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Use less memory for each entry in a hashtable

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Our hash-table implementation stored a copy of the hash code in each
element.  But as we were using it, all of our hash codes were
ridiculously easy to calculate: most of them were just a matter of a
load and a shift.

This patch lets ht-internal be built in either of two ways: one caches
the hash-code for each element, and one recalculates it each time it's
needed.

This patch also chooses a slightly better hash code for
event_debug_entry.
This commit is contained in:
Nick Mathewson 2010-01-25 13:44:56 -05:00
parent a19b4a05e6
commit a66e947b8b
3 changed files with 41 additions and 16 deletions
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2
event-internal.h
View File

@ -91,6 +91,8 @@ struct eventop {
#define EVMAP_USE_HT
#endif
/* #define HT_CACHE_HASH_VALS */
#ifdef EVMAP_USE_HT
#include "ht-internal.h"
struct event_map_entry;

5
event.c
View File

@ -157,7 +157,10 @@ struct event_debug_entry {
static inline unsigned
hash_debug_entry(const struct event_debug_entry *e)
{
return ((unsigned)e->ptr) >> 3;
/* Our hashtable implementation is pretty sensitive to low bits,
* and every struct event is over 64 bytes in size, so we can
* just say... */
return ((unsigned)e->ptr) >> 6;
}
static inline int

50
ht-internal.h
View File

@ -25,19 +25,22 @@
#define HT_INITIALIZER() \
{ NULL, 0, 0, 0, -1 }
#ifdef HT_CACHE_HASH_VALUES
#define HT_ENTRY(type) \
struct { \
struct type *hte_next; \
unsigned hte_hash; \
}
#else
#define HT_ENTRY(type) \
struct { \
struct type *hte_next; \
}
#endif
#define HT_EMPTY(head) \
((head)->hth_n_entries == 0)
/* Helper: alias for the bucket containing 'elm'. */
#define _HT_BUCKET(head, field, elm) \
((head)->hth_table[elm->field.hte_hash % head->hth_table_length])
/* How many elements in 'head'? */
#define HT_SIZE(head) \
((head)->hth_n_entries)
@ -94,8 +97,25 @@ ht_string_hash(const char *s)
return h;
}
#ifdef HT_CACHE_HASH_VALUES
#define _HT_SET_HASH(elm, field, hashfn) \
(elm)->field.hte_hash = hashfn(elm)
do { (elm)->field.hte_hash = hashfn(elm); } while(0)
#define _HT_SET_HASHVAL(elm, field, val) \
do { (elm)->field.hte_hash = (val); } while(0)
#define _HT_ELT_HASH(elm, field, hashfn) \
((elm)->field.hte_hash)
#else
#define _HT_SET_HASH(elm, field, hashfn) \
((void)0)
#define _HT_ELT_HASH(elm, field, hashfn) \
(hashfn(elm))
#define _HT_SET_HASHVAL(elm, field, val) \
((void)0)
#endif
/* Helper: alias for the bucket containing 'elm'. */
#define _HT_BUCKET(head, field, elm, hashfn) \
((head)->hth_table[_HT_ELT_HASH(elm,field,hashfn) % head->hth_table_length])
#define HT_FOREACH(x, name, head) \
for ((x) = HT_START(name, head); \
@ -122,7 +142,7 @@ ht_string_hash(const char *s)
struct type **p; \
if (!head->hth_table) \
return NULL; \
p = &_HT_BUCKET(head, field, elm); \
p = &_HT_BUCKET(head, field, elm, hashfn); \
while (*p) { \
if (eqfn(*p, elm)) \
return p; \
@ -151,7 +171,7 @@ ht_string_hash(const char *s)
name##_HT_GROW(head, head->hth_n_entries+1); \
++head->hth_n_entries; \
_HT_SET_HASH(elm, field, hashfn); \
p = &_HT_BUCKET(head, field, elm); \
p = &_HT_BUCKET(head, field, elm, hashfn); \
elm->field.hte_next = *p; \
*p = elm; \
} \
@ -247,7 +267,7 @@ ht_string_hash(const char *s)
if ((*elm)->field.hte_next) { \
return &(*elm)->field.hte_next; \
} else { \
unsigned b = ((*elm)->field.hte_hash % head->hth_table_length)+1; \
unsigned b = (_HT_ELT_HASH(*elm, field, hashfn) % head->hth_table_length)+1; \
while (b < head->hth_table_length) { \
if (head->hth_table[b]) \
return &head->hth_table[b]; \
@ -259,7 +279,7 @@ ht_string_hash(const char *s)
static inline struct type ** \
name##_HT_NEXT_RMV(struct name *head, struct type **elm) \
{ \
unsigned h = (*elm)->field.hte_hash; \
unsigned h = _HT_ELT_HASH(*elm, field, hashfn); \
*elm = (*elm)->field.hte_next; \
--head->hth_n_entries; \
if (*elm) { \
@ -316,7 +336,7 @@ ht_string_hash(const char *s)
elm = head->hth_table[b]; \
while (elm) { \
next = elm->field.hte_next; \
b2 = elm->field.hte_hash % new_len; \
b2 = _HT_ELT_HASH(elm, field, hashfn) % new_len; \
elm->field.hte_next = new_table[b2]; \
new_table[b2] = elm; \
elm = next; \
@ -334,7 +354,7 @@ ht_string_hash(const char *s)
for (b=0; b < head->hth_table_length; ++b) { \
struct type *e, **pE; \
for (pE = &new_table[b], e = *pE; e != NULL; e = *pE) { \
b2 = e->field.hte_hash % new_len; \
b2 = _HT_ELT_HASH(e, field, hashfn) % new_len; \
if (b2 == b) { \
pE = &e->field.hte_next; \
} else { \
@ -386,9 +406,9 @@ ht_string_hash(const char *s)
return 5; \
for (n = i = 0; i < head->hth_table_length; ++i) { \
for (elm = head->hth_table[i]; elm; elm = elm->field.hte_next) { \
if (elm->field.hte_hash != hashfn(elm)) \
if (_HT_ELT_HASH(elm, field, hashfn) != hashfn(elm)) \
return 1000 + i; \
if ((elm->field.hte_hash % head->hth_table_length) != i) \
if ((_HT_ELT_HASH(elm, field, hashfn) % head->hth_table_length) != i) \
return 10000 + i; \
++n; \
} \
@ -418,8 +438,8 @@ ht_string_hash(const char *s)
}
#define _HT_FOI_INSERT(field, head, elm, newent, var) \
{ \
newent->field.hte_hash = (elm)->field.hte_hash; \
newent->field.hte_next = NULL; \
_HT_SET_HASHVAL(newent, field, (elm)->field.hte_hash); \
newent->field.hte_next = NULL; \
*var = newent; \
++((head)->hth_n_entries); \
}