[PATCH 15/28] readahead: context based method

Wu Fengguang
2006-11-15T07:58:21Z

[-- Attachment #1: readahead-context-based-method.patch --]
[-- Type: text/plain, Size: 18079 bytes --]

This is the slow code path of adaptive read-ahead.

No valid state info is available, so the page cache is queried to obtain
the required position/timing info. This kind of estimation is more conservative
than the stateful method, and also fluctuates more on load variance.

HOW IT WORKS
============

It works by peeking into the file cache and check if there are any history
pages present or accessed. In this way it can detect almost all forms of
sequential / semi-sequential read patterns, e.g.
- parallel / interleaved sequential scans on one file
- sequential reads across file open/close
- mixed sequential / random accesses
- sparse / skimming sequential read

HOW DATABASES CAN BENEFIT FROM IT
=================================

The adaptive readahead might help db performance in the following cases:
- concurrent sequential scans
- sequential scan on a fragmented table
- index scan with clustered matches
- index scan on majority rows (in case the planner goes wrong)

However some databases do not rely on kernel readahead, so are unlikely to
benefit from it.

ALGORITHM STEPS
===============

- look back/forward to find the ra_index;
- look back to estimate a thrashing safe ra_size;
- assemble the next read-ahead request in file_ra_state;
- submit it.

ALGORITHM DYNAMICS
==================

* startup
When a sequential read is detected, chunk size is set to readahead-min
and grows up with each readahead. The grow speed is controlled by
readahead-ratio. When readahead-ratio == 100, the new logic grows chunk
sizes exponentially -- like the current logic, but lags behind it at
early steps.

* stabilize
When chunk size reaches readahead-max, or comes close to
(readahead-ratio * thrashing-threshold)
it stops growing and stay there.

The main difference with the stock readahead logic occurs at and after
the time chunk size stops growing:
- The current logic grows chunk size exponentially in normal and
decreases it by 2 each time thrashing is seen. That can lead to
thrashing with almost every readahead for very slow streams.
- The new logic can stop at a size below the thrashing-threshold,
and stay there stable.

* on stream speed up or system load fall
thrashing-threshold follows up and chunk size is likely to be enlarged.

* on stream slow down or system load rocket up
thrashing-threshold falls down.
If thrashing happened, the next read would be treated as a random read,
and with another read the chunk-size-growing-phase is restarted.

For a slow stream that has (thrashing-threshold < readahead-max):
- When readahead-ratio = 100, there is only one chunk in cache at
most time;
- When readahead-ratio = 50, there are two chunks in cache at most
time.
- Lowing readahead-ratio helps gracefully cut down the chunk size
without thrashing.

OVERHEADS
=========

The context based method has some overheads over the stateful method, due
to more lockings and memory scans.

Running oprofile on the following command shows the following differences:

# diff sparse sparse1

total oprofile samples run1 run2
stateful method 560482 558696
stateless method 564463 559413

So the average overhead is about 0.4%.

Detailed diffprofile data:

# diffprofile oprofile.50.stateful oprofile.50.stateless
2998 41.1% isolate_lru_pages
2669 26.4% shrink_zone
1822 14.7% system_call
1419 27.6% radix_tree_delete
1376 14.8% _raw_write_lock
1279 27.4% free_pages_bulk
1111 12.0% _raw_write_unlock
1035 43.3% free_hot_cold_page
849 15.3% unlock_page
786 29.6% page_referenced
710 4.6% kmap_atomic
651 26.4% __pagevec_release_nonlru
586 16.1% __rmqueue
578 11.3% find_get_page
481 15.5% page_waitqueue
440 6.6% add_to_page_cache
420 33.7% fget_light
260 4.3% get_page_from_freelist
223 13.7% find_busiest_group
221 35.1% mutex_debug_check_no_locks_freed
211 0.0% radix_tree_scan_hole
198 35.5% delay_tsc
195 14.8% ext3_get_branch
182 12.6% profile_tick
173 0.0% radix_tree_cache_lookup_node
164 22.9% find_next_bit
162 50.3% page_cache_readahead_adaptive
...
106 0.0% radix_tree_scan_hole_backward
...
-51 -7.6% radix_tree_preload
...
-68 -2.1% radix_tree_insert
...
-87 -2.0% mark_page_accessed
-88 -2.0% __pagevec_lru_add
-103 -7.7% softlockup_tick
-107 -71.8% free_block
-122 -77.7% do_IRQ
-132 -82.0% do_timer
-140 -47.1% ack_edge_ioapic_vector
-168 -81.2% handle_IRQ_event
-192 -35.2% irq_entries_start
-204 -14.8% rw_verify_area
-214 -13.2% account_system_time
-233 -9.5% radix_tree_lookup_node
-234 -16.6% scheduler_tick
-259 -58.7% __do_IRQ
-266 -6.8% put_page
-318 -29.3% rcu_pending
-333 -3.0% do_generic_mapping_read
-337 -28.3% hrtimer_run_queues
-493 -27.0% __rcu_pending
-1038 -9.4% default_idle
-3323 -3.5% __copy_to_user_ll
-10331 -5.9% do_mpage_readpage

# diffprofile oprofile.50.stateful2 oprofile.50.stateless2
1739 1.1% do_mpage_readpage
833 0.9% __copy_to_user_ll
340 21.3% find_busiest_group
288 9.5% free_hot_cold_page
261 4.6% _raw_read_unlock
239 3.9% get_page_from_freelist
201 0.0% radix_tree_scan_hole
163 14.3% raise_softirq
160 0.0% radix_tree_cache_lookup_node
160 11.8% update_process_times
136 9.3% fget_light
121 35.1% page_cache_readahead_adaptive
117 36.0% restore_all
117 2.8% mark_page_accessed
109 6.4% rebalance_tick
107 9.4% sys_read
102 0.0% radix_tree_scan_hole_backward
...
63 4.0% readahead_cache_hit
...
-10 -15.9% radix_tree_node_alloc
...
-39 -1.7% radix_tree_lookup_node
-39 -10.3% irq_entries_start
-43 -1.3% radix_tree_insert
...
-47 -4.6% __do_page_cache_readahead
-64 -9.3% radix_tree_preload
-65 -5.4% rw_verify_area
-65 -2.2% vfs_read
-70 -4.7% timer_interrupt
-71 -1.0% __wake_up_bit
-73 -1.1% radix_tree_delete
-79 -12.6% __mod_page_state_offset
-94 -1.8% __find_get_block
-94 -2.2% __pagevec_lru_add
-102 -1.7% free_pages_bulk
-116 -1.3% _raw_read_lock
-123 -7.4% do_sync_read
-130 -8.4% ext3_get_blocks_handle
-142 -3.8% put_page
-146 -7.9% mpage_readpages
-147 -5.6% apic_timer_interrupt
-168 -1.6% _raw_write_unlock
-172 -5.0% page_referenced
-206 -3.2% unlock_page
-212 -15.0% restore_nocheck
-213 -2.1% default_idle
-245 -5.0% __rmqueue
-278 -4.3% find_get_page
-282 -2.1% system_call
-287 -11.8% run_timer_softirq
-300 -2.7% _raw_write_lock
-420 -3.2% shrink_zone
-661 -5.7% isolate_lru_pages

Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
DESC
readahead: context based method - apply stream_shift size limits to contexta method
EDESC
From: Wu Fengguang <wfg@mail.ustc.edu.cn>

Apply size limits to the contexta method, so that the possible following
stateful method will not be presented too small stream_shift size.

Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
DESC
readahead: context based method - fix *remain counting
EDESC
From: Wu Fengguang <wfg@mail.ustc.edu.cn>

*remain in query_page_cache_segment() is over counted by 1, fix it.

Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
DESC
readahead: context based method - slow start
EDESC
From: Wu Fengguang <wfg@mail.ustc.edu.cn>

The context method will lead to noticable overhead(readahead miss) on very
sparse random reads.

Having the readahead window to start slowly makes it much better. But
still startup quick if the user prefers sparse readahead.

Benchmarks of reading randomly 100,000 pages on a 1000,000 pages _sparse_
file:

ARA before patch ARA STOCK
================ ================ ================
real 2.779s 2.782s 2.552s 2.606s 2.477s 2.521s
user 1.120s 1.184s 1.133s 1.155s 1.097s 1.159s
sys 1.248s 1.208s 1.093s 1.086s 1.079s 1.064s

Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
Signed-off-by: Andrew Morton <akpm@osdl.org>
--- linux-2.6.19-rc5-mm2.orig/mm/readahead.c
+++ linux-2.6.19-rc5-mm2/mm/readahead.c
@@ -1121,6 +1121,300 @@ state_based_readahead(struct address_spa
}

/*
+ * Page cache context based estimation of read-ahead/look-ahead size/index.
+ *
+ * The logic first looks around to find the start point of next read-ahead,
+ * and then, if necessary, looks backward in the inactive_list to get an
+ * estimation of the thrashing-threshold.
+ *
+ * The estimation theory can be illustrated with figure:
+ *
+ * chunk A chunk B chunk C head
+ *
+ * l01 l11 l12 l21 l22
+ *| |-->|-->| |------>|-->| |------>|
+ *| +-------+ +-----------+ +-------------+ |
+ *| | # | | # | | # | |
+ *| +-------+ +-----------+ +-------------+ |
+ *| |<==============|<===========================|<============================|
+ * L0 L1 L2
+ *
+ * Let f(l) = L be a map from
+ * l: the number of pages read by the stream
+ * to
+ * L: the number of pages pushed into inactive_list in the mean time
+ * then
+ * f(l01) <= L0
+ * f(l11 + l12) = L1
+ * f(l21 + l22) = L2
+ * ...
+ * f(l01 + l11 + ...) <= Sum(L0 + L1 + ...)
+ * <= Length(inactive_list) = f(thrashing-threshold)
+ *
+ * So the count of countinuous history pages left in the inactive_list is always
+ * a lower estimation of the true thrashing-threshold.
+ */
+
+#if PG_active < PG_referenced
+# error unexpected page flags order
+#endif
+
+#define PAGE_REFCNT_0 0
+#define PAGE_REFCNT_1 (1 << PG_referenced)
+#define PAGE_REFCNT_2 (1 << PG_active)
+#define PAGE_REFCNT_3 ((1 << PG_active) | (1 << PG_referenced))
+#define PAGE_REFCNT_MASK PAGE_REFCNT_3
+
+/*
+ * STATUS REFERENCE COUNT TYPE
+ * __ 0 fresh
+ * _R PAGE_REFCNT_1 stale
+ * A_ PAGE_REFCNT_2 disturbed once
+ * AR PAGE_REFCNT_3 disturbed twice
+ *
+ * A/R: Active / Referenced
+ */
+static inline unsigned long page_refcnt(struct page *page)
+{
+ return page->flags & PAGE_REFCNT_MASK;
+}
+
+/*
+ * Now that revisited pages are put into active_list immediately,
+ * we cannot get an accurate estimation of
+ *
+ * len(inactive_list) / speed(leader)
+ *
+ * on the situation of two sequential readers that come close enough:
+ *
+ * chunk 1 chunk 2 chunk 3
+ * ========== =============------- --------------------
+ * follower ^ leader ^
+ *
+ * In this case, using inactive_page_refcnt() in the context based method yields
+ * conservative read-ahead size, while page_refcnt() yields aggressive size.
+ */
+static inline unsigned long inactive_page_refcnt(struct page *page)
+{
+ if (!page || PageActive(page))
+ return 0;
+
+ return page_refcnt(page);
+}
+
+/*
+ * Count/estimate cache hits in range [begin, end).
+ * The estimation is simple and optimistic.
+ */
+#define CACHE_HIT_HASH_KEY 29 /* some prime number */
+static int count_cache_hit(struct address_space *mapping,
+ pgoff_t begin, pgoff_t end)
+{
+ int size = end - begin;
+ int count = 0;
+ int i;
+
+ /*
+ * The first page may well is chunk head and has been accessed,
+ * so it is index 0 that makes the estimation optimistic. This
+ * behavior guarantees a readahead when (size < ra_max) and
+ * (readahead_hit_rate >= 8).
+ */
+ read_lock_irq(&mapping->tree_lock);
+ for (i = 0; i < 8;) {
+ struct page *page = radix_tree_lookup(&mapping->page_tree,
+ begin + size * ((i++ * CACHE_HIT_HASH_KEY) & 7) / 8);
+ if (inactive_page_refcnt(page) >= PAGE_REFCNT_1 && ++count >= 2)
+ break;
+ }
+ read_unlock_irq(&mapping->tree_lock);
+
+ return size * count / i;
+}
+
+/*
+ * Look back and check history pages to estimate thrashing-threshold.
+ */
+static unsigned long count_history_pages(struct address_space *mapping,
+ struct file_ra_state *ra,
+ pgoff_t offset, unsigned long ra_max)
+{
+ pgoff_t head;
+ unsigned long count;
+ unsigned long lookback;
+ unsigned long hit_rate;
+
+ /*
+ * Scan backward and check the near @ra_max pages.
+ * The count here determines ra_size.
+ */
+ cond_resched();
+ read_lock_irq(&mapping->tree_lock);
+ head = 1 + radix_tree_scan_hole_backward(&mapping->page_tree,
+ offset - 1, ra_max);
+
+ count = offset - head;
+
+ /*
+ * Ensure readahead hit rate
+ */
+ hit_rate = max(readahead_hit_rate, 1);
+ if (count_cache_hit(mapping, head, offset) * hit_rate < count)
+ count = 0;
+
+ /*
+ * Unnecessary to count more?
+ */
+ if (count < ra_max)
+ goto out_unlock;
+
+ /*
+ * Check the far pages coarsely.
+ * The enlarged count will contribute to the look-ahead size.
+ */
+ lookback = ra_max * (LOOKAHEAD_RATIO + 1) *
+ 100 / (readahead_ratio | 1);
+
+ for (count += ra_max; count < lookback; count += ra_max)
+ if (!__probe_page(mapping, offset - count))
+ break;
+
+out_unlock:
+ read_unlock_irq(&mapping->tree_lock);
+
+ ddprintk("count_history_pages: ino=%lu, idx=%lu, count=%lu\n",
+ mapping->host->i_ino, offset, count);
+
+ return count;
+}
+
+/*
+ * Determine the request parameters for context based read-ahead that extends
+ * from start of file.
+ *
+ * One major weakness of stateless method is the slow scaling up of ra_size.
+ * The logic tries to make up for this in the important case of sequential
+ * reads that extend from start of file. In this case, the ra_size is not
+ * chosen to make the whole next chunk safe (as in normal ones). Only part of
+ * which is safe: the tailing look-ahead part is 'unsafe'. However it will be
+ * safeguarded by rescue_pages() when the previous chunks are lost.
+ */
+static void adjust_rala_aggressive(unsigned long ra_max,
+ unsigned long *ra_size, unsigned long *la_size)
+{
+ pgoff_t offset = *ra_size;
+
+ *ra_size -= min(*ra_size, *la_size);
+ *la_size = offset;
+ *ra_size += *la_size;
+}
+
+/*
+ * Main function for page context based read-ahead.
+ *
+ * RETURN VALUE HINT
+ * 1 @ra contains a valid ra-request, please submit it
+ * 0 no seq-pattern discovered, please try the next method
+ * -1 please don't do _any_ readahead
+ */
+static int
+try_context_based_readahead(struct address_space *mapping,
+ struct file_ra_state *ra,
+ struct page *page, pgoff_t offset,
+ unsigned long ra_min, unsigned long ra_max)
+{
+ pgoff_t start;
+ unsigned long ra_size;
+ unsigned long la_size;
+
+ /* Check if there is a segment of history pages, and its end index.
+ * Based on which we decide whether and where to start read-ahead.
+ *
+ * Case 1: we have a current page.
+ * Search forward for a nearby hole.
+ */
+ if (page) {
+ unsigned long max_scan = ra_max + ra_min;
+ start = radix_tree_scan_hole(&mapping->page_tree,
+ offset, max_scan);
+ if (start != 0 && start - offset < max_scan)
+ goto has_history_pages;
+ else
+ return -1;
+ }
+
+ /* Case 2: current page is missing; previous page is present.
+ * Just do read-ahead from the current index on.
+ * There's clear sign of sequential reading. It can be
+ * a) seek => read => this read
+ * b) cache hit read(s) => this read
+ * We either just detected a new sequence of sequential reads,
+ * or should quickly resume readahead after the cache hit.
+ */
+ if (offset == ra->prev_page + 1) {
+ start = offset;
+ goto has_history_pages;
+ }
+
+ /* Case 2x: the same context info as 2.
+ * It can be the early stage of semi-sequential reads(interleaved/nfsd),
+ * or an ugly random one. So be conservative.
+ */
+ if (readahead_hit_rate && probe_page(mapping, offset - 1)) {
+ start = offset;
+ if (ra_min > 2 * readahead_hit_rate)
+ ra_min = 2 * readahead_hit_rate;
+ goto has_history_pages;
+ }
+
+ /* Case 3: no current/previous pages;
+ * sparse read-ahead is enabled: ok, be aggressive.
+ * Check if there's any adjecent history pages.
+ */
+ if (readahead_hit_rate > 1) {
+ start = radix_tree_scan_data_backward(&mapping->page_tree,
+ offset, ra_min);
+ if (start != ULONG_MAX && offset - start < ra_min) {
+ ra_min += offset - start;
+ offset = ++start; /* pretend the request starts here */
+ goto has_history_pages;
+ }
+ }
+
+ return 0;
+
+has_history_pages:
+ ra_size = count_history_pages(mapping, ra, offset, ra_max);
+ if (!ra_size)
+ return 0;
+
+ la_size = start - offset;
+ if (page && ra_size < la_size) {
+ if (ra_size < offset)
+ rescue_pages(page, la_size);
+ return -1;
+ }
+
+ if (ra_size >= offset) {
+ ra_size = offset;
+ adjust_rala_aggressive(ra_max, &ra_size, &la_size);
+ ra_set_class(ra, RA_CLASS_CONTEXT_AGGRESSIVE);
+ } else {
+ ra_size = max(ra_min, ra_size * readahead_ratio / 100);
+ if (!adjust_rala(ra_max, &ra_size, &la_size))
+ return -1;
+ ra_set_class(ra, RA_CLASS_CONTEXT);
+ }
+
+ limit_rala(ra_max, start - offset, &ra_size, &la_size);
+
+ ra_set_index(ra, offset, start);
+ ra_set_size(ra, ra_size, la_size);
+
+ return 1;
+}
+
+/*
* ra_min is mainly determined by the size of cache memory. Reasonable?
*
* Table of concrete numbers for 4KB page size:

--