
+
+/*
+ * Something like:
+ * ra_cache_hit(ra, 1) += ra_cache_hit(ra, 0);
+ * ra_cache_hit(ra, 0) = 0;
+ */
+static inline void ra_addup_cache_hit(struct file_ra_state *ra)
+{
+	int n;
+
+	n = ra_cache_hit(ra, 0);
+	ra->cache_hit -= n;
+	n <<= 16;
+	ra->cache_hit += n;
+}
+
+/*
+ * The read-ahead is deemed success if cache-hit-rate > 50%.
+ */
+static inline int ra_cache_hit_ok(struct file_ra_state *ra)
+{
+	return ra_cache_hit(ra, 0) * readahead_hit_rate >=
+					(ra->lookahead_index - ra->la_index);
+}
+
+/*
+ * Check if @index falls in the ra request.
+ */
+static inline int ra_has_index(struct file_ra_state *ra, pgoff_t index)
+{
+	if (index < ra->la_index || index >= ra->readahead_index)
+		return 0;
+
+	if (index >= ra->ra_index)
+		return 1;
+	else
+		return -1;
+}
+
+/*
+ * Prepare file_ra_state for a new read-ahead sequence.
+ */
+static inline void ra_state_init(struct file_ra_state *ra,
+				pgoff_t la_index, pgoff_t ra_index)
+{
+	ra_addup_cache_hit(ra);
+	ra->cache_hit <<= 16;
+	ra->lookahead_index = la_index;
+	ra->readahead_index = ra_index;
+}
+
+/*
+ * Take down a new read-ahead request in file_ra_state.
+ */
+static inline void ra_state_update(struct file_ra_state *ra,
+				unsigned long ra_size, unsigned long la_size)
+{
+#ifdef DEBUG_READAHEAD
+	unsigned long old_ra = ra->readahead_index - ra->ra_index;
+	if (ra_size < old_ra && ra_cache_hit(ra, 0))
+		ra_account(ra, RA_EVENT_READAHEAD_SHRINK, old_ra - ra_size);
+#endif
+	ra_addup_cache_hit(ra);
+	ra->ra_index = ra->readahead_index;
+	ra->la_index = ra->lookahead_index;
+	ra->readahead_index += ra_size;
+	ra->lookahead_index = ra->readahead_index - la_size;
+	ra->age = nr_smooth_aging();
+}
+
+/*
+ * Adjust the read-ahead request in file_ra_state.
+ */
+static inline void ra_state_adjust(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->readahead_index - la_size;
+}
+
+/*
+ * Submit IO for the read-ahead request in file_ra_state.
+ */
+static int ra_dispatch(struct file_ra_state *ra,
+			struct address_space *mapping, struct file *filp)
+{
+	pgoff_t eof_index;
+	unsigned long ra_size;
+	unsigned long la_size;
+	int actual;
+	enum ra_class ra_class;
+
+	ra_class = (ra->flags & RA_CLASS_MASK);
+	BUG_ON(ra_class == 0 || ra_class > RA_CLASS_END);
+
+	eof_index = ((i_size_read(mapping->host) - 1) >> PAGE_CACHE_SHIFT) + 1;
+	ra_size = ra->readahead_index - ra->ra_index;
+	la_size = ra->readahead_index - ra->lookahead_index;
+
+	/* Snap to EOF. */
+	if (unlikely(ra->ra_index >= eof_index))
+		return 0;
+	if (ra->readahead_index + ra_size / 2 > eof_index) {
+		if (ra_class == RA_CLASS_CONTEXT_ACCELERATED &&
+				eof_index > ra->lookahead_index + 1)
+			la_size = eof_index - ra->lookahead_index;
+		else
+			la_size = 0;
+		ra_size = eof_index - ra->ra_index;
+		ra_state_adjust(ra, ra_size, la_size);
+	}
+
+	actual = __do_page_cache_readahead(mapping, filp,
+					ra->ra_index, ra_size, la_size);
+
+#ifdef READAHEAD_STREAMING
+	if (actual < ra_size) {
+		struct page *page = find_page(mapping, ra->ra_index + actual);
+		if (page)
+			rescue_pages(page, ra_size);
+	}
+#endif
+
+	if (ra->readahead_index == eof_index)
+		ra_account(ra, RA_EVENT_READAHEAD_EOF, actual);
+	if (la_size)
+		ra_account(ra, RA_EVENT_LOOKAHEAD, la_size);
+	ra_account(ra, RA_EVENT_READAHEAD, actual);
+
+	dprintk("readahead-%s(ino=%lu, index=%lu, ra=%lu+%lu-%lu) = %d\n",
+			ra_class_name[ra_class],
+			mapping->host->i_ino, ra->la_index,
+			ra->ra_index, ra_size, la_size, actual);
+
+	return actual;
+}
+
+/*
+ * Determine the request parameters from primitive values.
+ *
+ * It applies the following rules:
+ *   - Substract ra_size by the old look-ahead to get real safe read-ahead;
+ *   - Set new la_size according to the (still large) ra_size;
+ *   - Apply upper limits;
+ *   - Make sure stream_shift is not too small.
+ *     (So that the next global_shift will not be too small.)
+ *
+ * Input:
+ * ra_size stores the estimated thrashing-threshold.
+ * la_size stores the look-ahead size of previous request.
+ */
+static inline int adjust_rala(unsigned long ra_max,
+				unsigned long *ra_size, unsigned long *la_size)
+{
+	unsigned long stream_shift = *la_size;
+
+	if (*ra_size > *la_size)
+		*ra_size -= *la_size;
+	else
+		return 0;
+
+	*la_size = *ra_size / LOOKAHEAD_RATIO;
+
+	if (*ra_size > ra_max)
+		*ra_size = ra_max;
+	if (*la_size > *ra_size)
+		*la_size = *ra_size;
+
+	stream_shift += (*ra_size - *la_size);
+	if (stream_shift < *ra_size / 4)
+		*la_size -= (*ra_size / 4 - stream_shift);
+
+	return 1;
+}
+
+/*
+ * The function estimates two values:
+ * 1. thrashing-threshold for the current stream
+ *    It is returned to make the next read-ahead request.
+ * 2. the remained space for the current chunk
+ *    It will be checked to ensure that the current chunk is safe.
+ *
+ * The computation will be pretty accurate under heavy load, and will change
+ * vastly with light load(small global_shift), so the grow speed of ra_size
+ * must be limited, and a moderate large stream_shift must be insured.
+ *
+ * This figure illustrates the formula:
+ * While the stream reads stream_shift pages inside the chunks,
+ * the chunks are shifted global_shift pages inside inactive_list.
+ *
+ *      chunk A                    chunk B
+ *                          |<=============== global_shift ================|
+ *  +-------------+         +-------------------+                          |
+ *  |       #     |         |           #       |            inactive_list |
+ *  +-------------+         +-------------------+                     head |
+ *          |---->|         |---------->|
+ *             |                  |
+ *             +-- stream_shift --+
+ */
+static inline unsigned long compute_thrashing_threshold(
+						struct file_ra_state *ra,
+						unsigned long *remain)
+{
+	unsigned long global_size;
+	unsigned long global_shift;
+	unsigned long stream_shift;
+	unsigned long ra_size;
+
+	global_size = nr_free_inactive();
+	global_shift = nr_smooth_aging() - ra->age;
+	stream_shift = ra_cache_hit(ra, 0);
+
+	ra_size = stream_shift *
+			global_size * readahead_ratio / (100 * global_shift);
+
+	if (global_size > global_shift)
+		*remain = stream_shift *
+				(global_size - global_shift) / global_shift;
+	else
+		*remain = 0;
+
+	ddprintk("compute_thrashing_threshold: "
+			"ra=%lu=%lu*%lu/%lu, remain %lu for %lu\n",
+			ra_size, stream_shift, global_size, global_shift,
+			*remain, ra->readahead_index - ra->lookahead_index);
+
+	return ra_size;
+}
+
+/*
+ * Main function for file_ra_state based read-ahead.
+ */
+static inline unsigned long
+state_based_readahead(struct address_space *mapping, struct file *filp,
+			struct file_ra_state *ra, struct page *page,
+			unsigned long ra_max)
+{
+	unsigned long ra_old;
+	unsigned long ra_size;
+	unsigned long la_size;
+	unsigned long remain_space;
+
+	la_size = ra->readahead_index - ra->lookahead_index;
+	ra_old = ra->readahead_index - ra->ra_index;
+	ra_size = compute_thrashing_threshold(ra, &remain_space);
+
+	if (readahead_ratio < VM_READAHEAD_PROTECT_RATIO &&
+			remain_space <= la_size && la_size > 1) {
+		rescue_pages(page, la_size);
+		return 0;
+	}
+
+	if (!adjust_rala(min(ra_max, 2 * ra_old + (ra_max - ra_old) / 16),
+				&ra_size, &la_size))
+		return 0;
+
+	set_ra_class(ra, RA_CLASS_STATE);
+	ra_state_update(ra, ra_size, la_size);
+
+	return ra_dispatch(ra, mapping, filp);
+}
+
+
+/*
  * ra_size is mainly determined by:
  * 1. sequential-start: min(MIN_RA_PAGES + (pages>>14), KB(128))
  * 2. sequential-max:	min(ra->ra_pages, 0xFFFF)

--