[PATCH 14/28] readahead: state based method

Wu Fengguang
2006-11-15T07:51:31Z

[-- Attachment #1: readahead-state-based-method.patch --]
[-- Type: text/plain, Size: 6894 bytes --]

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

MAJOR STEPS
===========

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

THE REFERENCE MODEL
===================

1. inactive list has constant length and page flow speed
2. the observed stream receives a steady flow of read requests
3. no page activation, so that the inactive list forms a pipe

With that we get the picture showed below.

|<------------------------- constant length ------------------------->|
<<<<<<<<<<<<<<<<<<<<<<<<< steady flow of pages <<<<<<<<<<<<<<<<<<<<<<<<
+---------------------------------------------------------------------+
|tail inactive list head|
| ======= ==========---- |
| chunk A(stale pages) chunk B(stale + fresh pages) |
+---------------------------------------------------------------------+

REAL WORLD ISSUES
=================

Real world workloads will always have fluctuations (violation of assumption
1 and 2). To counteract it, a tunable parameter readahead_ratio is introduced
to make the estimation conservative enough. Violation of assumption 3 will
not lead to thrashing, it is there just for simplicity of discussion.

Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
DESC
readahead: state based method - stand-alone size limit code
EDESC
From: Wu Fengguang <wfg@mail.ustc.edu.cn>

Separate out the readahead/lookahead sizes limiting code, and put them to
stand-alone limit_rala() function.

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
@@ -18,6 +18,8 @@
#include <linux/pagevec.h>
#include <linux/buffer_head.h>

+#include <asm/div64.h>
+
/*
* Convienent macros for min/max read-ahead pages.
* Note that MAX_RA_PAGES is rounded down, while MIN_RA_PAGES is rounded up.
@@ -964,6 +966,161 @@ static int ra_submit(struct file_ra_stat
}

/*
+ * Deduce the read-ahead/look-ahead size from primitive values.
+ *
+ * Input:
+ * - @ra_size stores the estimated thrashing-threshold.
+ * - @la_size stores the look-ahead size of previous request.
+ */
+static int adjust_rala(unsigned long ra_max,
+ unsigned long *ra_size, unsigned long *la_size)
+{
+ /*
+ * Substract the old look-ahead to get real safe size for the next
+ * read-ahead request.
+ */
+ if (*ra_size > *la_size)
+ *ra_size -= *la_size;
+ else {
+ ra_account(NULL, RA_EVENT_READAHEAD_SHRINK, *ra_size);
+ return 0;
+ }
+
+ /*
+ * Set new la_size according to the (still large) ra_size.
+ */
+ *la_size = *ra_size / LOOKAHEAD_RATIO;
+
+ return 1;
+}
+
+static void limit_rala(unsigned long ra_max, unsigned long la_old,
+ unsigned long *ra_size, unsigned long *la_size)
+{
+ unsigned long stream_shift;
+
+ /*
+ * Apply basic upper limits.
+ */
+ if (*ra_size > ra_max)
+ *ra_size = ra_max;
+ if (*la_size > *ra_size)
+ *la_size = *ra_size;
+
+ /*
+ * Make sure stream_shift is not too small.
+ * (So that the next global_shift will not be too small.)
+ */
+ stream_shift = la_old + (*ra_size - *la_size);
+ if (stream_shift < *ra_size / 4)
+ *la_size -= (*ra_size / 4 - stream_shift);
+}
+
+/*
+ * 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 safe 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 vibrate
+ * more on light load(with small global_shift), so the grow speed of ra_size
+ * must be limited, and a moderate large stream_shift must be insured.
+ *
+ * The following figure illustrates the formula used in the function:
+ * While the stream reads stream_shift pages inside the chunks,
+ * the chunks are shifted global_shift pages inside inactive_list.
+ * So
+ * thrashing_threshold = free_mem * stream_shift / global_shift;
+ *
+ *
+ * chunk A chunk B
+ * |<=============== global_shift ================|
+ * +-------------+ +-------------------+ |
+ * | # | | # | inactive_list |
+ * +-------------+ +-------------------+ head |
+ * |---->| |---------->|
+ * | |
+ * +-- stream_shift --+
+ */
+static 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;
+ uint64_t ll;
+
+ global_size = nr_free_inactive_pages_node(numa_node_id());
+ global_shift = nr_scanned_pages_node(numa_node_id()) - ra->age;
+ global_shift |= 1UL;
+ stream_shift = ra_invoke_interval(ra);
+
+ /* future safe space */
+ ll = (uint64_t) stream_shift * (global_size >> 9) * readahead_ratio * 5;
+ do_div(ll, global_shift);
+ ra_size = ll;
+
+ /* remained safe space */
+ if (global_size > global_shift) {
+ ll = (uint64_t) stream_shift * (global_size - global_shift);
+ do_div(ll, global_shift);
+ *remain = ll;
+ } else
+ *remain = 0;
+
+ ddprintk("compute_thrashing_threshold: "
+ "at %lu ra %lu=%lu*%lu/%lu, remain %lu for %lu\n",
+ ra->readahead_index, ra_size,
+ stream_shift, global_size, global_shift,
+ *remain, ra_lookahead_size(ra));
+
+ return ra_size;
+}
+
+/*
+ * Main function for file_ra_state based read-ahead.
+ */
+static unsigned long
+state_based_readahead(struct address_space *mapping, struct file *filp,
+ struct file_ra_state *ra,
+ struct page *page, pgoff_t offset,
+ unsigned long req_size, unsigned long ra_max)
+{
+ unsigned long ra_old, ra_size;
+ unsigned long la_old, la_size;
+ unsigned long remain_space;
+ unsigned long growth_limit;
+
+ la_old = la_size = ra->readahead_index - offset;
+ ra_old = ra_readahead_size(ra);
+ ra_size = compute_thrashing_threshold(ra, &remain_space);
+
+ if (page && remain_space <= la_size) {
+ rescue_pages(page, la_size);
+ return 0;
+ }
+
+ growth_limit = req_size;
+ growth_limit += ra_max / 16;
+ growth_limit += (2 + readahead_ratio / 64) * ra_old;
+ if (growth_limit > ra_max)
+ growth_limit = ra_max;
+
+ if (!adjust_rala(growth_limit, &ra_size, &la_size))
+ return 0;
+
+ limit_rala(growth_limit, la_old, &ra_size, &la_size);
+
+ ra_set_class(ra, RA_CLASS_STATE);
+ ra_set_index(ra, offset, ra->readahead_index);
+ ra_set_size(ra, ra_size, la_size);
+
+ return ra_submit(ra, mapping, filp);
+}
+
+/*
* ra_min is mainly determined by the size of cache memory. Reasonable?
*
* Table of concrete numbers for 4KB page size:

--