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Improve accounting for memory used by shared hash tables

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pg_shmem_allocations tracks the memory allocated by ShmemInitStruct(),
but for shared hash tables that covered only the header and hash
directory.  The remaining parts (segments and buckets) were allocated
later using ShmemAlloc(), which does not update the shmem accounting.
Thus, these allocations were not shown in pg_shmem_allocations.

This commit improves the situation by allocating all the hash table
parts at once, using a single ShmemInitStruct() call. This way the
ShmemIndex entries (and thus pg_shmem_allocations) better reflect the
proper size of the hash table.

This affects allocations for private (non-shared) hash tables too, as
the hash_create() code is shared. For non-shared tables this however
makes no practical difference.

This changes the alignment a bit. ShmemAlloc() aligns the chunks using
CACHELINEALIGN(), which means some parts (header, directory, segments)
were aligned this way. Allocating all parts as a single chunk removes
this (implicit) alignment. We've considered adding explicit alignment,
but we've decided not to - it seems to be merely a coincidence due to
using the ShmemAlloc() API, not due to necessity.

Author: Rahila Syed <rahilasyed90@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Reviewed-by: Tomas Vondra <tomas@vondra.me>
Discussion: https://postgr.es/m/CAH2L28vHzRankszhqz7deXURxKncxfirnuW68zD7+hVAqaS5GQ@mail.gmail.com
pull/210/head
Tomas Vondra 9 months ago
parent bd178960c6
commit f5930f9a98
3 changed files with 222 additions and 66 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. 4
      src/backend/storage/ipc/shmem.c
  2. 281
      src/backend/utils/hash/dynahash.c
  3. 3
      src/include/utils/hsearch.h

4
src/backend/storage/ipc/shmem.c
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@ -73,6 +73,7 @@
#include "storage/shmem.h"
#include "storage/spin.h"
#include "utils/builtins.h"
#include "utils/dynahash.h"
static void *ShmemAllocRaw(Size size, Size *allocated_size);
@ -346,7 +347,8 @@ ShmemInitHash(const char *name, /* table string name for shmem index */
/* look it up in the shmem index */
location = ShmemInitStruct(name,
hash_get_shared_size(infoP, hash_flags),
hash_get_size(infoP, hash_flags,
init_size, true),
&found);
/*

281
src/backend/utils/hash/dynahash.c
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@ -260,12 +260,39 @@ 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 bool element_alloc(HTAB *hashp, int nelem, int freelist_idx);
static HASHELEMENT *element_alloc(HTAB *hashp, int nelem);
static void element_add(HTAB *hashp, HASHELEMENT *firstElement,
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);
@ -280,6 +307,9 @@ 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);
/*
@ -353,6 +383,8 @@ 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
@ -507,9 +539,31 @@ 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)
{
hashp->hctl = (HASHHDR *) hashp->alloc(sizeof(HASHHDR));
Size size = hash_get_size(info, flags, nelem, prealloc);
hashp->hctl = (HASHHDR *) hashp->alloc(size);
if (!hashp->hctl)
ereport(ERROR,
(errcode(ERRCODE_OUT_OF_MEMORY),
@ -558,6 +612,9 @@ 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;
@ -567,14 +624,6 @@ 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)
{
@ -582,6 +631,7 @@ 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
@ -606,14 +656,28 @@ 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;
if (!element_alloc(hashp, temp, i))
ereport(ERROR,
(errcode(ERRCODE_OUT_OF_MEMORY),
errmsg("out of memory")));
element_add(hashp, (HASHELEMENT *) ptr, temp, i);
ptr = HASH_ELEMENT_NEXT(hctl, temp, ptr);
}
}
@ -702,29 +766,16 @@ init_htab(HTAB *hashp, long nelem)
SpinLockInit(&(hctl->freeList[i].mutex));
/*
* 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.)
* 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.
*/
while (nbuckets < hctl->num_partitions)
nbuckets <<= 1;
compute_buckets_and_segs(nelem, hctl->num_partitions, hctl->ssize,
&nbuckets, &nsegs);
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).
@ -737,26 +788,25 @@ init_htab(HTAB *hashp, long nelem)
return false;
}
/* Allocate a directory */
/*
* Assign a directory by making it point to the correct location in the
* pre-allocated buffer.
*/
if (!(hashp->dir))
{
CurrentDynaHashCxt = hashp->hcxt;
hashp->dir = (HASHSEGMENT *)
hashp->alloc(hctl->dsize * sizeof(HASHSEGMENT));
if (!hashp->dir)
return false;
hashp->dir = (HASHSEGMENT *) HASH_DIRECTORY_PTR(hashp);
}
/* Allocate initial segments */
/* Assign initial segments, which are also pre-allocated */
i = 0;
for (segp = hashp->dir; hctl->nsegs < nsegs; hctl->nsegs++, segp++)
{
*segp = seg_alloc(hashp);
if (*segp == NULL)
return false;
*segp = (HASHSEGMENT) HASH_SEGMENT_PTR(hashp, i++);
MemSet(*segp, 0, HASH_SEGMENT_SIZE(hashp));
}
/* Choose number of entries to allocate at a time */
hctl->nelem_alloc = choose_nelem_alloc(hctl->entrysize);
Assert(i == nsegs);
#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",
@ -846,16 +896,75 @@ hash_select_dirsize(long num_entries)
}
/*
* 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.
* 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).
*/
Size
hash_get_shared_size(HASHCTL *info, int flags)
hash_get_size(const HASHCTL *info, int flags, long nelem, bool prealloc)
{
Assert(flags & HASH_DIRSIZE);
Assert(info->dsize == info->max_dsize);
return sizeof(HASHHDR) + info->dsize * sizeof(HASHSEGMENT);
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;
}
@ -1285,7 +1394,7 @@ get_hash_entry(HTAB *hashp, int freelist_idx)
* Failing because the needed element is in a different freelist is
* not acceptable.
*/
if (!element_alloc(hashp, hctl->nelem_alloc, freelist_idx))
if ((newElement = element_alloc(hashp, hctl->nelem_alloc)) == NULL)
{
int borrow_from_idx;
@ -1322,6 +1431,7 @@ 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 */
@ -1700,29 +1810,43 @@ seg_alloc(HTAB *hashp)
}
/*
* allocate some new elements and link them into the indicated free list
* allocate some new elements
*/
static bool
element_alloc(HTAB *hashp, int nelem, int freelist_idx)
static HASHELEMENT *
element_alloc(HTAB *hashp, int nelem)
{
HASHHDR *hctl = hashp->hctl;
Size elementSize;
HASHELEMENT *firstElement;
HASHELEMENT *tmpElement;
HASHELEMENT *prevElement;
int i;
HASHELEMENT *firstElement = NULL;
if (hashp->isfixed)
return false;
return NULL;
/* Each element has a HASHELEMENT header plus user data. */
elementSize = MAXALIGN(sizeof(HASHELEMENT)) + MAXALIGN(hctl->entrysize);
elementSize = HASH_ELEMENT_SIZE(hctl);
CurrentDynaHashCxt = hashp->hcxt;
firstElement = (HASHELEMENT *) hashp->alloc(nelem * elementSize);
if (!firstElement)
return false;
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)
{
HASHHDR *hctl = hashp->hctl;
Size elementSize;
HASHELEMENT *tmpElement;
HASHELEMENT *prevElement;
int i;
/* Each element has a HASHELEMENT header plus user data. */
elementSize = HASH_ELEMENT_SIZE(hctl);
/* prepare to link all the new entries into the freelist */
prevElement = NULL;
@ -1744,8 +1868,6 @@ element_alloc(HTAB *hashp, int nelem, int freelist_idx)
if (IS_PARTITIONED(hctl))
SpinLockRelease(&hctl->freeList[freelist_idx].mutex);
return true;
}
/*
@ -1957,3 +2079,34 @@ 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
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@ -151,7 +151,8 @@ 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_shared_size(HASHCTL *info, int flags);
extern Size hash_get_size(const HASHCTL *info, int flags,
long nelem, bool prealloc);
extern void AtEOXact_HashTables(bool isCommit);
extern void AtEOSubXact_HashTables(bool isCommit, int nestDepth);

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