COPY-SIMD-add-heuristic-to-avoid-regression-on-sm.txt

From aa55843b0c64bed9f72cf8cd7854df9df7ef989b Mon Sep 17 00:00:00 2001
From: Nazir Bilal Yavuz <byavuz81@gmail.com>
Date: Tue, 19 Aug 2025 15:16:02 +0300
Subject: [PATCH v1] COPY SIMD: add heuristic to avoid regression on small
 advances
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When SIMD advances fewer than 5 characters, performance regresses.
To mitigate this, introduce a heuristic:

- If advance < 5 -> insert a sleep penalty (n cycles).
- Each time advance < 5, n is doubled.
- Each time advance ≥ 5, n is halved.
---
 src/backend/commands/copyfromparse.c | 42 ++++++++++++++++++++++++++--
 1 file changed, 40 insertions(+), 2 deletions(-)

diff --git a/src/backend/commands/copyfromparse.c b/src/backend/commands/copyfromparse.c
index 5aba0fa6cb7..e58d7d4e353 100644
--- a/src/backend/commands/copyfromparse.c
+++ b/src/backend/commands/copyfromparse.c
@@ -1263,6 +1263,9 @@ CopyReadLineText(CopyFromState cstate, bool is_csv)
 	Vector8		bs = vector8_broadcast('\\');
 	Vector8		quote;
 	Vector8		escape;
+
+	int			sleep_cyle = 0;
+	int			last_sleep_cyle = 1;
 #endif
 
 	if (is_csv)
@@ -1359,7 +1362,7 @@ CopyReadLineText(CopyFromState cstate, bool is_csv)
 		 *   vector register, as SIMD operations require processing data in
 		 *   fixed-size chunks.
 		 */
-		if (!in_quote && copy_buf_len - input_buf_ptr >= sizeof(Vector8))
+		if (sleep_cyle <= 0 && !in_quote && copy_buf_len - input_buf_ptr >= sizeof(Vector8))
 		{
 			Vector8		chunk;
 			Vector8		match;
@@ -1390,14 +1393,49 @@ CopyReadLineText(CopyFromState cstate, bool is_csv)
 				 */
 				int advance = pg_rightmost_one_pos32(mask);
 				input_buf_ptr += advance;
+
+				/*
+				 * If we advance less than 5 characters we cause regression.
+				 * Sleep a bit then try again. Sleep time increases
+				 * exponentially.
+				 */
+				if (advance < 5)
+				{
+					if (last_sleep_cyle >= PG_INT16_MAX / 2)
+						last_sleep_cyle = PG_INT16_MAX;
+					else
+						last_sleep_cyle = last_sleep_cyle << 1;
+
+					sleep_cyle = last_sleep_cyle;
+				}
+
+				/*
+				 * If we advance more than 4 charactes this means we have
+				 * performance improvement. Halve sleep time for next sleep.
+				 */
+				else
+				{
+					last_sleep_cyle = Max(last_sleep_cyle >> 1, 1);
+					sleep_cyle = 0;
+				}
 			}
 			else
 			{
-				/* No special characters found, so skip the entire chunk */
+				/*
+				 * No special characters found, so skip the entire chunk and
+				 * halve sleep time for next sleep.
+				 */
 				input_buf_ptr += sizeof(Vector8);
+				last_sleep_cyle = Max(last_sleep_cyle >> 1, 1);
 				continue;
 			}
 		}
+
+		/*
+		 * Vulnerable to overflow if we are in quote for more than INT16_MAX
+		 * characters.
+		 */
+		sleep_cyle--;
 #endif
 
 		/* OK to fetch a character */
-- 
2.50.1