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Revert "Improve accounting for memory used by shared hash tables"

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This reverts commit f5930f9a98.

This broke the expansion of private hash tables, which reallocates the
directory. But that's impossible when it's allocated together with the
other fields, and dir_realloc() failed with BogusFree. Clearly, this
needs rethinking.

Discussion: https://postgr.es/m/CAApHDvriCiNkm=v521AP6PKPfyWkJ++jqZ9eqX4cXnhxLv8w-A@mail.gmail.com
This commit is contained in:
Tomas Vondra
2025-04-04 04:28:59 +02:00
parent 88f55bc976
commit 1aff1dc8df
3 changed files with 69 additions and 225 deletions
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4
src/backend/storage/ipc/shmem.c
View File

@ -73,7 +73,6 @@
#include "storage/shmem.h"
#include "storage/spin.h"
#include "utils/builtins.h"
#include "utils/dynahash.h"
static void *ShmemAllocRaw(Size size, Size *allocated_size);
@ -347,8 +346,7 @@ ShmemInitHash(const char *name, /* table string name for shmem index */
/* look it up in the shmem index */
location = ShmemInitStruct(name,
hash_get_size(infoP, hash_flags,
init_size, true),
hash_get_shared_size(infoP, hash_flags),
&found);
/*

287
src/backend/utils/hash/dynahash.c
View File

@ -260,39 +260,12 @@ static long hash_accesses,
hash_expansions;
#endif
/* access to parts of the hash table, allocated as a single chunk */
#define HASH_DIRECTORY_PTR(hashp) \
(((char *) (hashp)->hctl) + sizeof(HASHHDR))
#define HASH_SEGMENT_OFFSET(hctl, idx) \
(sizeof(HASHHDR) + \
((hctl)->dsize * sizeof(HASHSEGMENT)) + \
((hctl)->ssize * (idx) * sizeof(HASHBUCKET)))
#define HASH_SEGMENT_PTR(hashp, idx) \
((char *) (hashp)->hctl + HASH_SEGMENT_OFFSET((hashp)->hctl, (idx)))
#define HASH_SEGMENT_SIZE(hashp) \
((hashp)->ssize * sizeof(HASHBUCKET))
#define HASH_ELEMENTS_PTR(hashp, nsegs) \
((char *) (hashp)->hctl + HASH_SEGMENT_OFFSET((hashp)->hctl, nsegs))
/* Each element has a HASHELEMENT header plus user data. */
#define HASH_ELEMENT_SIZE(hctl) \
(MAXALIGN(sizeof(HASHELEMENT)) + MAXALIGN((hctl)->entrysize))
#define HASH_ELEMENT_NEXT(hctl, num, ptr) \
((char *) (ptr) + ((num) * HASH_ELEMENT_SIZE(hctl)))
/*
* Private function prototypes
*/
static void *DynaHashAlloc(Size size);
static HASHSEGMENT seg_alloc(HTAB *hashp);
static HASHELEMENT *element_alloc(HTAB *hashp, int nelem);
static void element_add(HTAB *hashp, HASHELEMENT *firstElement,
int nelem, int freelist_idx);
static bool element_alloc(HTAB *hashp, int nelem, int freelist_idx);
static bool dir_realloc(HTAB *hashp);
static bool expand_table(HTAB *hashp);
static HASHBUCKET get_hash_entry(HTAB *hashp, int freelist_idx);
@ -307,9 +280,6 @@ static int next_pow2_int(long num);
static void register_seq_scan(HTAB *hashp);
static void deregister_seq_scan(HTAB *hashp);
static bool has_seq_scans(HTAB *hashp);
static void compute_buckets_and_segs(long nelem, long num_partitions,
long ssize,
int *nbuckets, int *nsegments);
/*
@ -383,8 +353,6 @@ hash_create(const char *tabname, long nelem, const HASHCTL *info, int flags)
{
HTAB *hashp;
HASHHDR *hctl;
int nelem_batch;
bool prealloc;
/*
* Hash tables now allocate space for key and data, but you have to say
@ -539,31 +507,9 @@ hash_create(const char *tabname, long nelem, const HASHCTL *info, int flags)
hashp->isshared = false;
}
/* Choose the number of entries to allocate at a time. */
nelem_batch = choose_nelem_alloc(info->entrysize);
/*
* Decide whether to pre-allocate elements.
*
* For a private hash table, preallocate the requested number of elements
* if it's less than our chosen nelem_alloc. This avoids wasting space if
* the caller correctly estimates a small table size.
*
* For a shared hash table, preallocate the requested number of elements.
* This reduces problems with run-time out-of-shared-memory conditions.
*/
prealloc = (flags & HASH_SHARED_MEM) || (nelem < nelem_batch);
/*
* Allocate the memory needed for hash header, directory, segments and
* elements together. Use pointer arithmetic to arrive at the start of
* each of these structures later.
*/
if (!hashp->hctl)
{
Size size = hash_get_size(info, flags, nelem, prealloc);
hashp->hctl = (HASHHDR *) hashp->alloc(size);
hashp->hctl = (HASHHDR *) hashp->alloc(sizeof(HASHHDR));
if (!hashp->hctl)
ereport(ERROR,
(errcode(ERRCODE_OUT_OF_MEMORY),
@ -612,9 +558,6 @@ hash_create(const char *tabname, long nelem, const HASHCTL *info, int flags)
hctl->keysize = info->keysize;
hctl->entrysize = info->entrysize;
/* remember how many elements to allocate at once */
hctl->nelem_alloc = nelem_batch;
/* make local copies of heavily-used constant fields */
hashp->keysize = hctl->keysize;
hashp->ssize = hctl->ssize;
@ -624,6 +567,14 @@ hash_create(const char *tabname, long nelem, const HASHCTL *info, int flags)
if (!init_htab(hashp, nelem))
elog(ERROR, "failed to initialize hash table \"%s\"", hashp->tabname);
/*
* For a shared hash table, preallocate the requested number of elements.
* This reduces problems with run-time out-of-shared-memory conditions.
*
* For a non-shared hash table, preallocate the requested number of
* elements if it's less than our chosen nelem_alloc. This avoids wasting
* space if the caller correctly estimates a small table size.
*/
if ((flags & HASH_SHARED_MEM) ||
nelem < hctl->nelem_alloc)
{
@ -631,7 +582,6 @@ hash_create(const char *tabname, long nelem, const HASHCTL *info, int flags)
freelist_partitions,
nelem_alloc,
nelem_alloc_first;
void *ptr = NULL;
/*
* If hash table is partitioned, give each freelist an equal share of
@ -656,28 +606,14 @@ hash_create(const char *tabname, long nelem, const HASHCTL *info, int flags)
else
nelem_alloc_first = nelem_alloc;
/*
* Calculate the offset at which to find the first partition of
* elements. We have to skip space for the header, segments and
* buckets.
*/
ptr = HASH_ELEMENTS_PTR(hashp, hctl->nsegs);
/*
* Assign the correct location of each parition within a pre-allocated
* buffer.
*
* Actual memory allocation happens in ShmemInitHash for shared hash
* tables or earlier in this function for non-shared hash tables.
*
* We just need to split that allocation into per-batch freelists.
*/
for (i = 0; i < freelist_partitions; i++)
{
int temp = (i == 0) ? nelem_alloc_first : nelem_alloc;
element_add(hashp, (HASHELEMENT *) ptr, temp, i);
ptr = HASH_ELEMENT_NEXT(hctl, temp, ptr);
if (!element_alloc(hashp, temp, i))
ereport(ERROR,
(errcode(ERRCODE_OUT_OF_MEMORY),
errmsg("out of memory")));
}
}
@ -766,16 +702,29 @@ init_htab(HTAB *hashp, long nelem)
SpinLockInit(&(hctl->freeList[i].mutex));
/*
* We've already calculated these parameters when we calculated how much
* space to allocate in hash_get_size(). Be careful to keep these two
* places in sync, so that we get the same parameters.
* Allocate space for the next greater power of two number of buckets,
* assuming a desired maximum load factor of 1.
*/
compute_buckets_and_segs(nelem, hctl->num_partitions, hctl->ssize,
&nbuckets, &nsegs);
nbuckets = next_pow2_int(nelem);
/*
* In a partitioned table, nbuckets must be at least equal to
* num_partitions; were it less, keys with apparently different partition
* numbers would map to the same bucket, breaking partition independence.
* (Normally nbuckets will be much bigger; this is just a safety check.)
*/
while (nbuckets < hctl->num_partitions)
nbuckets <<= 1;
hctl->max_bucket = hctl->low_mask = nbuckets - 1;
hctl->high_mask = (nbuckets << 1) - 1;
/*
* Figure number of directory segments needed, round up to a power of 2
*/
nsegs = (nbuckets - 1) / hctl->ssize + 1;
nsegs = next_pow2_int(nsegs);
/*
* Make sure directory is big enough. If pre-allocated directory is too
* small, choke (caller screwed up).
@ -788,25 +737,26 @@ init_htab(HTAB *hashp, long nelem)
return false;
}
/*
* Assign a directory by making it point to the correct location in the
* pre-allocated buffer.
*/
/* Allocate a directory */
if (!(hashp->dir))
{
CurrentDynaHashCxt = hashp->hcxt;
hashp->dir = (HASHSEGMENT *) HASH_DIRECTORY_PTR(hashp);
hashp->dir = (HASHSEGMENT *)
hashp->alloc(hctl->dsize * sizeof(HASHSEGMENT));
if (!hashp->dir)
return false;
}
/* Assign initial segments, which are also pre-allocated */
i = 0;
/* Allocate initial segments */
for (segp = hashp->dir; hctl->nsegs < nsegs; hctl->nsegs++, segp++)
{
*segp = (HASHSEGMENT) HASH_SEGMENT_PTR(hashp, i++);
MemSet(*segp, 0, HASH_SEGMENT_SIZE(hashp));
*segp = seg_alloc(hashp);
if (*segp == NULL)
return false;
}
Assert(i == nsegs);
/* Choose number of entries to allocate at a time */
hctl->nelem_alloc = choose_nelem_alloc(hctl->entrysize);
#ifdef HASH_DEBUG
fprintf(stderr, "init_htab:\n%s%p\n%s%ld\n%s%ld\n%s%d\n%s%ld\n%s%u\n%s%x\n%s%x\n%s%ld\n",
@ -896,75 +846,16 @@ hash_select_dirsize(long num_entries)
}
/*
* hash_get_size -- determine memory needed for a new dynamic hash table
*
* info: hash table parameters
* flags: bitmask indicating which parameters to take from *info
* nelem: maximum number of elements expected
* prealloc: request preallocation of elements
*
* Compute the required initial memory allocation for a hashtable with the given
* parameters. We need space for the HASHHDR, for the directory, segments and
* preallocated elements.
*
* The hash table may be private or shared. For shared hash tables the directory
* size is non-expansive, and we preallocate all elements (nelem). For private
* hash tables, we preallocate elements only if the expected number of elements
* is small (less than nelem_alloc).
* Compute the required initial memory allocation for a shared-memory
* hashtable with the given parameters. We need space for the HASHHDR
* and for the (non expansible) directory.
*/
Size
hash_get_size(const HASHCTL *info, int flags, long nelem, bool prealloc)
hash_get_shared_size(HASHCTL *info, int flags)
{
int nbuckets;
int nsegs;
int num_partitions;
long ssize;
long dsize;
Size elementSize = HASH_ELEMENT_SIZE(info);
long nelem_prealloc = 0;
#ifdef USE_ASSERT_CHECKING
/* shared hash tables have non-expansive directory */
if (flags & HASH_SHARED_MEM)
{
Assert(flags & HASH_DIRSIZE);
Assert(info->dsize == info->max_dsize);
Assert(prealloc);
}
#endif
/* Non-shared hash tables may not specify dir size */
if (flags & HASH_DIRSIZE)
dsize = info->dsize;
else
dsize = DEF_DIRSIZE;
if (flags & HASH_SEGMENT)
ssize = info->ssize;
else
ssize = DEF_SEGSIZE;
if (flags & HASH_PARTITION)
{
num_partitions = info->num_partitions;
/* Number of entries should be atleast equal to the freelists */
if (nelem < NUM_FREELISTS)
nelem = NUM_FREELISTS;
}
else
num_partitions = 0;
compute_buckets_and_segs(nelem, num_partitions, ssize,
&nbuckets, &nsegs);
/* initial_elems as false indicates no elements are to be pre-allocated */
if (prealloc)
nelem_prealloc = nelem;
return sizeof(HASHHDR) + dsize * sizeof(HASHSEGMENT)
+ sizeof(HASHBUCKET) * ssize * nsegs
+ nelem_prealloc * elementSize;
Assert(flags & HASH_DIRSIZE);
Assert(info->dsize == info->max_dsize);
return sizeof(HASHHDR) + info->dsize * sizeof(HASHSEGMENT);
}
@ -1394,7 +1285,7 @@ get_hash_entry(HTAB *hashp, int freelist_idx)
* Failing because the needed element is in a different freelist is
* not acceptable.
*/
if ((newElement = element_alloc(hashp, hctl->nelem_alloc)) == NULL)
if (!element_alloc(hashp, hctl->nelem_alloc, freelist_idx))
{
int borrow_from_idx;
@ -1431,7 +1322,6 @@ get_hash_entry(HTAB *hashp, int freelist_idx)
/* no elements available to borrow either, so out of memory */
return NULL;
}
element_add(hashp, newElement, hctl->nelem_alloc, freelist_idx);
}
/* remove entry from freelist, bump nentries */
@ -1810,43 +1700,29 @@ seg_alloc(HTAB *hashp)
}
/*
* allocate some new elements
* allocate some new elements and link them into the indicated free list
*/
static HASHELEMENT *
element_alloc(HTAB *hashp, int nelem)
{
HASHHDR *hctl = hashp->hctl;
Size elementSize;
HASHELEMENT *firstElement = NULL;
if (hashp->isfixed)
return NULL;
/* Each element has a HASHELEMENT header plus user data. */
elementSize = HASH_ELEMENT_SIZE(hctl);
CurrentDynaHashCxt = hashp->hcxt;
firstElement = (HASHELEMENT *) hashp->alloc(nelem * elementSize);
if (!firstElement)
return NULL;
return firstElement;
}
/*
* link the elements allocated by element_alloc into the indicated free list
*/
static void
element_add(HTAB *hashp, HASHELEMENT *firstElement, int nelem, int freelist_idx)
static bool
element_alloc(HTAB *hashp, int nelem, int freelist_idx)
{
HASHHDR *hctl = hashp->hctl;
Size elementSize;
HASHELEMENT *firstElement;
HASHELEMENT *tmpElement;
HASHELEMENT *prevElement;
int i;
if (hashp->isfixed)
return false;
/* Each element has a HASHELEMENT header plus user data. */
elementSize = HASH_ELEMENT_SIZE(hctl);
elementSize = MAXALIGN(sizeof(HASHELEMENT)) + MAXALIGN(hctl->entrysize);
CurrentDynaHashCxt = hashp->hcxt;
firstElement = (HASHELEMENT *) hashp->alloc(nelem * elementSize);
if (!firstElement)
return false;
/* prepare to link all the new entries into the freelist */
prevElement = NULL;
@ -1868,6 +1744,8 @@ element_add(HTAB *hashp, HASHELEMENT *firstElement, int nelem, int freelist_idx)
if (IS_PARTITIONED(hctl))
SpinLockRelease(&hctl->freeList[freelist_idx].mutex);
return true;
}
/*
@ -2079,34 +1957,3 @@ AtEOSubXact_HashTables(bool isCommit, int nestDepth)
}
}
}
/*
* Calculate the number of buckets and segments to store the given
* number of elements in a hash table. Segments contain buckets which
* in turn contain elements.
*/
static void
compute_buckets_and_segs(long nelem, long num_partitions, long ssize,
int *nbuckets, int *nsegments)
{
/*
* Allocate space for the next greater power of two number of buckets,
* assuming a desired maximum load factor of 1.
*/
*nbuckets = next_pow2_int(nelem);
/*
* In a partitioned table, nbuckets must be at least equal to
* num_partitions; were it less, keys with apparently different partition
* numbers would map to the same bucket, breaking partition independence.
* (Normally nbuckets will be much bigger; this is just a safety check.)
*/
while ((*nbuckets) < num_partitions)
(*nbuckets) <<= 1;
/*
* Figure number of directory segments needed, round up to a power of 2
*/
*nsegments = ((*nbuckets) - 1) / ssize + 1;
*nsegments = next_pow2_int(*nsegments);
}

3
src/include/utils/hsearch.h
View File

@ -151,8 +151,7 @@ extern void hash_seq_term(HASH_SEQ_STATUS *status);
extern void hash_freeze(HTAB *hashp);
extern Size hash_estimate_size(long num_entries, Size entrysize);
extern long hash_select_dirsize(long num_entries);
extern Size hash_get_size(const HASHCTL *info, int flags,
long nelem, bool prealloc);
extern Size hash_get_shared_size(HASHCTL *info, int flags);
extern void AtEOXact_HashTables(bool isCommit);
extern void AtEOSubXact_HashTables(bool isCommit, int nestDepth);