On Mon, Aug 08, 2022 at 01:46:48PM +0700, John Naylor wrote:
> Okay, I think it's basically in good shape. Since it should be a bit
> faster than a couple versions ago, would you be up for retesting with
> the original test having 8 to 512 writers?
Sure thing. The results are similar. As before, the improvements are most
visible when the arrays are large.
writers head patch
8 672 680
16 639 664
32 701 689
64 705 703
128 628 653
256 576 627
512 530 584
768 450 536
1024 350 494
> And also add the const
> markers we discussed upthread?
Oops, sorry about that. This is done in v9.
> Aside from that, I plan to commit this
> week unless there is further bikeshedding.
Great, thanks.
--
Nathan Bossart
Amazon Web Services: https://aws.amazon.com
>From 816c56bc8779a1e8ab85db8ca61ba8d3438957d7 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathandboss...@gmail.com>
Date: Wed, 3 Aug 2022 09:49:04 -0700
Subject: [PATCH v9 1/2] Introduce optimized routine for linear searches
through an array of integers.
If SSE2 is available, this function uses it to speed up the search. Otherwise,
it uses a simple 'for' loop. This is a prerequisite for a follow-up commit
that will use this function to optimize [sub]xip lookups in
XidInMVCCSnapshot(), but it can be used anywhere that might benefit from such
an optimization.
It might be worthwhile to add an ARM-specific code path to this function in the
future.
Author: Nathan Bossart
Reviewed by: Andres Freund, John Naylor
Discussion: https://postgr.es/m/20220713170950.GA3116318%40nathanxps13
---
src/include/port/pg_lfind.h | 69 +++++++++++++++++++++++++++++++++++++
1 file changed, 69 insertions(+)
create mode 100644 src/include/port/pg_lfind.h
diff --git a/src/include/port/pg_lfind.h b/src/include/port/pg_lfind.h
new file mode 100644
index 0000000000..4a9484a16d
--- /dev/null
+++ b/src/include/port/pg_lfind.h
@@ -0,0 +1,69 @@
+/*-------------------------------------------------------------------------
+ *
+ * pg_lfind.h
+ * Optimized linear search routines.
+ *
+ * Copyright (c) 2022, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ * src/port/pg_lfind.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef PG_LFIND_H
+#define PG_LFIND_H
+
+#include "port/simd.h"
+
+/*
+ * pg_lfind32
+ *
+ * Returns true if there is an element in 'base' that equals 'key'. Otherwise,
+ * returns false.
+ */
+static inline bool
+pg_lfind32(uint32 key, uint32 *base, uint32 nelem)
+{
+ uint32 i = 0;
+
+ /* If possible, use SSE2 intrinsics to speed up the search. */
+#ifdef USE_SSE2
+ const __m128i keys = _mm_set1_epi32(key); /* load 4 copies of key */
+ uint32 iterations = nelem & ~0xF; /* round down to multiple of 16 */
+
+ for (; i < iterations; i += 16)
+ {
+ /* load the next 16 values into __m128i variables */
+ const __m128i vals1 = _mm_loadu_si128((__m128i *) &base[i]);
+ const __m128i vals2 = _mm_loadu_si128((__m128i *) &base[i + 4]);
+ const __m128i vals3 = _mm_loadu_si128((__m128i *) &base[i + 8]);
+ const __m128i vals4 = _mm_loadu_si128((__m128i *) &base[i + 12]);
+
+ /* perform the comparisons */
+ const __m128i result1 = _mm_cmpeq_epi32(keys, vals1);
+ const __m128i result2 = _mm_cmpeq_epi32(keys, vals2);
+ const __m128i result3 = _mm_cmpeq_epi32(keys, vals3);
+ const __m128i result4 = _mm_cmpeq_epi32(keys, vals4);
+
+ /* shrink the results into a single variable */
+ const __m128i tmp1 = _mm_or_si128(result1, result2);
+ const __m128i tmp2 = _mm_or_si128(result3, result4);
+ const __m128i result = _mm_or_si128(tmp1, tmp2);
+
+ /* see if there was a match */
+ if (_mm_movemask_epi8(result) != 0)
+ return true;
+ }
+#endif
+
+ /* Process the remaining elements the slow way. */
+ for (; i < nelem; i++)
+ {
+ if (key == base[i])
+ return true;
+ }
+
+ return false;
+}
+
+#endif /* PG_LFIND_H */
--
2.25.1
>From 6942097a6406bca2c52851bbad40e5f679cc18ef Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathandboss...@gmail.com>
Date: Wed, 3 Aug 2022 09:59:28 -0700
Subject: [PATCH v9 2/2] Optimize linear searches in XidInMVCCSnapshot().
This change makes use of the recently-introduced optimized linear search
routine to speed up searches through the [sub]xip arrays when possible, which
should improve performance significantly when the arrays are large.
Author: Nathan Bossart
Reviewed by: Andres Freund, John Naylor
Discussion: https://postgr.es/m/20220713170950.GA3116318%40nathanxps13
---
src/backend/utils/time/snapmgr.c | 28 +++++++---------------------
1 file changed, 7 insertions(+), 21 deletions(-)
diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
index 5bc2a15160..9b504c9745 100644
--- a/src/backend/utils/time/snapmgr.c
+++ b/src/backend/utils/time/snapmgr.c
@@ -56,6 +56,7 @@
#include "datatype/timestamp.h"
#include "lib/pairingheap.h"
#include "miscadmin.h"
+#include "port/pg_lfind.h"
#include "storage/predicate.h"
#include "storage/proc.h"
#include "storage/procarray.h"
@@ -2284,8 +2285,6 @@ RestoreTransactionSnapshot(Snapshot snapshot, void *source_pgproc)
bool
XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot)
{
- uint32 i;
-
/*
* Make a quick range check to eliminate most XIDs without looking at the
* xip arrays. Note that this is OK even if we convert a subxact XID to
@@ -2317,13 +2316,8 @@ XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot)
if (!snapshot->suboverflowed)
{
/* we have full data, so search subxip */
- int32 j;
-
- for (j = 0; j < snapshot->subxcnt; j++)
- {
- if (TransactionIdEquals(xid, snapshot->subxip[j]))
- return true;
- }
+ if (pg_lfind32(xid, snapshot->subxip, snapshot->subxcnt))
+ return true;
/* not there, fall through to search xip[] */
}
@@ -2344,16 +2338,11 @@ XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot)
return false;
}
- for (i = 0; i < snapshot->xcnt; i++)
- {
- if (TransactionIdEquals(xid, snapshot->xip[i]))
- return true;
- }
+ if (pg_lfind32(xid, snapshot->xip, snapshot->xcnt))
+ return true;
}
else
{
- int32 j;
-
/*
* In recovery we store all xids in the subxact array because it is by
* far the bigger array, and we mostly don't know which xids are
@@ -2383,11 +2372,8 @@ XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot)
* indeterminate xid. We don't know whether it's top level or subxact
* but it doesn't matter. If it's present, the xid is visible.
*/
- for (j = 0; j < snapshot->subxcnt; j++)
- {
- if (TransactionIdEquals(xid, snapshot->subxip[j]))
- return true;
- }
+ if (pg_lfind32(xid, snapshot->subxip, snapshot->subxcnt))
+ return true;
}
return false;
--
2.25.1
