diff --minimal -Nru a/include/linux/mm.h b/include/linux/mm.h
--- a/include/linux/mm.h Fri Jan 4 09:14:28 2002
+++ b/include/linux/mm.h Fri Jan 4 09:14:28 2002
@@ -161,7 +161,6 @@
unsigned char age; /* Page aging counter. */
unsigned char zone; /* Memory zone the page belongs to. */
struct pte_chain * pte_chain; /* Reverse pte mapping pointer. */
- wait_queue_head_t wait; /* Page locked? Stand in line... */
struct page **pprev_hash; /* Complement to *next_hash. */
struct buffer_head * buffers; /* Buffer maps us to a disk block. */
diff --minimal -Nru a/include/linux/mm_inline.h b/include/linux/mm_inline.h
--- a/include/linux/mm_inline.h Fri Jan 4 09:14:28 2002
+++ b/include/linux/mm_inline.h Fri Jan 4 09:14:28 2002
@@ -4,6 +4,39 @@
#include <linux/mm.h>
/*
+ * Knuth recommends primes in approximately golden ratio to the maximum
+ * integer representable by a machine word for multiplicative hashing.
+ * Chuck Lever verified the effectiveness of this technique for several
+ * hash tables in his paper documenting the benchmark results:
+ * http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf
+ */
+#define GOLDEN_RATIO_PRIME 2654435761UL
+
+/*
+ * In order to wait for pages to become available there must be
+ * waitqueues associated with pages. By using a hash table of
+ * waitqueues where the bucket discipline is to maintain all
+ * waiters on the same queue and wake all when any of the pages
+ * become available, and for the woken contexts to check to be
+ * sure the appropriate page became available, this saves space
+ * at a cost of "thundering herd" phenomena during rare hash
+ * collisions. This cost is great enough that effective hashing
+ * is necessary to maintain performance.
+ */
+static inline wait_queue_head_t *page_waitqueue(struct page *page)
+{
+ const zone_t *zone = page_zone(page);
+ wait_queue_head_t *wait = zone->wait_table;
+ unsigned long hash = (unsigned long)page;
+
+ hash *= GOLDEN_RATIO_PRIME;
+ hash >>= BITS_PER_LONG - zone->wait_table_bits;
+ hash &= zone->wait_table_size - 1;
+
+ return &wait[hash];
+}
+
+/*
* These inline functions tend to need bits and pieces of all the
* other VM include files, meaning they cannot be defined inside
* one of the other VM include files.
diff --minimal -Nru a/include/linux/mmzone.h b/include/linux/mmzone.h
--- a/include/linux/mmzone.h Fri Jan 4 09:14:28 2002
+++ b/include/linux/mmzone.h Fri Jan 4 09:14:28 2002
@@ -7,6 +7,7 @@
#include <linux/config.h>
#include <linux/spinlock.h>
#include <linux/list.h>
+#include <linux/wait.h>
/*
* Free memory management - zoned buddy allocator.
@@ -52,6 +53,35 @@
struct list_head inactive_dirty_list;
struct list_head inactive_clean_list;
free_area_t free_area[MAX_ORDER];
+
+ /*
+ * wait_table -- the array holding the hash table
+ * wait_table_size -- the size of the hash table array
+ * wait_table_bits -- wait_table_size == (1 << wait_table_bits)
+ * so it's the integer logarithm of the size
+ *
+ * The purpose of all these is to keep track of the people
+ * waiting for a page to become available and make them
+ * runnable again when possible. The trouble is that this
+ * consumes a lot of space, especially when so few things
+ * wait on pages at a given time. So instead of using
+ * per-page waitqueues, we use a waitqueue hash table.
+ *
+ * The bucket discipline is to sleep on the same queue when
+ * colliding and wake all in that wait queue when removing.
+ * When something wakes, it must check to be sure its page is
+ * truly available, a la thundering herd. The cost of a
+ * collision is great, but given the expected load of the
+ * table, they should be so rare as to be outweighed by the
+ * benefits from the saved space.
+ *
+ *__wait_on_page() and unlock_page() in mm/filemap.c, are the
+ * primary users of these fields, and in mm/page_alloc.c
+ * free_area_init_core() performs the initialization of them.
+ */
+ wait_queue_head_t *wait_table;
+ unsigned long wait_table_size;
+ unsigned long wait_table_bits;
/*
* Discontig memory support fields.
diff --minimal -Nru a/mm/filemap.c b/mm/filemap.c
--- a/mm/filemap.c Fri Jan 4 09:14:28 2002
+++ b/mm/filemap.c Fri Jan 4 09:14:28 2002
@@ -782,13 +782,28 @@
* This must be called with the caller "holding" the page,
* ie with increased "page->count" so that the page won't
* go away during the wait..
+ *
+ * The waiting strategy is to get on a waitqueue determined
+ * by hashing. Waiters will then collide, and the newly woken
+ * task must then determine whether it was woken for the page
+ * it really wanted, and go back to sleep on the waitqueue if
+ * that wasn't it. With the waitqueue semantics, it never leaves
+ * the waitqueue unless it calls, so the loop moves forward one
+ * iteration every time there is
+ * (1) a collision
+ * and
+ * (2) one of the colliding pages is woken
+ *
+ * This is the thundering herd problem, but it is expected to
+ * be very rare due to the few pages that are actually being
+ * waited on at any given time and the quality of the hash function.
*/
void ___wait_on_page(struct page *page)
{
struct task_struct *tsk = current;
DECLARE_WAITQUEUE(wait, tsk);
- add_wait_queue(&page->wait, &wait);
+ add_wait_queue(page_waitqueue(page), &wait);
do {
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
if (!PageLocked(page))
@@ -796,10 +811,17 @@
sync_page(page);
schedule();
} while (PageLocked(page));
- tsk->state = TASK_RUNNING;
- remove_wait_queue(&page->wait, &wait);
+ __set_task_state(tsk, TASK_RUNNING);
+ remove_wait_queue(page_waitqueue(page), &wait);
}
+/*
+ * unlock_page() is the other half of the story just above
+ * __wait_on_page(). Here a couple of quick checks are done
+ * and a couple of flags are set on the page, and then all
+ * of the waiters for all of the pages in the appropriate
+ * wait queue are woken.
+ */
void unlock_page(struct page *page)
{
clear_bit(PG_launder, &(page)->flags);
@@ -807,8 +829,18 @@
if (!test_and_clear_bit(PG_locked, &(page)->flags))
BUG();
smp_mb__after_clear_bit();
- if (waitqueue_active(&(page)->wait))
- wake_up(&(page)->wait);
+
+ if(!page_waitqueue(page))
+ BUG();
+
+ /*
+ * Although the default semantics of wake_up() are
+ * to wake all, here the specific function is used
+ * to make it even more explicit that a number of
+ * pages are being waited on here.
+ */
+ if(waitqueue_active(page_waitqueue(page)))
+ wake_up_all(page_waitqueue(page));
}
/*
@@ -820,7 +852,7 @@
struct task_struct *tsk = current;
DECLARE_WAITQUEUE(wait, tsk);
- add_wait_queue_exclusive(&page->wait, &wait);
+ add_wait_queue_exclusive(page_waitqueue(page), &wait);
for (;;) {
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
if (PageLocked(page)) {
@@ -830,8 +862,8 @@
if (!TryLockPage(page))
break;
}
- tsk->state = TASK_RUNNING;
- remove_wait_queue(&page->wait, &wait);
+ __set_task_state(tsk, TASK_RUNNING);
+ remove_wait_queue(page_waitqueue(page), &wait);
}
diff --minimal -Nru a/mm/page_alloc.c b/mm/page_alloc.c
--- a/mm/page_alloc.c Fri Jan 4 09:14:28 2002
+++ b/mm/page_alloc.c Fri Jan 4 09:14:28 2002
@@ -809,6 +809,44 @@
}
}
+/*
+ * Helper functions to size the waitqueue hash table.
+ * Essentially these want to choose hash table sizes sufficiently
+ * large so that collisions trying to wait on pages are rare.
+ * But in fact, the number of active page waitqueues on typical
+ * systems is ridiculously low, less than 200. So this is even
+ * conservative, even though it seems large.
+ *
+ * The constant PAGES_PER_WAITQUEUE specifies the ratio of pages to
+ * waitqueues, i.e. the size of the waitq table given the number of pages.
+ */
+
+#define PAGES_PER_WAITQUEUE 256
+
+static inline unsigned long wait_table_size(unsigned long pages)
+{
+ unsigned long size = 1;
+
+ pages /= PAGES_PER_WAITQUEUE;
+
+ while(size < pages)
+ size <<= 1;
+
+ return size;
+}
+
+
+/*
+ * This is an integer logarithm so that shifts can be used later
+ * to extract the more random high bits from the multiplicative
+ * hash function before the remainder is taken.
+ */
+static inline unsigned long wait_table_bits(unsigned long size)
+{
+ return ffz(~size);
+}
+
+
#define LONG_ALIGN(x) (((x)+(sizeof(long))-1)&~((sizeof(long))-1))
/*
@@ -883,9 +921,28 @@
INIT_LIST_HEAD(&zone->active_list);
INIT_LIST_HEAD(&zone->inactive_dirty_list);
INIT_LIST_HEAD(&zone->inactive_clean_list);
+
if (!size)
continue;
+ /*
+ * The per-page waitqueue mechanism requires hash tables
+ * whose buckets are waitqueues. These hash tables are
+ * per-zone, and dynamically sized according to the size
+ * of the zone so as to maintain a good ratio of waiters
+ * to hash table buckets. Right here we just allocate
+ * and initialize them for later use (in filemap.c)
+ */
+ zone->wait_table_size = wait_table_size(size);
+ zone->wait_table_bits = wait_table_bits(zone->wait_table_size);
+ zone->wait_table = (wait_queue_head_t *)
+ alloc_bootmem_node(pgdat,
+ zone->wait_table_size
+ * sizeof(wait_queue_head_t));
+
+ for(i = 0; i < zone->wait_table_size; ++i)
+ init_waitqueue_head(zone->wait_table + i);
+
pgdat->nr_zones = j+1;
mask = (realsize / zone_balance_ratio[j]);
@@ -926,7 +983,6 @@
set_page_zone(page, pgdat->node_id * MAX_NR_ZONES + j);
init_page_count(page);
__SetPageReserved(page);
- init_waitqueue_head(&page->wait);
memlist_init(&page->list);
if (j != ZONE_HIGHMEM)
set_page_address(page, __va(zone_start_paddr));