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Remove large fill factor support from dynahash.c.

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Since ancient times we have had support for a fill factor (maximum load
factor) to be set for a dynahash hash table, but:

1.  It was an integer, whereas for in-memory hash tables interesting
load factor targets are probably somewhere near the 0.75-1.0 range.

2.  It was implemented in a way that performed an expensive division
operation that regularly showed up in profiles.

3.  We are not aware of anyone ever having used a non-default value.

Therefore, remove support, effectively fixing it at 1.

Author: Jakub Wartak <Jakub.Wartak@tomtom.com>
Reviewed-by: Alvaro Herrera <alvherre@2ndquadrant.com>
Reviewed-by: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/VI1PR0701MB696044FC35013A96FECC7AC8F62D0%40VI1PR0701MB6960.eurprd07.prod.outlook.com
pull/57/head
Thomas Munro 5 years ago
parent 06a7c3154f
commit be0a666665
2 changed files with 6 additions and 16 deletions
Show all changes Ignore whitespace when comparing lines Ignore changes in amount of whitespace Ignore changes in whitespace at EOL
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  1. 20
      src/backend/utils/hash/dynahash.c
  2. 2
      src/include/utils/hsearch.h

20
src/backend/utils/hash/dynahash.c
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@ -122,7 +122,6 @@
#define DEF_SEGSIZE 256 #define DEF_SEGSIZE 256
#define DEF_SEGSIZE_SHIFT 8 /* must be log2(DEF_SEGSIZE) */ #define DEF_SEGSIZE_SHIFT 8 /* must be log2(DEF_SEGSIZE) */
#define DEF_DIRSIZE 256 #define DEF_DIRSIZE 256
#define DEF_FFACTOR 1 /* default fill factor */
/* Number of freelists to be used for a partitioned hash table. */ /* Number of freelists to be used for a partitioned hash table. */
#define NUM_FREELISTS 32 #define NUM_FREELISTS 32
@ -191,7 +190,6 @@ struct HASHHDR
Size keysize; /* hash key length in bytes */ Size keysize; /* hash key length in bytes */
Size entrysize; /* total user element size in bytes */ Size entrysize; /* total user element size in bytes */
long num_partitions; /* # partitions (must be power of 2), or 0 */ long num_partitions; /* # partitions (must be power of 2), or 0 */
long ffactor; /* target fill factor */
long max_dsize; /* 'dsize' limit if directory is fixed size */ long max_dsize; /* 'dsize' limit if directory is fixed size */
long ssize; /* segment size --- must be power of 2 */ long ssize; /* segment size --- must be power of 2 */
int sshift; /* segment shift = log2(ssize) */ int sshift; /* segment shift = log2(ssize) */
@ -497,8 +495,6 @@ hash_create(const char *tabname, long nelem, HASHCTL *info, int flags)
/* ssize had better be a power of 2 */ /* ssize had better be a power of 2 */
Assert(hctl->ssize == (1L << hctl->sshift)); Assert(hctl->ssize == (1L << hctl->sshift));
} }
if (flags & HASH_FFACTOR)
hctl->ffactor = info->ffactor;
/* /*
* SHM hash tables have fixed directory size passed by the caller. * SHM hash tables have fixed directory size passed by the caller.
@ -603,8 +599,6 @@ hdefault(HTAB *hashp)
hctl->num_partitions = 0; /* not partitioned */ hctl->num_partitions = 0; /* not partitioned */
hctl->ffactor = DEF_FFACTOR;
/* table has no fixed maximum size */ /* table has no fixed maximum size */
hctl->max_dsize = NO_MAX_DSIZE; hctl->max_dsize = NO_MAX_DSIZE;
@ -670,11 +664,10 @@ init_htab(HTAB *hashp, long nelem)
SpinLockInit(&(hctl->freeList[i].mutex)); SpinLockInit(&(hctl->freeList[i].mutex));
/* /*
* Divide number of elements by the fill factor to determine a desired * Allocate space for the next greater power of two number of buckets,
* number of buckets. Allocate space for the next greater power of two * assuming a desired maximum load factor of 1.
* number of buckets
*/ */
nbuckets = next_pow2_int((nelem - 1) / hctl->ffactor + 1); nbuckets = next_pow2_int(nelem);
/* /*
* In a partitioned table, nbuckets must be at least equal to * In a partitioned table, nbuckets must be at least equal to
@ -733,7 +726,6 @@ init_htab(HTAB *hashp, long nelem)
"DIRECTORY SIZE ", hctl->dsize, "DIRECTORY SIZE ", hctl->dsize,
"SEGMENT SIZE ", hctl->ssize, "SEGMENT SIZE ", hctl->ssize,
"SEGMENT SHIFT ", hctl->sshift, "SEGMENT SHIFT ", hctl->sshift,
"FILL FACTOR ", hctl->ffactor,
"MAX BUCKET ", hctl->max_bucket, "MAX BUCKET ", hctl->max_bucket,
"HIGH MASK ", hctl->high_mask, "HIGH MASK ", hctl->high_mask,
"LOW MASK ", hctl->low_mask, "LOW MASK ", hctl->low_mask,
@ -761,7 +753,7 @@ hash_estimate_size(long num_entries, Size entrysize)
elementAllocCnt; elementAllocCnt;
/* estimate number of buckets wanted */ /* estimate number of buckets wanted */
nBuckets = next_pow2_long((num_entries - 1) / DEF_FFACTOR + 1); nBuckets = next_pow2_long(num_entries);
/* # of segments needed for nBuckets */ /* # of segments needed for nBuckets */
nSegments = next_pow2_long((nBuckets - 1) / DEF_SEGSIZE + 1); nSegments = next_pow2_long((nBuckets - 1) / DEF_SEGSIZE + 1);
/* directory entries */ /* directory entries */
@ -804,7 +796,7 @@ hash_select_dirsize(long num_entries)
nDirEntries; nDirEntries;
/* estimate number of buckets wanted */ /* estimate number of buckets wanted */
nBuckets = next_pow2_long((num_entries - 1) / DEF_FFACTOR + 1); nBuckets = next_pow2_long(num_entries);
/* # of segments needed for nBuckets */ /* # of segments needed for nBuckets */
nSegments = next_pow2_long((nBuckets - 1) / DEF_SEGSIZE + 1); nSegments = next_pow2_long((nBuckets - 1) / DEF_SEGSIZE + 1);
/* directory entries */ /* directory entries */
@ -975,7 +967,7 @@ hash_search_with_hash_value(HTAB *hashp,
* order of these tests is to try to check cheaper conditions first. * order of these tests is to try to check cheaper conditions first.
*/ */
if (!IS_PARTITIONED(hctl) && !hashp->frozen && if (!IS_PARTITIONED(hctl) && !hashp->frozen &&
hctl->freeList[0].nentries / (long) (hctl->max_bucket + 1) >= hctl->ffactor && hctl->freeList[0].nentries > (long) (hctl->max_bucket + 1) &&
!has_seq_scans(hashp)) !has_seq_scans(hashp))
(void) expand_table(hashp); (void) expand_table(hashp);
} }

2
src/include/utils/hsearch.h
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@ -68,7 +68,6 @@ typedef struct HASHCTL
long ssize; /* segment size */ long ssize; /* segment size */
long dsize; /* (initial) directory size */ long dsize; /* (initial) directory size */
long max_dsize; /* limit to dsize if dir size is limited */ long max_dsize; /* limit to dsize if dir size is limited */
long ffactor; /* fill factor */
Size keysize; /* hash key length in bytes */ Size keysize; /* hash key length in bytes */
Size entrysize; /* total user element size in bytes */ Size entrysize; /* total user element size in bytes */
HashValueFunc hash; /* hash function */ HashValueFunc hash; /* hash function */
@ -83,7 +82,6 @@ typedef struct HASHCTL
#define HASH_PARTITION 0x0001 /* Hashtable is used w/partitioned locking */ #define HASH_PARTITION 0x0001 /* Hashtable is used w/partitioned locking */
#define HASH_SEGMENT 0x0002 /* Set segment size */ #define HASH_SEGMENT 0x0002 /* Set segment size */
#define HASH_DIRSIZE 0x0004 /* Set directory size (initial and max) */ #define HASH_DIRSIZE 0x0004 /* Set directory size (initial and max) */
#define HASH_FFACTOR 0x0008 /* Set fill factor */
#define HASH_ELEM 0x0010 /* Set keysize and entrysize */ #define HASH_ELEM 0x0010 /* Set keysize and entrysize */
#define HASH_BLOBS 0x0020 /* Select support functions for binary keys */ #define HASH_BLOBS 0x0020 /* Select support functions for binary keys */
#define HASH_FUNCTION 0x0040 /* Set user defined hash function */ #define HASH_FUNCTION 0x0040 /* Set user defined hash function */

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