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Issue 18771: Make it possible to set the number linear probes at compile-time.

This commit is contained in:
Raymond Hettinger 2013-09-15 14:57:15 -07:00
parent e6d35dba3c
commit 8408dc581e
2 changed files with 28 additions and 7 deletions
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11
Doc/whatsnew/3.4.rst
View File

@ -444,8 +444,15 @@ Major performance enhancements have been added:
* The UTF-32 decoder is now 3x to 4x faster.
* The cost of hash collisions for sets is now reduced. Each hash table
probe now checks a second key/hash pair for each cache line retrieved.
This exploits cache locality to make collision resolution less expensive.
probe now checks a series of consecutive, adjacent key/hash pairs before
continuing to make random probes through the hash table. This exploits
cache locality to make collision resolution less expensive.
The collision resolution scheme can be described as a hybrid of linear
probing and open addressing. The number of additional linear probes
defaults to nine. This can be changed at compile-time by defining
LINEAR_PROBES to be any value. Set LINEAR_PROBES=0 to turn-off
linear probing entirely.
(Contributed by Raymond Hettinger in :issue"`18771`.)

24
Objects/setobject.c
View File

@ -44,7 +44,9 @@ PyObject *_PySet_Dummy = dummy;
/* ======= Begin logic for probing the hash table ========================= */
/* This should be >= PySet_MINSIZE - 1 */
#ifndef LINEAR_PROBES
#define LINEAR_PROBES 9
#endif
/* This must be >= 1 */
#define PERTURB_SHIFT 5
@ -55,12 +57,14 @@ set_lookkey(PySetObject *so, PyObject *key, Py_hash_t hash)
setentry *table = so->table;
setentry *freeslot = NULL;
setentry *entry;
setentry *limit;
size_t perturb = hash;
size_t mask = so->mask;
size_t i = (size_t)hash; /* Unsigned for defined overflow behavior. */
size_t j;
int cmp;
#if LINEAR_PROBES
setentry *limit;
size_t j;
#endif
entry = &table[i & mask];
if (entry->key == NULL)
@ -84,6 +88,7 @@ set_lookkey(PySetObject *so, PyObject *key, Py_hash_t hash)
if (entry->key == dummy && freeslot == NULL)
freeslot = entry;
#if LINEAR_PROBES
limit = &table[mask];
for (j = 0 ; j < LINEAR_PROBES ; j++) {
entry = (entry == limit) ? &table[0] : entry + 1;
@ -106,13 +111,14 @@ set_lookkey(PySetObject *so, PyObject *key, Py_hash_t hash)
if (entry->key == dummy && freeslot == NULL)
freeslot = entry;
}
#endif
perturb >>= PERTURB_SHIFT;
i = i * 5 + 1 + perturb;
entry = &table[i & mask];
if (entry->key == NULL)
break;
goto found_null;
}
found_null:
return freeslot == NULL ? entry : freeslot;
@ -129,11 +135,13 @@ set_lookkey_unicode(PySetObject *so, PyObject *key, Py_hash_t hash)
setentry *table = so->table;
setentry *freeslot = NULL;
setentry *entry;
setentry *limit;
size_t perturb = hash;
size_t mask = so->mask;
size_t i = (size_t)hash;
#if LINEAR_PROBES
setentry *limit;
size_t j;
#endif
/* Make sure this function doesn't have to handle non-unicode keys,
including subclasses of str; e.g., one reason to subclass
@ -157,6 +165,7 @@ set_lookkey_unicode(PySetObject *so, PyObject *key, Py_hash_t hash)
if (entry->key == dummy && freeslot == NULL)
freeslot = entry;
#if LINEAR_PROBES
limit = &table[mask];
for (j = 0 ; j < LINEAR_PROBES ; j++) {
entry = (entry == limit) ? &table[0] : entry + 1;
@ -170,13 +179,14 @@ set_lookkey_unicode(PySetObject *so, PyObject *key, Py_hash_t hash)
if (entry->key == dummy && freeslot == NULL)
freeslot = entry;
}
#endif
perturb >>= PERTURB_SHIFT;
i = i * 5 + 1 + perturb;
entry = &table[i & mask];
if (entry->key == NULL)
break;
goto found_null;
}
found_null:
return freeslot == NULL ? entry : freeslot;
@ -198,17 +208,21 @@ set_insert_clean(PySetObject *so, PyObject *key, Py_hash_t hash)
size_t perturb = hash;
size_t mask = (size_t)so->mask;
size_t i = (size_t)hash;
#if LINEAR_PROBES
size_t j;
#endif
while (1) {
entry = &table[i & mask];
if (entry->key == NULL)
goto found_null;
#if LINEAR_PROBES
for (j = 1 ; j <= LINEAR_PROBES ; j++) {
entry = &table[(i + j) & mask];
if (entry->key == NULL)
goto found_null;
}
#endif
perturb >>= PERTURB_SHIFT;
i = i * 5 + 1 + perturb;
}