Shrink NET::buff to reduce memory usage

include/mysql_com_server.h

@@ -63,6 +63,9 @@ typedef struct NET_SERVER {
   struct compression_attributes compression;
   mysql_compress_context compress_ctx;
   bool timeout_on_full_packet;
+
+  /** Number of consecutive requests to shrink NET::buff. */
+  unsigned int shrink_requests;
 } NET_SERVER;
 
 inline void net_server_ext_init(NET_SERVER *ns) {
@@ -71,6 +74,7 @@ inline void net_server_ext_init(NET_SERVER *ns) {
   ns->m_after_header = nullptr;
   ns->compress_ctx.algorithm = MYSQL_UNCOMPRESSED;
   ns->timeout_on_full_packet = false;
+  ns->shrink_requests = 0;
 }
-
+bool net_shrink(struct NET *net, size_t length);
 #endif

mysql-test/r/all_persisted_variables.result

@@ -48,17 +48,17 @@ include/assert.inc [Expect 500+ variables in the table. Due to open Bugs, we are
 
 # Test SET PERSIST
 
-include/assert.inc [Expect 451 persisted variables in the table.]
+include/assert.inc [Expect 452 persisted variables in the table.]
 
 ************************************************************
 * 3. Restart server, it must preserve the persisted variable
 *    settings. Verify persisted configuration.
 ************************************************************
 # restart
 
-include/assert.inc [Expect 451 persisted variables in persisted_variables table.]
-include/assert.inc [Expect 451 persisted variables shown as PERSISTED in variables_info table.]
-include/assert.inc [Expect 451 persisted variables with matching peristed and global values.]
+include/assert.inc [Expect 452 persisted variables in persisted_variables table.]
+include/assert.inc [Expect 452 persisted variables shown as PERSISTED in variables_info table.]
+include/assert.inc [Expect 452 persisted variables with matching peristed and global values.]
 
 ************************************************************
 * 4. Test RESET PERSIST IF EXISTS. Verify persisted variable

mysql-test/r/mysqld--help-notwin.result

@@ -775,6 +775,12 @@ The following options may be given as the first argument:
  --myisam-use-mmap   Use memory mapping for reading and writing MyISAM tables
  --net-buffer-length=# 
  Buffer length for TCP/IP and socket communication
+ --net-buffer-shrink-threshold=# 
+ If the size currently required for net buffer is less
+ than half of the allocated size, a shrink is requested.
+ Once the number of consecutive requests reaches this
+ threshold, the buffer is halved, but never below
+ net_buffer_length. A value of 0 disables shrink
  --net-read-timeout=# 
  Number of seconds to wait for more data from a connection
  before aborting the read
@@ -1867,6 +1873,7 @@ myisam-sort-buffer-size 8388608
 myisam-stats-method nulls_unequal
 myisam-use-mmap FALSE
 net-buffer-length 16384
+net-buffer-shrink-threshold 5
 net-read-timeout 30
 net-retry-count 10
 net-write-timeout 60

mysql-test/r/shrink_net_buffer.result

@@ -0,0 +1,36 @@
+SHOW VARIABLES LIKE 'net_buffer_length';
+Variable_name	Value
+net_buffer_length	16384
+CREATE USER 'shrink_test_user'@'localhost';
+SET GLOBAL net_buffer_shrink_threshold = 5;
+[connection con1]
+[connection default]
+"NET::buff after a large query; capacity should have grown."
+CURRENT_ALLOCATIONS	CURRENT_NUMBER_OF_BYTES_USED - 39
+1	36864
+[connection con1]
+[connection default]
+"NET::buff after four small queries; capacity should not be shrunk."
+CURRENT_ALLOCATIONS	CURRENT_NUMBER_OF_BYTES_USED - 39
+1	36864
+[connection con1]
+[connection default]
+"NET::buff after five small queries; capacity should be about half."
+CURRENT_ALLOCATIONS	CURRENT_NUMBER_OF_BYTES_USED - 39
+1	20480
+[connection con1]
+[connection default]
+"NET::buff after ten small queries; capacity should reach its minimum."
+CURRENT_ALLOCATIONS	CURRENT_NUMBER_OF_BYTES_USED - 39
+1	16384
+[connection con1]
+[connection default]
+"NET::buff remains at its minimum after five more small queries."
+CURRENT_ALLOCATIONS	CURRENT_NUMBER_OF_BYTES_USED - 39
+1	16384
+[connection con1]
+[connection default]
+"NET::buff after another large query; capacity should grow again."
+CURRENT_ALLOCATIONS	CURRENT_NUMBER_OF_BYTES_USED - 39
+1	36864
+DROP USER 'shrink_test_user'@'localhost';

mysql-test/r/variables.result

@@ -245,24 +245,28 @@ set global net_buffer_length=1024, net_write_timeout=200, net_read_timeout=300;
 show global variables like 'net_%';
 Variable_name	Value
 net_buffer_length	1024
+net_buffer_shrink_threshold	5
 net_read_timeout	300
 net_retry_count	10
 net_write_timeout	200
 select * from performance_schema.global_variables where variable_name like 'net_%' order by 1;
 VARIABLE_NAME	VARIABLE_VALUE
 net_buffer_length	1024
+net_buffer_shrink_threshold	5
 net_read_timeout	300
 net_retry_count	10
 net_write_timeout	200
 show session variables like 'net_%';
 Variable_name	Value
 net_buffer_length	16384
+net_buffer_shrink_threshold	5
 net_read_timeout	30
 net_retry_count	10
 net_write_timeout	60
 select * from performance_schema.session_variables where variable_name like 'net_%' order by 1;
 VARIABLE_NAME	VARIABLE_VALUE
 net_buffer_length	16384
+net_buffer_shrink_threshold	5
 net_read_timeout	30
 net_retry_count	10
 net_write_timeout	60
@@ -272,12 +276,14 @@ Warning	1292	Truncated incorrect net_buffer_length value: '8000'
 show global variables like 'net_%';
 Variable_name	Value
 net_buffer_length	7168
+net_buffer_shrink_threshold	5
 net_read_timeout	900
 net_retry_count	10
 net_write_timeout	1000
 select * from performance_schema.global_variables where variable_name like 'net_%' order by 1;
 VARIABLE_NAME	VARIABLE_VALUE
 net_buffer_length	7168
+net_buffer_shrink_threshold	5
 net_read_timeout	900
 net_retry_count	10
 net_write_timeout	1000

mysql-test/suite/sys_vars/r/all_vars.result

@@ -72,6 +72,8 @@ master_verify_checksum
 master_verify_checksum
 max_length_for_sort_data
 max_length_for_sort_data
+net_buffer_shrink_threshold
+net_buffer_shrink_threshold
 optimizer_max_subgraph_pairs
 optimizer_max_subgraph_pairs
 partial_revokes

mysql-test/t/all_persisted_variables.test

@@ -58,7 +58,7 @@ let $total_global_vars=`SELECT COUNT(*)
    AND variable_name NOT LIKE '%pqc%'
    AND variable_name NOT LIKE '%tls_kex%'`;
 
-let $total_persistent_vars=451;
+let $total_persistent_vars=452;
 
 --echo ***************************************************************
 --echo * 0. Verify that variables present in performance_schema.global

mysql-test/t/shrink_net_buffer.test

@@ -0,0 +1,173 @@
+# Test NET::buff shrinking for classic protocol connections.
+#
+# The test verifies that:
+#   1. A query larger than net_buffer_length grows NET::buff.
+#   2. Five consecutive small queries shrink the buffer by approximately half.
+#   3. Further small queries shrink it to, but never below, net_buffer_length.
+#   4. A later large query can grow the buffer again after it was shrunk.
+#
+# Expected result: PFS allocation statistics show grow, gradual shrink, a
+# stable lower bound, and successful regrowth in that order.
+
+--source include/have_nodebug.inc
+
+# =============================================================================
+# Prerequisites
+# =============================================================================
+
+if (`SELECT @@GLOBAL.performance_schema = FALSE`) {
+  skip Test requires @@GLOBAL.performance_schema = TRUE.;
+}
+
+let $instrument = `SELECT ENABLED FROM performance_schema.setup_instruments
+                    WHERE NAME = 'memory/sql/NET::buff'`;
+if ($instrument != YES) {
+  skip Test requires memory/sql/NET::buff instrumentation.;
+}
+
+# =============================================================================
+# Setup
+# =============================================================================
+
+let $old_net_buffer_shrink_threshold = query_get_value(
+  SHOW VARIABLES LIKE "net_buffer_shrink_threshold", Value, 1);
+SHOW VARIABLES LIKE 'net_buffer_length';
+
+# Generate padding for a query larger than net_buffer_length. The padding is
+# produced on the default connection so setup cannot affect con1's shrink count.
+let $query_padding = `SELECT REPEAT('x', 32768)`;
+
+CREATE USER 'shrink_test_user'@'localhost';
+SET GLOBAL net_buffer_shrink_threshold = 5;
+--connect(con1, localhost, shrink_test_user, , )
+let $con1_processlist_id = `SELECT CONNECTION_ID()`;
+--connection default
+--disable_query_log
+--eval SET @con1_processlist_id = $con1_processlist_id
+
+# Extra 39 bytes are allocated for
+# PSI header (32 B), net header (4 B), compression header (4 B)
+# They are not counted.
+
+let $pfs_net_buff_stmt =
+  SELECT COUNT_ALLOC - COUNT_FREE AS CURRENT_ALLOCATIONS,
+         CURRENT_NUMBER_OF_BYTES_USED - 39
+  FROM performance_schema.memory_summary_by_thread_by_event_name
+  WHERE EVENT_NAME = 'memory/sql/NET::buff'
+    AND THREAD_ID = (SELECT THREAD_ID
+                     FROM performance_schema.threads
+                     WHERE PROCESSLIST_ID = @con1_processlist_id);
+
+# =============================================================================
+# Test 1: A large incoming query grows NET::buff
+# =============================================================================
+
+--connection con1
+--echo [connection con1]
+--disable_result_log
+--eval SELECT 1 /* $query_padding */
+--enable_result_log
+
+--connection default
+--echo [connection default]
+--echo "NET::buff after a large query; capacity should have grown."
+--eval $pfs_net_buff_stmt
+
+# =============================================================================
+# Test 2: Five small queries shrink the grown buffer by about half
+# =============================================================================
+
+--connection con1
+--echo [connection con1]
+--disable_result_log
+SELECT 1;
+SELECT 1;
+SELECT 1;
+SELECT 1;
+--enable_result_log
+
+--connection default
+--echo [connection default]
+--echo "NET::buff after four small queries; capacity should not be shrunk."
+--eval $pfs_net_buff_stmt
+
+--connection con1
+--echo [connection con1]
+--disable_result_log
+SELECT 1;
+--enable_result_log
+
+--connection default
+--echo [connection default]
+--echo "NET::buff after five small queries; capacity should be about half."
+--eval $pfs_net_buff_stmt
+
+# =============================================================================
+# Test 3: Five more small queries reach net_buffer_length
+# =============================================================================
+
+--connection con1
+--echo [connection con1]
+--disable_result_log
+SELECT 1;
+SELECT 1;
+SELECT 1;
+SELECT 1;
+SELECT 1;
+--enable_result_log
+
+--connection default
+--echo [connection default]
+--echo "NET::buff after ten small queries; capacity should reach its minimum."
+--eval $pfs_net_buff_stmt
+
+let $net_buffer_length = query_get_value(
+  SHOW VARIABLES LIKE 'net_buffer_length', Value, 1);
+let $current_buffer_capacity = `$pfs_net_buff_stmt`;
+
+if ($current_buffer_capacity == $net_buffer_length) {
+  --echo NET::buff capacity is equal to net_buffer_length.
+}
+
+# =============================================================================
+# Test 4: Further small queries do not shrink below net_buffer_length
+# =============================================================================
+
+--connection con1
+--echo [connection con1]
+--disable_result_log
+SELECT 1;
+SELECT 1;
+SELECT 1;
+SELECT 1;
+SELECT 1;
+--enable_result_log
+
+--connection default
+--echo [connection default]
+--echo "NET::buff remains at its minimum after five more small queries."
+--eval $pfs_net_buff_stmt
+
+# =============================================================================
+# Test 5: A later large query grows the buffer again
+# =============================================================================
+
+--connection con1
+--echo [connection con1]
+--disable_result_log
+--eval SELECT 1 /* $query_padding */
+--enable_result_log
+
+--connection default
+--echo [connection default]
+--echo "NET::buff after another large query; capacity should grow again."
+--eval $pfs_net_buff_stmt
+
+# =============================================================================
+# Cleanup
+# =============================================================================
+
+--disconnect con1
+--eval SET GLOBAL net_buffer_shrink_threshold = $old_net_buffer_shrink_threshold
+--enable_query_log
+DROP USER 'shrink_test_user'@'localhost';

sql-common/net_serv.cc

@@ -258,6 +258,36 @@ bool net_realloc(NET *net, size_t length) {
   return false;
 }
 
+#ifdef MYSQL_SERVER
+/**
+  Shrink net buffer.
+
+  @param net      NET handler
+  @param length   shrink net buffer to length. length should smaller
+                  than the original buffer size
+
+  @retval  true   failed to shrink
+  @retval  false  Shrunk to length successfully
+*/
+bool net_shrink(NET *net, size_t length) {
+  uchar *buff;
+  size_t pkt_length;
+
+  assert(length < net->max_packet);
+  pkt_length = (length + IO_SIZE - 1) & ~(IO_SIZE - 1);
+
+  if (!(buff = (uchar *)my_realloc(
+            key_memory_NET_buff, (char *)net->buff,
+            pkt_length + NET_HEADER_SIZE + COMP_HEADER_SIZE, MYF(0))))
+    return true;
+
+  net->buff = net->write_pos = buff;
+  net->buff_end = buff + pkt_length;
+  net->max_packet = ulong(pkt_length);
+  return false;
+}
+#endif
+
 /**
   Clear (reinitialize) the NET structure for a new command.
 

sql/conn_handler/init_net_server_extension.cc

@@ -132,6 +132,7 @@ void init_net_server_extension(THD *thd) {
   thd->m_net_server_extension.m_after_header = net_after_header_psi;
   thd->m_net_server_extension.compress_ctx.algorithm = MYSQL_UNCOMPRESSED;
   thd->m_net_server_extension.timeout_on_full_packet = false;
+  thd->m_net_server_extension.shrink_requests = 0;
   /* Activate this private extension for the mysqld server. */
   thd->get_protocol_classic()->get_net()->extension =
       &thd->m_net_server_extension;