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Subject[PATCH 11/33] 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 = 1
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 <= 9 select the (old) stock read-ahead logic
readahead_ratio >= 10 select the (new) adaptive read-ahead logic
Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
---
Documentation/sysctl/vm.txt | 37 +++++++++++++++++++++++++++++++++++++
include/linux/sysctl.h | 2 ++
kernel/sysctl.c | 28 ++++++++++++++++++++++++++++
mm/readahead.c | 17 +++++++++++++++++
4 files changed, 84 insertions(+)
--- linux-2.6.17-rc4-mm3.orig/mm/readahead.c
+++ linux-2.6.17-rc4-mm3/mm/readahead.c
@@ -20,6 +20,23 @@
#include <linux/nfsd/const.h>

/*
+ * 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
+
+/* Set read-ahead size to ##% of the thrashing-threshold. */
+int readahead_ratio = 50;
+EXPORT_SYMBOL_GPL(readahead_ratio);
+
+/* Readahead as long as cache hit ratio keeps above 1/##. */
+int readahead_hit_rate = 1;
+
+/*
* Detailed classification of read-ahead behaviors.
*/
#define RA_CLASS_SHIFT 4
--- linux-2.6.17-rc4-mm3.orig/include/linux/sysctl.h
+++ linux-2.6.17-rc4-mm3/include/linux/sysctl.h
@@ -194,6 +194,8 @@ enum
VM_ZONE_RECLAIM_INTERVAL=32, /* time period to wait after reclaim failure */
VM_PANIC_ON_OOM=33, /* panic at out-of-memory */
VM_SWAP_PREFETCH=34, /* swap prefetch */
+ VM_READAHEAD_RATIO=35, /* percent of read-ahead size to thrashing-threshold */
+ VM_READAHEAD_HIT_RATE=36, /* one accessed page legitimizes so many read-ahead pages */
};

/* CTL_NET names: */
--- linux-2.6.17-rc4-mm3.orig/kernel/sysctl.c
+++ linux-2.6.17-rc4-mm3/kernel/sysctl.c
@@ -77,6 +77,12 @@ extern int percpu_pagelist_fraction;
extern int compat_log;
extern int print_fatal_signals;

+#if defined(CONFIG_ADAPTIVE_READAHEAD)
+extern int readahead_ratio;
+extern int readahead_hit_rate;
+static int one = 1;
+#endif
+
#if defined(CONFIG_X86_LOCAL_APIC) && defined(CONFIG_X86)
int unknown_nmi_panic;
int nmi_watchdog_enabled;
@@ -987,6 +993,28 @@ 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 = &one,
+ },
+#endif
{ .ctl_name = 0 }
};

--- linux-2.6.17-rc4-mm3.orig/Documentation/sysctl/vm.txt
+++ linux-2.6.17-rc4-mm3/Documentation/sysctl/vm.txt
@@ -31,6 +31,8 @@ Currently, these files are in /proc/sys/
- zone_reclaim_interval
- panic_on_oom
- swap_prefetch
+- readahead_ratio
+- readahead_hit_rate

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

@@ -202,3 +204,38 @@ 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 dynamicly 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
+ 1-9 select the stock readahead logic
+ 10-inf select the adaptive readahead logic
+
+The default value is 50. Reasonable values would be [50, 100].
+
+==============================================================
+
+readahead_hit_rate
+
+This is the max allowed value of (readahead-pages : accessed-pages).
+Useful only when (readahead_ratio >= 10). If the previous readahead
+request has bad hit rate, the kernel will be reluctant to do the next
+readahead.
+
+Larger values help catch more sparse access patterns. Be aware that
+readahead of the sparse patterns sacrifices memory for speed.
+
+The default value is 1. It is recommended to keep the value below 16.
--
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