On Mon, Sep 14, 2020 at 11:35 PM David Rowley <[email protected]> wrote:
> I just did some benchmarking with this patch using the same recovery
> benchmark that I used in [1] and also the two patches that I posted in
> [2]. Additionally, I added a PANIC at the end of recovery so that I
> could repeat the recovery over and over again with the same WAL.
>
> [data]
N Min Max Median Avg Stddev
x 10 62.15 67.06 64.86 64.132 1.6188528
+ 10 59.6 63.81 63.13 62.233 1.4983031
Difference at 95.0% confidence
-1.899 +/- 1.46553
-2.96108% +/- 2.28517%
(Student's t, pooled s = 1.55974)
Thanks! Hmm, small but apparently significant and in line with
Jakub's report, and I suppose the effect will be greater with other
nearby recovery performance patches applied that halve the times.
Annoyingly, I can't reproduce this speedup on my local i9-9900; maybe
it requires a different CPU...
> I looked over the patch and the only thing I saw was that we might
> also want to remove the following line:
>
> #define DEF_FFACTOR 1 /* default fill factor */
Right, thanks. Fixed in the attached.
> The 2nd most costly call to hash_search_with_hash_value() came in via
> hash_search() via smgropen(). That does use HASH_ENTER, which could
> have triggered the divide code. The main caller of smgropen() was
> XLogReadBufferExtended().
>
> So, it looks unlikely that any gains we are seeing are from improved
> buffer lookups. It's more likely they're coming from more optimal
> XLogReadBufferExtended()
I think we call smgropen() twice for every buffer referenced in the
WAL: XLogReadBufferExtended() and again in
ReadBufferWithoutRelcache(). We could reduce it to once with some
refactoring, but I am looking into whether I can reduce it to zero as
a side-effect of another change, more soon...
From efecf68b159a3c65517e91076009cb4e5cc6f157 Mon Sep 17 00:00:00 2001
From: Thomas Munro <[email protected]>
Date: Thu, 10 Sep 2020 12:27:25 +1200
Subject: [PATCH v2] Remove custom fill factor support from dynahash.c.
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 had to be an integer value >= 1, 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, making fill factor 1 as the implicit value.
Author: Jakub Wartak <[email protected]>
Reviewed-by: Alvaro Herrera <[email protected]>
Reviewed-by: Tomas Vondra <[email protected]>
Reviewed-by: Thomas Munro <[email protected]>
Reviewed-by: David Rowley <[email protected]>
Discussion: https://postgr.es/m/VI1PR0701MB696044FC35013A96FECC7AC8F62D0%40VI1PR0701MB6960.eurprd07.prod.outlook.com
---
src/backend/utils/hash/dynahash.c | 20 ++++++--------------
src/include/utils/hsearch.h | 2 --
2 files changed, 6 insertions(+), 16 deletions(-)
diff --git a/src/backend/utils/hash/dynahash.c b/src/backend/utils/hash/dynahash.c
index f4fbccdd7e..1122e2e5e5 100644
--- a/src/backend/utils/hash/dynahash.c
+++ b/src/backend/utils/hash/dynahash.c
@@ -122,7 +122,6 @@
#define DEF_SEGSIZE 256
#define DEF_SEGSIZE_SHIFT 8 /* must be log2(DEF_SEGSIZE) */
#define DEF_DIRSIZE 256
-#define DEF_FFACTOR 1 /* default fill factor */
/* Number of freelists to be used for a partitioned hash table. */
#define NUM_FREELISTS 32
@@ -191,7 +190,6 @@ struct HASHHDR
Size keysize; /* hash key length in bytes */
Size entrysize; /* total user element size in bytes */
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 ssize; /* segment size --- must be power of 2 */
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 */
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.
@@ -603,8 +599,6 @@ hdefault(HTAB *hashp)
hctl->num_partitions = 0; /* not partitioned */
- hctl->ffactor = DEF_FFACTOR;
-
/* table has no fixed maximum size */
hctl->max_dsize = NO_MAX_DSIZE;
@@ -670,11 +664,10 @@ init_htab(HTAB *hashp, long nelem)
SpinLockInit(&(hctl->freeList[i].mutex));
/*
- * Divide number of elements by the fill factor to determine a desired
- * number of buckets. Allocate space for the next greater power of two
- * number of buckets
+ * 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 - 1) / hctl->ffactor + 1);
+ nbuckets = next_pow2_int(nelem);
/*
* 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,
"SEGMENT SIZE ", hctl->ssize,
"SEGMENT SHIFT ", hctl->sshift,
- "FILL FACTOR ", hctl->ffactor,
"MAX BUCKET ", hctl->max_bucket,
"HIGH MASK ", hctl->high_mask,
"LOW MASK ", hctl->low_mask,
@@ -761,7 +753,7 @@ hash_estimate_size(long num_entries, Size entrysize)
elementAllocCnt;
/* 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 */
nSegments = next_pow2_long((nBuckets - 1) / DEF_SEGSIZE + 1);
/* directory entries */
@@ -804,7 +796,7 @@ hash_select_dirsize(long num_entries)
nDirEntries;
/* 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 */
nSegments = next_pow2_long((nBuckets - 1) / DEF_SEGSIZE + 1);
/* 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.
*/
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))
(void) expand_table(hashp);
}
diff --git a/src/include/utils/hsearch.h b/src/include/utils/hsearch.h
index f1deb9beab..bebf89b3c4 100644
--- a/src/include/utils/hsearch.h
+++ b/src/include/utils/hsearch.h
@@ -68,7 +68,6 @@ typedef struct HASHCTL
long ssize; /* segment size */
long dsize; /* (initial) directory size */
long max_dsize; /* limit to dsize if dir size is limited */
- long ffactor; /* fill factor */
Size keysize; /* hash key length in bytes */
Size entrysize; /* total user element size in bytes */
HashValueFunc hash; /* hash function */
@@ -83,7 +82,6 @@ typedef struct HASHCTL
#define HASH_PARTITION 0x0001 /* Hashtable is used w/partitioned locking */
#define HASH_SEGMENT 0x0002 /* Set segment size */
#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_BLOBS 0x0020 /* Select support functions for binary keys */
#define HASH_FUNCTION 0x0040 /* Set user defined hash function */
--
2.20.1
