Everything is working nicely now, surviving a serious thrashing.
Overall the behaviour I would say has improved as a result of all the
modifications.
Many thanks go to Nick and Andrew for suggestions.
Andrew please consider this version for -mm.
Cheers,
Con
---
This patch implements swap prefetching when the vm is relatively idle and
there is free ram available. The code is based on some preliminary code by
Thomas Schlichter.
This stores a list of swapped entries in a list ordered most recently used
and a radix tree. It generates a low priority kernel thread running at nice 19
to do the prefetching at a later stage.
Once pages have been added to the swapped list, a timer is started, testing
for conditions suitable to prefetch swap pages every 5 seconds. Suitable
conditions are defined as lack of swapping out or in any pages, and no
watermark tests failing. Significant amounts of dirtied ram and changes in
free ram representing disk writes or reads also prevent prefetching.
It then checks that we have spare ram looking for at least 3* pages_high free
per zone and if it succeeds that will prefetch pages from swap into the swap
cache. The pages are added to the tail of the inactive list to preserve LRU
ordering.
Pages are prefetched until the list is empty or the vm is seen as busy
according to the previously described criteria. Node data on numa is stored
with the entries and an appropriate zonelist based on this is used when
allocating ram.
The pages are copied to swap cache and kept on backing store. This allows
pressure on either physical ram or swap to readily find free pages without
further I/O.
Prefetching can be enabled/disabled via the tunable in
/proc/sys/vm/swap_prefetch initially set to 1 (enabled).
In testing on modern pc hardware this results in wall-clock time activation of
the firefox browser to speed up 5 fold after a worst case complete swap-out
of the browser on a static web page.
Signed-off-by: Con Kolivas <[email protected]>
Documentation/sysctl/vm.txt | 11
include/linux/mm_inline.h | 7
include/linux/swap.h | 55 ++++
include/linux/sysctl.h | 1
init/Kconfig | 22 +
kernel/sysctl.c | 10
mm/Makefile | 1
mm/swap.c | 42 +++
mm/swap_prefetch.c | 488 ++++++++++++++++++++++++++++++++++++++++++++
mm/swap_state.c | 10
mm/vmscan.c | 5
11 files changed, 651 insertions(+), 1 deletion(-)
Index: linux-2.6.16-rc2-sp/Documentation/sysctl/vm.txt
===================================================================
--- linux-2.6.16-rc2-sp.orig/Documentation/sysctl/vm.txt 2006-02-08 23:22:30.000000000 +1100
+++ linux-2.6.16-rc2-sp/Documentation/sysctl/vm.txt 2006-02-09 22:51:24.000000000 +1100
@@ -29,6 +29,7 @@ Currently, these files are in /proc/sys/
- drop-caches
- zone_reclaim_mode
- zone_reclaim_interval
+- swap_prefetch
==============================================================
@@ -178,3 +179,13 @@ Time is set in seconds and set by defaul
Reduce the interval if undesired off node allocations occur. However, too
frequent scans will have a negative impact onoff node allocation performance.
+==============================================================
+
+swap_prefetch
+
+This enables or disables the swap prefetching feature. When the virtual
+memory subsystem has been extremely idle for at least 5 seconds it will start
+copying back pages from swap into the swapcache and keep a copy in swap. In
+practice it can take many minutes before the vm is idle enough.
+
+The default value is 1.
Index: linux-2.6.16-rc2-sp/include/linux/swap.h
===================================================================
--- linux-2.6.16-rc2-sp.orig/include/linux/swap.h 2006-02-08 23:22:30.000000000 +1100
+++ linux-2.6.16-rc2-sp/include/linux/swap.h 2006-02-12 01:25:22.000000000 +1100
@@ -7,6 +7,7 @@
#include <linux/mmzone.h>
#include <linux/list.h>
#include <linux/sched.h>
+#include <linux/mm.h>
#include <asm/atomic.h>
#include <asm/page.h>
@@ -164,6 +165,7 @@ extern unsigned int nr_free_pagecache_pa
/* linux/mm/swap.c */
extern void FASTCALL(lru_cache_add(struct page *));
extern void FASTCALL(lru_cache_add_active(struct page *));
+extern void FASTCALL(lru_cache_add_tail(struct page *));
extern void FASTCALL(activate_page(struct page *));
extern void FASTCALL(mark_page_accessed(struct page *));
extern void lru_add_drain(void);
@@ -214,6 +216,58 @@ extern int shmem_unuse(swp_entry_t entry
extern void swap_unplug_io_fn(struct backing_dev_info *, struct page *);
+#ifdef CONFIG_SWAP_PREFETCH
+/* mm/swap_prefetch.c */
+extern int swap_prefetch;
+struct swapped_entry {
+ swp_entry_t swp_entry; /* The actual swap entry */
+ struct list_head swapped_list; /* Linked list of entries */
+#if MAX_NUMNODES > 1
+ int node; /* Node id */
+#endif
+} __attribute__((packed));
+
+static inline void store_swap_entry_node(struct swapped_entry *entry,
+ struct page *page)
+{
+#if MAX_NUMNODES > 1
+ entry->node = page_to_nid(page);
+#endif
+}
+
+static inline int get_swap_entry_node(struct swapped_entry *entry)
+{
+#if MAX_NUMNODES > 1
+ return entry->node;
+#else
+ return 0;
+#endif
+}
+
+extern void add_to_swapped_list(struct page *page);
+extern void remove_from_swapped_list(const unsigned long index);
+extern void delay_swap_prefetch(void);
+extern void prepare_swap_prefetch(void);
+
+#else /* CONFIG_SWAP_PREFETCH */
+static inline void add_to_swapped_list(struct page *__unused)
+{
+}
+
+static inline void prepare_swap_prefetch(void)
+{
+}
+
+static inline void remove_from_swapped_list(const unsigned long __unused)
+{
+}
+
+static inline void delay_swap_prefetch(void)
+{
+}
+
+#endif /* CONFIG_SWAP_PREFETCH */
+
#ifdef CONFIG_SWAP
/* linux/mm/page_io.c */
extern int swap_readpage(struct file *, struct page *);
@@ -235,6 +289,7 @@ extern void free_pages_and_swap_cache(st
extern struct page * lookup_swap_cache(swp_entry_t);
extern struct page * read_swap_cache_async(swp_entry_t, struct vm_area_struct *vma,
unsigned long addr);
+extern int add_to_swap_cache(struct page *page, swp_entry_t entry);
/* linux/mm/swapfile.c */
extern long total_swap_pages;
extern unsigned int nr_swapfiles;
Index: linux-2.6.16-rc2-sp/include/linux/sysctl.h
===================================================================
--- linux-2.6.16-rc2-sp.orig/include/linux/sysctl.h 2006-02-08 23:22:30.000000000 +1100
+++ linux-2.6.16-rc2-sp/include/linux/sysctl.h 2006-02-08 23:28:01.000000000 +1100
@@ -184,6 +184,7 @@ enum
VM_PERCPU_PAGELIST_FRACTION=30,/* int: fraction of pages in each percpu_pagelist */
VM_ZONE_RECLAIM_MODE=31, /* reclaim local zone memory before going off node */
VM_ZONE_RECLAIM_INTERVAL=32, /* time period to wait after reclaim failure */
+ VM_SWAP_PREFETCH=33, /* swap prefetch */
};
Index: linux-2.6.16-rc2-sp/init/Kconfig
===================================================================
--- linux-2.6.16-rc2-sp.orig/init/Kconfig 2006-02-08 23:22:30.000000000 +1100
+++ linux-2.6.16-rc2-sp/init/Kconfig 2006-02-08 23:28:01.000000000 +1100
@@ -103,6 +103,28 @@ config SWAP
used to provide more virtual memory than the actual RAM present
in your computer. If unsure say Y.
+config SWAP_PREFETCH
+ bool "Support for prefetching swapped memory"
+ depends on SWAP
+ default y
+ ---help---
+ This option will allow the kernel to prefetch swapped memory pages
+ when idle. The pages will be kept on both swap and in swap_cache
+ thus avoiding the need for further I/O if either ram or swap space
+ is required.
+
+ What this will do on workstations is slowly bring back applications
+ that have swapped out after memory intensive workloads back into
+ physical ram if you have free ram at a later stage and the machine
+ is relatively idle. This means that when you come back to your
+ computer after leaving it idle for a while, applications will come
+ to life faster. Note that your swap usage will appear to increase
+ but these are cached pages, can be dropped freely by the vm, and it
+ should stabilise around 50% swap usage maximum.
+
+ Workstations and multiuser workstation servers will most likely want
+ to say Y.
+
config SYSVIPC
bool "System V IPC"
---help---
Index: linux-2.6.16-rc2-sp/kernel/sysctl.c
===================================================================
--- linux-2.6.16-rc2-sp.orig/kernel/sysctl.c 2006-02-08 23:22:30.000000000 +1100
+++ linux-2.6.16-rc2-sp/kernel/sysctl.c 2006-02-08 23:28:01.000000000 +1100
@@ -891,6 +891,16 @@ static ctl_table vm_table[] = {
.strategy = &sysctl_jiffies,
},
#endif
+#ifdef CONFIG_SWAP_PREFETCH
+ {
+ .ctl_name = VM_SWAP_PREFETCH,
+ .procname = "swap_prefetch",
+ .data = &swap_prefetch,
+ .maxlen = sizeof(swap_prefetch),
+ .mode = 0644,
+ .proc_handler = &proc_dointvec,
+ },
+#endif
{ .ctl_name = 0 }
};
Index: linux-2.6.16-rc2-sp/mm/Makefile
===================================================================
--- linux-2.6.16-rc2-sp.orig/mm/Makefile 2006-02-08 23:22:30.000000000 +1100
+++ linux-2.6.16-rc2-sp/mm/Makefile 2006-02-08 23:28:01.000000000 +1100
@@ -13,6 +13,7 @@ obj-y := bootmem.o filemap.o mempool.o
prio_tree.o util.o $(mmu-y)
obj-$(CONFIG_SWAP) += page_io.o swap_state.o swapfile.o thrash.o
+obj-$(CONFIG_SWAP_PREFETCH) += swap_prefetch.o
obj-$(CONFIG_HUGETLBFS) += hugetlb.o
obj-$(CONFIG_NUMA) += mempolicy.o
obj-$(CONFIG_SPARSEMEM) += sparse.o
Index: linux-2.6.16-rc2-sp/mm/swap.c
===================================================================
--- linux-2.6.16-rc2-sp.orig/mm/swap.c 2006-02-08 23:22:30.000000000 +1100
+++ linux-2.6.16-rc2-sp/mm/swap.c 2006-02-12 01:25:22.000000000 +1100
@@ -370,6 +370,45 @@ void __pagevec_lru_add_active(struct pag
pagevec_reinit(pvec);
}
+static inline void __pagevec_lru_add_tail(struct pagevec *pvec)
+{
+ int i;
+ struct zone *zone = NULL;
+
+ for (i = 0; i < pagevec_count(pvec); i++) {
+ struct page *page = pvec->pages[i];
+ struct zone *pagezone = page_zone(page);
+
+ if (pagezone != zone) {
+ if (zone)
+ spin_unlock_irq(&zone->lru_lock);
+ zone = pagezone;
+ spin_lock_irq(&zone->lru_lock);
+ }
+ if (TestSetPageLRU(page))
+ BUG();
+ add_page_to_inactive_list_tail(zone, page);
+ }
+ if (zone)
+ spin_unlock_irq(&zone->lru_lock);
+ release_pages(pvec->pages, pvec->nr, pvec->cold);
+ pagevec_reinit(pvec);
+}
+
+/*
+ * Function used uniquely to put pages back to the lru at the end of the
+ * inactive list currently only used by swap prefetch.
+ */
+void fastcall lru_cache_add_tail(struct page *page)
+{
+ struct pagevec *pvec = &get_cpu_var(lru_add_pvecs);
+
+ page_cache_get(page);
+ if (!pagevec_add(pvec, page))
+ __pagevec_lru_add_tail(pvec);
+ put_cpu_var(lru_add_pvecs);
+}
+
/*
* Try to drop buffers from the pages in a pagevec
*/
@@ -502,5 +541,8 @@ void __init swap_setup(void)
* Right now other parts of the system means that we
* _really_ don't want to cluster much more
*/
+
+ prepare_swap_prefetch();
+
hotcpu_notifier(cpu_swap_callback, 0);
}
Index: linux-2.6.16-rc2-sp/mm/swap_prefetch.c
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6.16-rc2-sp/mm/swap_prefetch.c 2006-02-12 01:25:22.000000000 +1100
@@ -0,0 +1,488 @@
+/*
+ * linux/mm/swap_prefetch.c
+ *
+ * Copyright (C) 2005-2006 Con Kolivas
+ *
+ * Written by Con Kolivas <[email protected]>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#include <linux/fs.h>
+#include <linux/swap.h>
+#include <linux/ioprio.h>
+#include <linux/kthread.h>
+#include <linux/pagemap.h>
+#include <linux/syscalls.h>
+#include <linux/writeback.h>
+
+/*
+ * Time to delay prefetching if vm is busy or prefetching unsuccessful. There
+ * needs to be at least this duration of idle time meaning in practice it can
+ * be much longer
+ */
+#define PREFETCH_DELAY (HZ * 5)
+
+/* sysctl - enable/disable swap prefetching */
+int swap_prefetch __read_mostly = 1;
+
+struct swapped_root {
+ unsigned long busy; /* vm busy */
+ spinlock_t lock; /* protects all data */
+ struct list_head list; /* MRU list of swapped pages */
+ struct radix_tree_root swap_tree; /* Lookup tree of pages */
+ unsigned int count; /* Number of entries */
+ unsigned int maxcount; /* Maximum entries allowed */
+ kmem_cache_t *cache; /* Of struct swapped_entry */
+};
+
+static struct swapped_root swapped = {
+ .busy = 0, /* Any vm activity */
+ .lock = SPIN_LOCK_UNLOCKED,
+ .list = LIST_HEAD_INIT(swapped.list),
+ .swap_tree = RADIX_TREE_INIT(GFP_ATOMIC),
+ .count = 0, /* Number of swapped entries */
+};
+
+static task_t *kprefetchd_task;
+
+/*
+ * We check to see no part of the vm is busy. If it is this will interrupt
+ * trickle_swap and wait another PREFETCH_DELAY. Purposefully racy.
+ */
+inline void delay_swap_prefetch(void)
+{
+ if (!test_bit(0, &swapped.busy))
+ __set_bit(0, &swapped.busy);
+}
+
+/*
+ * Drop behind accounting which keeps a list of the most recently used swap
+ * entries.
+ */
+void add_to_swapped_list(struct page *page)
+{
+ struct swapped_entry *entry;
+ unsigned long index;
+ int wakeup;
+
+ if (!swap_prefetch)
+ return;
+
+ wakeup = 0;
+
+ spin_lock(&swapped.lock);
+ if (swapped.count >= swapped.maxcount) {
+ /*
+ * We limit the number of entries to 2/3 of physical ram.
+ * Once the number of entries exceeds this we start removing
+ * the least recently used entries.
+ */
+ entry = list_entry(swapped.list.next,
+ struct swapped_entry, swapped_list);
+ radix_tree_delete(&swapped.swap_tree, entry->swp_entry.val);
+ list_del(&entry->swapped_list);
+ swapped.count--;
+ } else {
+ entry = kmem_cache_alloc(swapped.cache, GFP_ATOMIC);
+ if (unlikely(!entry))
+ /* bad, can't allocate more mem */
+ goto out_locked;
+ }
+
+ index = page_private(page);
+ entry->swp_entry.val = index;
+ /*
+ * On numa we need to store the node id to ensure that we prefetch to
+ * the same node it came from.
+ */
+ store_swap_entry_node(entry, page);
+
+ if (likely(!radix_tree_insert(&swapped.swap_tree, index, entry))) {
+ /*
+ * If this is the first entry, kprefetchd needs to be
+ * (re)started.
+ */
+ if (list_empty(&swapped.list))
+ wakeup = 1;
+ list_add(&entry->swapped_list, &swapped.list);
+ swapped.count++;
+ }
+
+out_locked:
+ spin_unlock(&swapped.lock);
+
+ /* Do the wakeup outside the lock to shorten lock hold time. */
+ if (wakeup)
+ wake_up_process(kprefetchd_task);
+
+ return;
+}
+
+/*
+ * Removes entries from the swapped_list. The radix tree allows us to quickly
+ * look up the entry from the index without having to iterate over the whole
+ * list.
+ */
+void remove_from_swapped_list(const unsigned long index)
+{
+ struct swapped_entry *entry;
+ unsigned long flags;
+
+ if (list_empty(&swapped.list))
+ return;
+
+ spin_lock_irqsave(&swapped.lock, flags);
+ entry = radix_tree_delete(&swapped.swap_tree, index);
+ if (likely(entry)) {
+ list_del_init(&entry->swapped_list);
+ swapped.count--;
+ kmem_cache_free(swapped.cache, entry);
+ }
+ spin_unlock_irqrestore(&swapped.lock, flags);
+}
+
+enum trickle_return {
+ TRICKLE_SUCCESS,
+ TRICKLE_FAILED,
+ TRICKLE_DELAY,
+};
+
+/*
+ * prefetch_stats stores the free ram data of each node and this is used to
+ * determine if a node is suitable for prefetching into.
+ */
+struct prefetch_stats{
+ unsigned long last_free[MAX_NUMNODES];
+ /* Free ram after a cycle of prefetching */
+ unsigned long current_free[MAX_NUMNODES];
+ /* Free ram on this cycle of checking prefetch_suitable */
+ unsigned long prefetch_watermark[MAX_NUMNODES];
+ /* Maximum amount we will prefetch to */
+ nodemask_t prefetch_nodes;
+ /* Which nodes are currently suited to prefetching */
+ unsigned long prefetched_pages;
+ /* Total pages we've prefetched on this wakeup of kprefetchd */
+};
+
+static struct prefetch_stats sp_stat;
+
+/*
+ * This tries to read a swp_entry_t into swap cache for swap prefetching.
+ * If it returns TRICKLE_DELAY we should delay further prefetching.
+ */
+static enum trickle_return trickle_swap_cache_async(const swp_entry_t entry,
+ const int node)
+{
+ enum trickle_return ret = TRICKLE_FAILED;
+ struct page *page;
+
+ read_lock_irq(&swapper_space.tree_lock);
+ /* Entry may already exist */
+ page = radix_tree_lookup(&swapper_space.page_tree, entry.val);
+ read_unlock_irq(&swapper_space.tree_lock);
+ if (page) {
+ remove_from_swapped_list(entry.val);
+ goto out;
+ }
+
+ /*
+ * Get a new page to read from swap. We have already checked the
+ * watermarks so __alloc_pages will not call on reclaim.
+ */
+ page = alloc_pages_node(node, GFP_HIGHUSER & ~__GFP_WAIT, 0);
+ if (unlikely(!page)) {
+ ret = TRICKLE_DELAY;
+ goto out;
+ }
+
+ if (add_to_swap_cache(page, entry)) {
+ /* Failed to add to swap cache */
+ goto out_release;
+ }
+
+ /* Add them to the tail of the inactive list to preserve LRU order */
+ lru_cache_add_tail(page);
+ if (unlikely(swap_readpage(NULL, page))) {
+ ret = TRICKLE_DELAY;
+ goto out_release;
+ }
+
+ sp_stat.prefetched_pages++;
+ sp_stat.last_free[node]--;
+
+ ret = TRICKLE_SUCCESS;
+out_release:
+ page_cache_release(page);
+out:
+ return ret;
+}
+
+static void clear_last_prefetch_free(void)
+{
+ int node;
+
+ /*
+ * Reset the nodes suitable for prefetching to all nodes. We could
+ * update the data to take into account memory hotplug if desired..
+ */
+ sp_stat.prefetch_nodes = node_online_map;
+ for_each_node_mask(node, sp_stat.prefetch_nodes)
+ sp_stat.last_free[node] = 0;
+}
+
+static void clear_current_prefetch_free(void)
+{
+ int node;
+
+ sp_stat.prefetch_nodes = node_online_map;
+ for_each_node_mask(node, sp_stat.prefetch_nodes)
+ sp_stat.current_free[node] = 0;
+}
+
+/*
+ * We want to be absolutely certain it's ok to start prefetching.
+ */
+static int prefetch_suitable(void)
+{
+ struct page_state ps;
+ unsigned long limit;
+ struct zone *z;
+ int node, ret = 0;
+
+ /* Purposefully racy and might return false positive which is ok */
+ if (__test_and_clear_bit(0, &swapped.busy))
+ goto out;
+
+ clear_current_prefetch_free();
+
+ /*
+ * Have some hysteresis between where page reclaiming and prefetching
+ * will occur to prevent ping-ponging between them.
+ */
+ for_each_zone(z) {
+ unsigned long free;
+
+ if (!populated_zone(z))
+ continue;
+ node = z->zone_pgdat->node_id;
+
+ free = z->free_pages;
+ if (z->pages_high * 3 + z->lowmem_reserve[zone_idx(z)] > free) {
+ node_clear(node, sp_stat.prefetch_nodes);
+ continue;
+ }
+ sp_stat.current_free[node] += free;
+ }
+
+ /*
+ * We iterate over each node testing to see if it is suitable for
+ * prefetching and clear the nodemask if it is not.
+ */
+ for_each_node_mask(node, sp_stat.prefetch_nodes) {
+ /*
+ * We check to see that pages are not being allocated
+ * elsewhere at any significant rate implying any
+ * degree of memory pressure (eg during file reads)
+ */
+ if (sp_stat.last_free[node]) {
+ if (sp_stat.current_free[node] + SWAP_CLUSTER_MAX <
+ sp_stat.last_free[node]) {
+ sp_stat.last_free[node] =
+ sp_stat.current_free[node];
+ node_clear(node,
+ sp_stat.prefetch_nodes);
+ continue;
+ }
+ } else
+ sp_stat.last_free[node] = sp_stat.current_free[node];
+
+ /*
+ * get_page_state is super expensive so we only perform it
+ * every SWAP_CLUSTER_MAX prefetched_pages
+ */
+ if (sp_stat.prefetched_pages % SWAP_CLUSTER_MAX)
+ continue;
+
+ get_page_state_node(&ps, node);
+
+ /* We shouldn't prefetch when we are doing writeback */
+ if (ps.nr_writeback) {
+ node_clear(node, sp_stat.prefetch_nodes);
+ continue;
+ }
+
+ /*
+ * >2/3 of the ram on this node is mapped, slab, swapcache or
+ * dirty, we need to leave some free for pagecache.
+ * Note that currently nr_slab is innacurate on numa because
+ * nr_slab is incremented on the node doing the accounting
+ * even if the slab is being allocated on a remote node. This
+ * would be expensive to fix and not of great significance.
+ */
+ limit = ps.nr_mapped + ps.nr_slab + ps.nr_dirty +
+ ps.nr_unstable + total_swapcache_pages;
+ if (limit > sp_stat.prefetch_watermark[node]) {
+ node_clear(node, sp_stat.prefetch_nodes);
+ continue;
+ }
+ }
+
+ if (nodes_empty(sp_stat.prefetch_nodes))
+ goto out;
+
+ /* Survived all that? Hooray we can prefetch! */
+ ret = 1;
+out:
+ return ret;
+}
+
+/*
+ * Get next swapped entry when iterating over all entries. swapped.lock
+ * should be held and we should already ensure that entry exists.
+ */
+static inline struct swapped_entry *next_swapped_entry
+ (struct swapped_entry *entry)
+{
+ return list_entry(entry->swapped_list.next->next,
+ struct swapped_entry, swapped_list);
+}
+
+/*
+ * trickle_swap is the main function that initiates the swap prefetching. It
+ * first checks to see if the busy flag is set, and does not prefetch if it
+ * is, as the flag implied we are low on memory or swapping in currently.
+ * Otherwise it runs until prefetch_suitable fails which occurs when the
+ * vm is busy, we prefetch to the watermark, or the list is empty.
+ */
+static enum trickle_return trickle_swap(void)
+{
+ enum trickle_return ret = TRICKLE_DELAY;
+ struct swapped_entry *entry;
+
+ if (!swap_prefetch)
+ return ret;
+
+ entry = NULL;
+
+ for ( ; ; ) {
+ swp_entry_t swp_entry;
+ int node;
+
+ if (!prefetch_suitable())
+ break;
+
+ spin_lock(&swapped.lock);
+ if (list_empty(&swapped.list)) {
+ ret = TRICKLE_FAILED;
+ spin_unlock(&swapped.lock);
+ break;
+ }
+
+ if (!entry) {
+ /*
+ * This sets the entry for the first iteration. It
+ * also is a safeguard against the entry disappearing
+ * while the lock is not held.
+ */
+ entry = list_entry(swapped.list.next,
+ struct swapped_entry, swapped_list);
+ } else if (entry->swapped_list.next == swapped.list.next) {
+ /* Have we iterated over all entries? */
+ spin_unlock(&swapped.lock);
+ break;
+ }
+
+ node = get_swap_entry_node(entry);
+ if (!node_isset(node, sp_stat.prefetch_nodes)) {
+ /*
+ * We found an entry that belongs to a node that is
+ * not suitable for prefetching so skip it.
+ */
+ entry = next_swapped_entry(entry);
+ spin_unlock(&swapped.lock);
+ continue;
+ }
+ swp_entry = entry->swp_entry;
+ entry = next_swapped_entry(entry);
+ spin_unlock(&swapped.lock);
+
+ if (trickle_swap_cache_async(swp_entry, node) == TRICKLE_DELAY)
+ break;
+ }
+
+ if (sp_stat.prefetched_pages) {
+ lru_add_drain();
+ sp_stat.prefetched_pages = 0;
+ }
+ return ret;
+}
+
+static int kprefetchd(void *__unused)
+{
+ set_user_nice(current, 19);
+ /* Set ioprio to lowest if supported by i/o scheduler */
+ sys_ioprio_set(IOPRIO_WHO_PROCESS, 0, IOPRIO_CLASS_IDLE);
+
+ do {
+ try_to_freeze();
+
+ /*
+ * TRICKLE_FAILED implies no entries left - we do not schedule
+ * a wakeup, and further delay the next one.
+ */
+ if (trickle_swap() == TRICKLE_FAILED) {
+ set_current_state(TASK_INTERRUPTIBLE);
+ schedule();
+ }
+ clear_last_prefetch_free();
+ schedule_timeout_interruptible(PREFETCH_DELAY);
+ } while (!kthread_should_stop());
+
+ return 0;
+}
+
+/*
+ * Create kmem cache for swapped entries
+ */
+void __init prepare_swap_prefetch(void)
+{
+ pg_data_t *pgdat;
+ int node;
+
+ swapped.cache = kmem_cache_create("swapped_entry",
+ sizeof(struct swapped_entry), 0, SLAB_PANIC, NULL, NULL);
+
+ /*
+ * Set max number of entries to 2/3 the size of physical ram as we
+ * only ever prefetch to consume 2/3 of the ram.
+ */
+ swapped.maxcount = nr_free_pagecache_pages() / 3 * 2;
+
+ for_each_pgdat(pgdat) {
+ unsigned long present;
+
+ present = pgdat->node_present_pages;
+ if (!present)
+ continue;
+ node = pgdat->node_id;
+ sp_stat.prefetch_watermark[node] += present / 3 * 2;
+ }
+}
+
+static int __init kprefetchd_init(void)
+{
+ kprefetchd_task = kthread_run(kprefetchd, NULL, "kprefetchd");
+
+ return 0;
+}
+
+static void __exit kprefetchd_exit(void)
+{
+ kthread_stop(kprefetchd_task);
+}
+
+module_init(kprefetchd_init);
+module_exit(kprefetchd_exit);
Index: linux-2.6.16-rc2-sp/mm/swap_state.c
===================================================================
--- linux-2.6.16-rc2-sp.orig/mm/swap_state.c 2006-02-08 23:22:30.000000000 +1100
+++ linux-2.6.16-rc2-sp/mm/swap_state.c 2006-02-08 23:28:01.000000000 +1100
@@ -81,6 +81,7 @@ static int __add_to_swap_cache(struct pa
error = radix_tree_insert(&swapper_space.page_tree,
entry.val, page);
if (!error) {
+ remove_from_swapped_list(entry.val);
page_cache_get(page);
SetPageLocked(page);
SetPageSwapCache(page);
@@ -94,11 +95,12 @@ static int __add_to_swap_cache(struct pa
return error;
}
-static int add_to_swap_cache(struct page *page, swp_entry_t entry)
+int add_to_swap_cache(struct page *page, swp_entry_t entry)
{
int error;
if (!swap_duplicate(entry)) {
+ remove_from_swapped_list(entry.val);
INC_CACHE_INFO(noent_race);
return -ENOENT;
}
@@ -147,6 +149,9 @@ int add_to_swap(struct page * page, gfp_
swp_entry_t entry;
int err;
+ /* Swap prefetching is delayed if we're swapping pages */
+ delay_swap_prefetch();
+
if (!PageLocked(page))
BUG();
@@ -320,6 +325,9 @@ struct page *read_swap_cache_async(swp_e
struct page *found_page, *new_page = NULL;
int err;
+ /* Swap prefetching is delayed if we're already reading from swap */
+ delay_swap_prefetch();
+
do {
/*
* First check the swap cache. Since this is normally
Index: linux-2.6.16-rc2-sp/mm/vmscan.c
===================================================================
--- linux-2.6.16-rc2-sp.orig/mm/vmscan.c 2006-02-08 23:22:30.000000000 +1100
+++ linux-2.6.16-rc2-sp/mm/vmscan.c 2006-02-09 22:51:24.000000000 +1100
@@ -396,6 +396,7 @@ static int remove_mapping(struct address
if (PageSwapCache(page)) {
swp_entry_t swap = { .val = page_private(page) };
+ add_to_swapped_list(page);
__delete_from_swap_cache(page);
write_unlock_irq(&mapping->tree_lock);
swap_free(swap);
@@ -1406,6 +1407,8 @@ int try_to_free_pages(struct zone **zone
unsigned long lru_pages = 0;
int i;
+ delay_swap_prefetch();
+
sc.gfp_mask = gfp_mask;
sc.may_writepage = !laptop_mode;
sc.may_swap = 1;
@@ -1758,6 +1761,8 @@ int shrink_all_memory(int nr_pages)
.reclaimed_slab = 0,
};
+ delay_swap_prefetch();
+
current->reclaim_state = &reclaim_state;
for_each_pgdat(pgdat) {
int freed;
Index: linux-2.6.16-rc2-sp/include/linux/mm_inline.h
===================================================================
--- linux-2.6.16-rc2-sp.orig/include/linux/mm_inline.h 2006-02-12 01:25:13.000000000 +1100
+++ linux-2.6.16-rc2-sp/include/linux/mm_inline.h 2006-02-12 01:25:22.000000000 +1100
@@ -14,6 +14,13 @@ add_page_to_inactive_list(struct zone *z
}
static inline void
+add_page_to_inactive_list_tail(struct zone *zone, struct page *page)
+{
+ list_add_tail(&page->lru, &zone->inactive_list);
+ zone->nr_inactive++;
+}
+
+static inline void
del_page_from_active_list(struct zone *zone, struct page *page)
{
list_del(&page->lru);
On Sun, 12 Feb 2006, Con Kolivas wrote:
> Once pages have been added to the swapped list, a timer is started, testing
> for conditions suitable to prefetch swap pages every 5 seconds. Suitable
> conditions are defined as lack of swapping out or in any pages, and no
> watermark tests failing. Significant amounts of dirtied ram and changes in
> free ram representing disk writes or reads also prevent prefetching.
>
> It then checks that we have spare ram looking for at least 3* pages_high free
> per zone and if it succeeds that will prefetch pages from swap into the swap
> cache. The pages are added to the tail of the inactive list to preserve LRU
> ordering.
spare ram when swapping??? We are already under memory pressure. Why make
it worse by getting rid of the few bits of available memory? If a system
swaps then we are per definition in the bad performance range. Add more
memory.
On Mon, 13 Feb 2006, Christoph Lameter wrote:
> spare ram when swapping??? We are already under memory pressure. Why make
> it worse by getting rid of the few bits of available memory? If a system
> swaps then we are per definition in the bad performance range. Add more
> memory.
when a program exits it's memory is now free, rather then just waiting
until something uses this memory up normally, this patch attempts to fill
that memory with things that are expected to be useful (things that were
swapped out)
this won't be a win in all cases by any means, but if it uses disk
bandwidth that would otherwise be idle and memory that is empty (and is at
the tail of the LRU list so it's the first to be thrown away) the cost of
doing this should be close to zero.
David Lang
--
There are two ways of constructing a software design. One way is to make it so simple that there are obviously no deficiencies. And the other way is to make it so complicated that there are no obvious deficiencies.
-- C.A.R. Hoare
On Mon, 13 Feb 2006, David Lang wrote:
> On Mon, 13 Feb 2006, Christoph Lameter wrote:
> > spare ram when swapping??? We are already under memory pressure. Why make
> > it worse by getting rid of the few bits of available memory? If a system
> > swaps then we are per definition in the bad performance range. Add more
> > memory.
> when a program exits it's memory is now free, rather then just waiting until
> something uses this memory up normally, this patch attempts to fill that
> memory with things that are expected to be useful (things that were swapped
> out)
Then trigger this action when a program exits and when you know there was
enough freed up to justify such an action. However, this is still a
heuristic. No one knows if the pages read from swap will be of any use at
all. For all I know a process may want to allocate some memory and fail
because the memory was needlessly spend to read in pages that no one
needs.
On Mon, 13 Feb 2006, Christoph Lameter wrote:
> Date: Mon, 13 Feb 2006 13:16:00 -0800 (PST)
> From: Christoph Lameter <[email protected]>
> To: David Lang <[email protected]>
> Cc: Con Kolivas <[email protected]>,
> linux kernel mailing list <[email protected]>,
> Andrew Morton <[email protected]>, ck list <[email protected]>
> Subject: Re: [PATCH] mm: Implement Swap Prefetching v25
>
> On Mon, 13 Feb 2006, David Lang wrote:
>
>> On Mon, 13 Feb 2006, Christoph Lameter wrote:
>>> spare ram when swapping??? We are already under memory pressure. Why make
>>> it worse by getting rid of the few bits of available memory? If a system
>>> swaps then we are per definition in the bad performance range. Add more
>>> memory.
>> when a program exits it's memory is now free, rather then just waiting until
>> something uses this memory up normally, this patch attempts to fill that
>> memory with things that are expected to be useful (things that were swapped
>> out)
>
> Then trigger this action when a program exits and when you know there was
> enough freed up to justify such an action. However, this is still a
> heuristic. No one knows if the pages read from swap will be of any use at
> all. For all I know a process may want to allocate some memory and fail
> because the memory was needlessly spend to read in pages that no one
> needs.
this won't happen, becouse if a program requests memory the kernel will
through away data from the swap cache that's unchanged on disk (like the
memory just filled by this) to free up memory for the program.
you don't want to trigger this on a program exit becouse the system is
likly to be busy doing other things at that time, you want to try and do
this when the system is otherwise idle.
David Lang
--
There are two ways of constructing a software design. One way is to make it so simple that there are obviously no deficiencies. And the other way is to make it so complicated that there are no obvious deficiencies.
-- C.A.R. Hoare
On Tue, 14 Feb 2006 07:58 am, Christoph Lameter wrote:
> On Sun, 12 Feb 2006, Con Kolivas wrote:
> > Once pages have been added to the swapped list, a timer is started,
> > testing for conditions suitable to prefetch swap pages every 5 seconds.
> > Suitable conditions are defined as lack of swapping out or in any pages,
> > and no watermark tests failing. Significant amounts of dirtied ram and
> > changes in free ram representing disk writes or reads also prevent
> > prefetching.
> >
> > It then checks that we have spare ram looking for at least 3* pages_high
> > free per zone and if it succeeds that will prefetch pages from swap into
> > the swap cache. The pages are added to the tail of the inactive list to
> > preserve LRU ordering.
>
> spare ram when swapping??? We are already under memory pressure. Why make
> it worse by getting rid of the few bits of available memory? If a system
> swaps then we are per definition in the bad performance range. Add more
> memory.
This patch only prefetches the swapped pages when idle and tacks them onto the
end of the inactive list so they will be the first thing that is removed if
we need ram in the future.
Ordinary desktops swap under ordinary workloads day to day on normal pcs. It
is extraordinary common that one workload will swap out other applications
and then when complete we have free ram, yet our applications are floundering
on swapspace.
Your solution of add more memory is unrealistic. Desktops do have a limit on
how much ram we are likely to have/can afford and our workloads have, do, and
will for ever more outstrip all available ram. A huge chunk of linux in the
wild is on these sort of desktops. This patch makes a substantial difference
to ordinary desktops, is configurable on or off and it can be runtime
enabled/disabled.
Cheers,
Con
Hello,
Con Kolivas wrote:
> Everything is working nicely now, surviving a serious thrashing.
> Overall the behaviour I would say has improved as a result of all the
> modifications.
>
> Many thanks go to Nick and Andrew for suggestions.
>
> Andrew please consider this version for -mm.
>
> Cheers,
> Con
I'm what one would call a typical desktop user. I also very much a
layman with kernels. The layman is very interested in believing, if not
knowing, that the kernel is doing everything within its power to provide
application responsiveness, especially during heavy application loads.
Swap Prefetch has quantifiable and visible results when an application
is swapped out and then swapped back in after a idle period (a very
logical time to do things). That /is/ an obvious interaction benefit.
I play World of Warcraft daily and it forces many applications into
swap. That said, I now regularly experience desktop conditions that are
_very_ favorable with the Swap Prefetch patch soon as WoW is cleared out
and resources are regained for productive desktop use (browser, email,
chat, media, etc). Hard disk activity, after an idle period, almost
never get in the way of interactivity (likely due to swapping out) --
because they're back in physical memory. That is _very_ good use of
time and resources.
I'm very interested in seeing this patch applied to MM for major
testing, so that others with typical desktop demands can get an idea of
how this patch is beneficial.
Thanks for your attention,
Best of regards,
Ryan M.