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Subject[PATCH 10/28] readahead: sysctl parameters
Add new sysctl entries in /proc/sys/vm:

- readahead_ratio = 50
i.e. set read-ahead size to <= readahead_ratio% thrashing threshold
- readahead_hit_rate = 0
i.e. read-ahead hit ratio >= 1/readahead_hit_rate is deemed ok

readahead_ratio also provides a way to select read-ahead logic at runtime:

condition action
==========================================================================
readahead_ratio == 0 disable read-ahead
readahead_ratio == 1 select the (old) stock read-ahead logic
readahead_ratio >= 2 select the (new) adaptive read-ahead logic

readahead_hit_rate controls the features provided by context based read-ahead:

condition enabled function
==========================================================================
readahead_hit_rate == 0 handle only known good cases
i.e. nfsd read and seek/cache hit recovery
readahead_hit_rate == 1 also detect interleaved sequential reads
readahead_hit_rate >= 2 further handle sparse access patterns


Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
DESC
readahead-sysctl-parameters-fix
EDESC
From: Andrew Morton <akpm@osdl.org>

mm/readahead.c:41: parse error before numeric constant

Cc: Wu Fengguang <wfg@mail.ustc.edu.cn>
Signed-off-by: Andrew Morton <akpm@osdl.org>
--- linux-2.6.19-rc4-mm1.orig/Documentation/sysctl/vm.txt
+++ linux-2.6.19-rc4-mm1/Documentation/sysctl/vm.txt
@@ -32,6 +32,8 @@ Currently, these files are in /proc/sys/
- min_slab_ratio
- panic_on_oom
- swap_prefetch
+- readahead_ratio
+- readahead_hit_rate

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

@@ -216,3 +218,47 @@ copying back pages from swap into the sw
practice it can take many minutes before the vm is idle enough.

The default value is 1.
+
+==============================================================
+
+readahead_ratio
+
+This limits readahead size to percent of the thrashing threshold.
+The thrashing threshold is dynamically estimated from the _history_ read
+speed and system load, to deduce the _future_ readahead request size.
+
+Set it to a smaller value if you have not enough memory for all the
+concurrent readers, or the I/O loads fluctuate a lot. But if there's
+plenty of memory(>>2MB per reader), a bigger value may help performance.
+
+readahead_ratio also selects the readahead logic:
+ VALUE CODE PATH
+ -------------------------------------------
+ 0 disable readahead totally
+ 1 select the stock readahead logic
+ 2-100 select the adaptive readahead logic
+
+The default value is 50. Reasonable values would be [50, 100].
+
+==============================================================
+
+readahead_hit_rate
+
+This is the allowed sparseness(readahead-pages:accessed-pages) of the
+context based readahead. If the previous readahead has bad hit rate,
+the kernel will be reluctant to do the next readahead.
+
+The context based readahead logic can catch some semi-sequential patterns,
+i.e. interleaved/intermixed reading. They are subtle and therefore missed by
+the state based logic. However the logic can be overzealous and may hurt the
+performance of pure random reads.
+
+Possible values can be:
+0 only handle some known good cases, i.e. nfsd reads
+1 detect semi-sequential read patterns, found in some postgresql
+ applications and video streaming services
+2-8 detect sparse access patterns
+
+The larger value, the more capabilities, with more possible overheads.
+
+The default value is 0.
--- linux-2.6.19-rc4-mm1.orig/include/linux/mm.h
+++ linux-2.6.19-rc4-mm1/include/linux/mm.h
@@ -1064,6 +1064,17 @@ void handle_ra_miss(struct address_space
struct file_ra_state *ra, pgoff_t offset);
unsigned long max_sane_readahead(unsigned long nr);

+#ifdef CONFIG_ADAPTIVE_READAHEAD
+extern int readahead_ratio;
+#else
+#define readahead_ratio 1
+#endif /* CONFIG_ADAPTIVE_READAHEAD */
+
+static inline int prefer_adaptive_readahead(void)
+{
+ return readahead_ratio > 1;
+}
+
/* Do stack extension */
extern int expand_stack(struct vm_area_struct *vma, unsigned long address);
#ifdef CONFIG_IA64
--- linux-2.6.19-rc4-mm1.orig/include/linux/sysctl.h
+++ linux-2.6.19-rc4-mm1/include/linux/sysctl.h
@@ -204,6 +204,8 @@ enum
VM_VDSO_ENABLED=34, /* map VDSO into new processes? */
VM_MIN_SLAB=35, /* Percent pages ignored by zone reclaim */
VM_SWAP_PREFETCH=36, /* swap prefetch */
+ VM_READAHEAD_RATIO=37, /* percent of read-ahead size to thrashing-threshold */
+ VM_READAHEAD_HIT_RATE=38, /* one accessed page legitimizes so many read-ahead pages */
};


--- linux-2.6.19-rc4-mm1.orig/kernel/sysctl.c
+++ linux-2.6.19-rc4-mm1/kernel/sysctl.c
@@ -78,6 +78,11 @@ extern int sysctl_drop_caches;
extern int percpu_pagelist_fraction;
extern int compat_log;

+#if defined(CONFIG_ADAPTIVE_READAHEAD)
+extern int readahead_ratio;
+extern int readahead_hit_rate;
+#endif
+
/* this is needed for the proc_dointvec_minmax for [fs_]overflow UID and GID */
static int maxolduid = 65535;
static int minolduid;
@@ -1034,6 +1039,28 @@ static ctl_table vm_table[] = {
.proc_handler = &proc_dointvec,
},
#endif
+#ifdef CONFIG_ADAPTIVE_READAHEAD
+ {
+ .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 = &zero,
+ },
+#endif
{ .ctl_name = 0 }
};

--- linux-2.6.19-rc4-mm1.orig/mm/readahead.c
+++ linux-2.6.19-rc4-mm1/mm/readahead.c
@@ -27,6 +27,25 @@
#define MIN_RA_PAGES DIV_ROUND_UP(VM_MIN_READAHEAD*1024, PAGE_CACHE_SIZE)

/*
+ * Adaptive read-ahead parameters.
+ */
+
+/* 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
+
+#ifdef CONFIG_ADAPTIVE_READAHEAD
+/* Set read-ahead size to ##% of the thrashing-threshold. */
+int readahead_ratio = 50;
+EXPORT_SYMBOL_GPL(readahead_ratio);
+#endif
+
+/* Readahead as long as cache hit ratio keeps above 1/##. */
+int readahead_hit_rate = 0;
+
+/*
* Detailed classification of read-ahead behaviors.
*/
#define RA_CLASS_SHIFT 4
--
-
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