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Subject[PATCH 09/19] readahead: parameters
- new entry in /proc/sys/vm/readahead_ratio with default value of 50;
- new entry in /proc/sys/vm/readahead_hit_rate with default value of 2;
- new entry in /proc/sys/vm/readahead_live_chunk with default value of 2M;
- limit mmap read-around size to 256kb;
- dynamic minimal/initial read-ahead size.

readahead_ratio is divided into several ranges:

condition action
===================================================================
readahead_ratio == 0 disable read-ahead
readahead_ratio < 9 select old read-ahead logic
readahead_ratio >= 9 select new read-ahead logic
readahead_ratio >= 80 enable live pages protection

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

Documentation/sysctl/vm.txt | 51 ++++++++++++++++++++++++++++++++++++++++++++
include/linux/mm.h | 5 +++-
include/linux/sysctl.h | 3 ++
kernel/sysctl.c | 34 +++++++++++++++++++++++++++++
mm/filemap.c | 7 ++++++
mm/readahead.c | 39 +++++++++++++++++++++++++++++++++
6 files changed, 138 insertions(+), 1 deletion(-)

--- linux-2.6.15-rc2-mm1.orig/Documentation/sysctl/vm.txt
+++ linux-2.6.15-rc2-mm1/Documentation/sysctl/vm.txt
@@ -27,6 +27,9 @@ Currently, these files are in /proc/sys/
- laptop_mode
- block_dump
- swap_prefetch
+- readahead_ratio
+- readahead_hit_rate
+- readahead_live_chunk

==============================================================

@@ -114,3 +117,51 @@ except when laptop_mode is enabled and t
Setting it to 0 disables prefetching entirely.

The default value is dependant on ramsize.
+
+==============================================================
+
+readahead_ratio
+
+This limits read-ahead size to percent of the thrashing-threshold.
+The thrashing-threshold is dynamicly estimated according to the
+_history_ read speed and system load, and used to limit the
+_future_ read-ahead request size.
+
+Set it to a low value if you have not enough memory to counteract
+the I/O load fluctuations. But if there's plenty of memory, set it
+to a larger value might help increase read speed. Also note that a
+value >= 80 activates mandatory thrashing protection(see
+readahead_live_chunk).
+
+The default value is 50.
+
+==============================================================
+
+readahead_hit_rate
+
+This is the max allowed value of (read-ahead-pages : accessed-pages).
+If the previous read-ahead request has bad hit rate, kernel will be
+very conservative to issue the next read-ahead.
+
+A large value helps speedup some sparse access patterns, at the cost
+of more memory consumption. It is recommended to keep the value below
+(max-readahead-pages / 8).
+
+The default value is 2.
+
+==============================================================
+
+readahead_live_chunk
+
+In a file server, there are typically one or more sequential
+readers working on a file. The kernel can detect most live
+chunks(a sequence of pages to be accessed by an active reader),
+and save them for their imminent readers. This is called
+mandatory thrashing protection, and is only in effect when
+(readahead_ratio >= 80).
+
+This parameter controls the max allowed chunk size, i.e. the max
+number of pages pinned for an active reader.
+
+The default value is 2MB size of pages. That is 512 on most archs.
+Increase it if you have enough memory.
--- linux-2.6.15-rc2-mm1.orig/include/linux/mm.h
+++ linux-2.6.15-rc2-mm1/include/linux/mm.h
@@ -968,11 +968,14 @@ extern int filemap_populate(struct vm_ar
int write_one_page(struct page *page, int wait);

/* readahead.c */
-#define VM_MAX_READAHEAD 128 /* kbytes */
+#define VM_MAX_READAHEAD 1024 /* kbytes */
#define VM_MIN_READAHEAD 16 /* kbytes (includes current page) */
#define VM_MAX_CACHE_HIT 256 /* max pages in a row in cache before
* turning readahead off */

+/* turn on read-ahead thrashing protection if (readahead_ratio >= ##) */
+#define VM_READAHEAD_PROTECT_RATIO 80
+
int do_page_cache_readahead(struct address_space *mapping, struct file *filp,
pgoff_t offset, unsigned long nr_to_read);
int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
--- linux-2.6.15-rc2-mm1.orig/include/linux/sysctl.h
+++ linux-2.6.15-rc2-mm1/include/linux/sysctl.h
@@ -182,6 +182,9 @@ enum
VM_LEGACY_VA_LAYOUT=27, /* legacy/compatibility virtual address space layout */
VM_SWAP_TOKEN_TIMEOUT=28, /* default time for token time out */
VM_SWAP_PREFETCH=29, /* int: amount to swap prefetch */
+ VM_READAHEAD_RATIO=30, /* percent of read-ahead size to thrashing-threshold */
+ VM_READAHEAD_HIT_RATE=31, /* one accessed page legitimizes so many read-ahead pages */
+ VM_READAHEAD_LIVE_CHUNK=32, /* pin no more than that many pages for a live reader */
};


--- linux-2.6.15-rc2-mm1.orig/kernel/sysctl.c
+++ linux-2.6.15-rc2-mm1/kernel/sysctl.c
@@ -68,6 +68,9 @@ extern int min_free_kbytes;
extern int printk_ratelimit_jiffies;
extern int printk_ratelimit_burst;
extern int pid_max_min, pid_max_max;
+extern int readahead_ratio;
+extern int readahead_hit_rate;
+extern int readahead_live_chunk;

#if defined(CONFIG_X86_LOCAL_APIC) && defined(CONFIG_X86)
int unknown_nmi_panic;
@@ -668,6 +671,7 @@ static ctl_table kern_table[] = {
/* Constants for minimum and maximum testing in vm_table.
We use these as one-element integer vectors. */
static int zero;
+static int one = 1;
static int one_hundred = 100;


@@ -867,6 +871,36 @@ static ctl_table vm_table[] = {
},
#endif
#endif
+ {
+ .ctl_name = VM_READAHEAD_RATIO,
+ .procname = "readahead_ratio",
+ .data = &readahead_ratio,
+ .maxlen = sizeof(readahead_ratio),
+ .mode = 0644,
+ .proc_handler = &proc_dointvec,
+ .strategy = &sysctl_intvec,
+ .extra1 = &zero,
+ },
+ {
+ .ctl_name = VM_READAHEAD_HIT_RATE,
+ .procname = "readahead_hit_rate",
+ .data = &readahead_hit_rate,
+ .maxlen = sizeof(readahead_hit_rate),
+ .mode = 0644,
+ .proc_handler = &proc_dointvec,
+ .strategy = &sysctl_intvec,
+ .extra1 = &one,
+ },
+ {
+ .ctl_name = VM_READAHEAD_LIVE_CHUNK,
+ .procname = "readahead_live_chunk",
+ .data = &readahead_live_chunk,
+ .maxlen = sizeof(readahead_live_chunk),
+ .mode = 0644,
+ .proc_handler = &proc_dointvec,
+ .strategy = &sysctl_intvec,
+ .extra1 = &zero,
+ },
{ .ctl_name = 0 }
};

--- linux-2.6.15-rc2-mm1.orig/mm/filemap.c
+++ linux-2.6.15-rc2-mm1/mm/filemap.c
@@ -1336,6 +1336,13 @@ retry_find:
if (ra_pages) {
pgoff_t start = 0;

+ /*
+ * Max read-around should be much smaller than
+ * max read-ahead.
+ * How about adding a tunable parameter for this?
+ */
+ if (ra_pages > 64)
+ ra_pages = 64;
if (pgoff > ra_pages / 2)
start = pgoff - ra_pages / 2;
do_page_cache_readahead(mapping, file, start, ra_pages);
--- linux-2.6.15-rc2-mm1.orig/mm/readahead.c
+++ linux-2.6.15-rc2-mm1/mm/readahead.c
@@ -20,6 +20,24 @@
#define MAX_RA_PAGES KB(VM_MAX_READAHEAD)
#define MIN_RA_PAGES KB(VM_MIN_READAHEAD)

+/* In laptop mode, poll delayed look-ahead on every ## pages read. */
+#define LAPTOP_POLL_INTERVAL 16
+
+/* Set look-ahead size to 1/# of the thrashing-threshold. */
+#define LOOKAHEAD_RATIO 8
+
+/* Set read-ahead size to ##% of the thrashing-threshold. */
+int readahead_ratio = 50;
+EXPORT_SYMBOL(readahead_ratio);
+
+/* Readahead as long as cache hit ratio keeps above 1/##. */
+int readahead_hit_rate = 2;
+EXPORT_SYMBOL(readahead_hit_rate);
+
+/* Scan backward ## pages to find a live reader. */
+int readahead_live_chunk = 2 * MAX_RA_PAGES;
+EXPORT_SYMBOL(readahead_live_chunk);
+
/* Detailed classification of read-ahead behaviors. */
#define RA_CLASS_SHIFT 3
#define RA_CLASS_MASK ((1 << RA_CLASS_SHIFT) - 1)
@@ -789,6 +807,27 @@ out:
}

/*
+ * 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)
+ * 3. sequential: (thrashing-threshold) * readahead_ratio / 100
+ *
+ * Table of concrete numbers for 4KB page size:
+ * (inactive + free) (in MB): 4 8 16 32 64 128 256 512 1024
+ * initial ra_size (in KB): 16 16 16 16 20 24 32 48 64
+ */
+static inline void get_readahead_bounds(struct file_ra_state *ra,
+ unsigned long *ra_min,
+ unsigned long *ra_max)
+{
+ unsigned long pages;
+
+ pages = node_free_and_cold_pages();
+ *ra_max = min(min(pages/2, 0xFFFFUL), ra->ra_pages);
+ *ra_min = min(min(MIN_RA_PAGES + (pages>>14), KB(128)), *ra_max/2);
+}
+
+/*
* This is the entry point of the adaptive read-ahead logic.
*
* It is only called on two conditions:
--
-
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