- new sysctl entries in /proc/sys/vm:
- readahead_ratio = 1
- readahead_hit_rate = 2
- dynamic minimal/initial read-ahead size.
For now, different ranges of readahead_ratio select different read-ahead
code path:
condition action
===========================================================
readahead_ratio == 0 disable read-ahead
readahead_ratio < 9 select old read-ahead logic
readahead_ratio >= 9 select new read-ahead logic
Signed-off-by: Wu Fengguang <[email protected]>
---
Documentation/sysctl/vm.txt | 38 ++++++++++++++++++++++++++++++++++++++
include/linux/sysctl.h | 2 ++
kernel/sysctl.c | 23 +++++++++++++++++++++++
mm/readahead.c | 35 +++++++++++++++++++++++++++++++++++
4 files changed, 98 insertions(+)
--- linux.orig/Documentation/sysctl/vm.txt
+++ linux/Documentation/sysctl/vm.txt
@@ -28,6 +28,8 @@ Currently, these files are in /proc/sys/
- block_dump
- drop-caches
- swap_prefetch
+- readahead_ratio
+- readahead_hit_rate
==============================================================
@@ -132,3 +134,39 @@ laptop_mode is enabled and then it is te
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 speedup reads.
+
+readahead_ratio also selects the read-ahead logic:
+0: disable read-ahead totally
+1-9: select the well tested old read-ahead logic
+10-inf: select the new adaptive read-ahead logic
+
+The default value is 1.
+For the new adaptive read-ahead, reasonable values would be 50-100.
+
+==============================================================
+
+readahead_hit_rate
+
+This is the max allowed value of (read-ahead-pages : accessed-pages).
+Useful only when (readahead_ratio >= 10). 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.
--- linux.orig/include/linux/sysctl.h
+++ linux/include/linux/sysctl.h
@@ -184,6 +184,8 @@ enum
VM_DROP_PAGECACHE=29, /* int: nuke lots of pagecache */
VM_PERCPU_PAGELIST_FRACTION=30,/* int: fraction of pages in each percpu_pagelist */
VM_SWAP_PREFETCH=31, /* int: amount to swap prefetch */
+ VM_READAHEAD_RATIO=32, /* percent of read-ahead size to thrashing-threshold */
+ VM_READAHEAD_HIT_RATE=33, /* one accessed page legitimizes so many read-ahead pages */
};
/* CTL_NET names: */
--- linux.orig/kernel/sysctl.c
+++ linux/kernel/sysctl.c
@@ -70,6 +70,8 @@ extern int printk_ratelimit_burst;
extern int pid_max_min, pid_max_max;
extern int sysctl_drop_caches;
extern int percpu_pagelist_fraction;
+extern int readahead_ratio;
+extern int readahead_hit_rate;
#if defined(CONFIG_X86_LOCAL_APIC) && defined(CONFIG_X86)
int unknown_nmi_panic;
@@ -671,6 +673,7 @@ 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;
@@ -889,6 +892,26 @@ 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 = 0 }
};
--- linux.orig/mm/readahead.c
+++ linux/mm/readahead.c
@@ -20,6 +20,20 @@
#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 = 1;
+EXPORT_SYMBOL(readahead_ratio);
+
+/* Readahead as long as cache hit ratio keeps above 1/##. */
+int readahead_hit_rate = 2;
+EXPORT_SYMBOL(readahead_hit_rate);
+
/* Detailed classification of read-ahead behaviors. */
#define RA_CLASS_SHIFT 4
#define RA_CLASS_MASK ((1 << RA_CLASS_SHIFT) - 1)
@@ -794,6 +808,27 @@ 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:
--