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 /* + * 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 #include #include +#include /* * 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));