Re: [tarantool-patches] Re: [PATCH v2 4/4] Allow to configure TX fiber pool size

[Vladislav Shpilevoy <v.shpilevoy@tarantool.org>]

Hello. Thanks for review!

On 24/04/2018 18:43, Vladimir Davydov wrote:
> On Mon, Apr 23, 2018 at 08:05:04PM +0300, Vladislav Shpilevoy wrote:
>> TX fiber pool size provides fibers to execute transactions and
>> remote requests, so it is linked with maximal remote request
>> count, that is allowed to be altered in the previous patch. Lets
>> do the same for fiber pool size.
>>
>> Follow up #3320
>> ---
> 
> I don't think that having a separate option for configuring the fiber
> pool size is reasonable, because iproto_msg_max and fiber_pool_size are
> interconnected. For example, it's pointless to set iproto_msg_max to
> 1000 and fiber_pool_size to 100 or vice versa. I guess we should leave
> just one option that would tweak both iproto_msg_max and fiber_pool_size
> under the hood. Not sure about that though.

You are right. I squashed last 2 commits on the branch, and removed
fiber_pool_size option.

> 
>>   src/box/box.cc                  |  9 ++++++++-
>>   src/box/lua/load_cfg.lua        |  2 ++
>>   src/fiber_pool.h                |  6 +++++-
>>   test/box/fat_tx.lua             | 30 ++++++++++++++++++++++++++++
>>   test/box/request_limit.result   | 43 +++++++++++++++++++++++++++++++++++++++++
>>   test/box/request_limit.test.lua | 15 ++++++++++++++
>>   6 files changed, 103 insertions(+), 2 deletions(-)
>>   create mode 100644 test/box/fat_tx.lua
>>
>> diff --git a/src/box/box.cc b/src/box/box.cc
>> index 80684ad48..d2552709a 100644
>> --- a/src/box/box.cc
>> +++ b/src/box/box.cc
>> @@ -1716,8 +1716,15 @@ box_is_configured(void)
>>   static inline void
>>   box_cfg_xc(void)
>>   {
>> +	int fiber_pool_size =
>> +		cfg_geti_default("fiber_pool_size", FIBER_POOL_SIZE_DEFAULT);
>> +	if (fiber_pool_size < FIBER_POOL_SIZE_MIN) {
>> +		tnt_raise(ClientError, ER_CFG, "fiber_pool_size",
>> +			  tt_sprintf("minimal value is %d",
>> +				     FIBER_POOL_SIZE_MIN));
>> +	}
>>   	/* Join the cord interconnect as "tx" endpoint. */
>> -	fiber_pool_create(&tx_fiber_pool, "tx", FIBER_POOL_SIZE,
>> +	fiber_pool_create(&tx_fiber_pool, "tx", fiber_pool_size,
>>   			  FIBER_POOL_IDLE_TIMEOUT);
> 
> Can't you make this option dynamic?

Sure, done on the branch. Full diff of last 2 squashed commits is below.

iproto: allow to configure IPROTO_MSG_MAX

IPROTO_MSG_MAX is a constant that restricts count of requests in
fly. It allows to do not produce too many fibers in TX thread,
that would lead to too big overhead on fibers switching, their
stack storing.

But some users have powerful metal on which Tarantool
IPROTO_MSG_MAX constant is not serious. The patch exposes it as
a configuration runtime parameter.

'iproto_msg_max' is its name. If a user sees that IProto thread
is stuck due to too many requests, it can change iproto_msg_max
in runtime, and IProto thread immediately starts processing
pending requests.

'iproto_msg_max' can be decreased, but obviously it can not stop
already runned requests, so if now in IProto thread request count
is > new 'iproto_msg_max' value, then it takes some time until
some requests will be finished.

`iproto_msg_max` automatically increases fiber pool size, when
needed.

Closes #3320
---
  src/box/box.cc                  |  11 +++-
  src/box/box.h                   |   1 +
  src/box/iproto.cc               |  77 +++++++++++++++++-----
  src/box/iproto.h                |   3 +
  src/box/lua/cfg.cc              |  12 ++++
  src/box/lua/load_cfg.lua        |   3 +
  src/fiber_pool.c                |  14 ++++
  src/fiber_pool.h                |  14 +++-
  test/app-tap/init_script.result |  73 +++++++++++----------
  test/box/admin.result           |   2 +
  test/box/cfg.result             |  27 ++++++++
  test/box/cfg.test.lua           |  10 +++
  test/box/request_limit.result   | 141 +++++++++++++++++++++++++++++++++++++++-
  test/box/request_limit.test.lua |  65 +++++++++++++++++-
  14 files changed, 393 insertions(+), 60 deletions(-)

diff --git a/src/box/box.cc b/src/box/box.cc
index d2dfc5b5f..6e2dddcf8 100644
--- a/src/box/box.cc
+++ b/src/box/box.cc
@@ -759,6 +759,14 @@ box_set_vinyl_timeout(void)
  	vinyl_engine_set_timeout(vinyl,	cfg_getd("vinyl_timeout"));
  }
  
+void
+box_set_iproto_msg_max(void)
+{
+	int new_iproto_msg_max = cfg_geti("iproto_msg_max");
+	iproto_set_msg_max(new_iproto_msg_max);
+	fiber_pool_set_max_size(&tx_fiber_pool, new_iproto_msg_max);
+}
+
  /* }}} configuration bindings */
  
  /**
@@ -1711,7 +1719,7 @@ static inline void
  box_cfg_xc(void)
  {
  	/* Join the cord interconnect as "tx" endpoint. */
-	fiber_pool_create(&tx_fiber_pool, "tx", FIBER_POOL_SIZE,
+	fiber_pool_create(&tx_fiber_pool, "tx", FIBER_POOL_SIZE_DEFAULT,
  			  FIBER_POOL_IDLE_TIMEOUT);
  	/* Add an extra endpoint for WAL wake up/rollback messages. */
  	cbus_endpoint_create(&tx_prio_endpoint, "tx_prio", tx_prio_cb, &tx_prio_endpoint);
@@ -1735,6 +1743,7 @@ box_cfg_xc(void)
  	box_check_instance_uuid(&instance_uuid);
  	box_check_replicaset_uuid(&replicaset_uuid);
  
+	box_set_iproto_msg_max();
  	box_set_checkpoint_count();
  	box_set_too_long_threshold();
  	box_set_replication_timeout();
diff --git a/src/box/box.h b/src/box/box.h
index c9b5aad01..712e21191 100644
--- a/src/box/box.h
+++ b/src/box/box.h
@@ -181,6 +181,7 @@ void box_set_vinyl_cache(void);
  void box_set_vinyl_timeout(void);
  void box_set_replication_timeout(void);
  void box_set_replication_connect_quorum(void);
+void box_set_iproto_msg_max(void);
  
  extern "C" {
  #endif /* defined(__cplusplus) */
diff --git a/src/box/iproto.cc b/src/box/iproto.cc
index be3c5a1a6..51add7d41 100644
--- a/src/box/iproto.cc
+++ b/src/box/iproto.cc
@@ -63,8 +63,10 @@
  #include "applier.h"
  #include "cfg.h"
  
-/* The number of iproto messages in flight */
-enum { IPROTO_MSG_MAX = 768 };
+enum {
+	IPROTO_MSG_MAX_DEFAULT = 768,
+	IPROTO_MSG_MAX_MIN = 2,
+};
  
  /**
   * Network readahead. A signed integer to avoid
@@ -83,6 +85,9 @@ enum { IPROTO_MSG_MAX = 768 };
   */
  unsigned iproto_readahead = 16320;
  
+/* The maximal number of iproto messages in fly. */
+static int iproto_msg_max = IPROTO_MSG_MAX_DEFAULT;
+
  /**
   * How big is a buffer which needs to be shrunk before
   * it is put back into buffer cache.
@@ -381,7 +386,7 @@ iproto_must_stop_input()
  {
  	size_t connection_count = mempool_count(&iproto_connection_pool);
  	size_t request_count = mempool_count(&iproto_msg_pool);
-	return request_count > connection_count + IPROTO_MSG_MAX;
+	return request_count > connection_count + iproto_msg_max;
  }
  
  /**
@@ -1632,7 +1637,7 @@ net_cord_f(va_list /* ap */)
  	cbus_endpoint_create(&endpoint, "net", fiber_schedule_cb, fiber());
  	/* Create a pipe to "tx" thread. */
  	cpipe_create(&tx_pipe, "tx");
-	cpipe_set_max_input(&tx_pipe, IPROTO_MSG_MAX/2);
+	cpipe_set_max_input(&tx_pipe, iproto_msg_max / 2);
  	/* Process incomming messages. */
  	cbus_loop(&endpoint);
  
@@ -1660,29 +1665,47 @@ iproto_init()
  
  	/* Create a pipe to "net" thread. */
  	cpipe_create(&net_pipe, "net");
-	cpipe_set_max_input(&net_pipe, IPROTO_MSG_MAX/2);
+	cpipe_set_max_input(&net_pipe, iproto_msg_max / 2);
  }
  
  /**
   * Since there is no way to "synchronously" change the
- * state of the io thread, to change the listen port
- * we need to bounce a couple of messages to and
- * from this thread.
+ * state of the io thread, to change the listen port or max
+ * message count in fly it is needed to bounce a couple of
+ * messages to and from this thread.
   */
-struct iproto_bind_msg: public cbus_call_msg
+struct iproto_cfg_msg: public cbus_call_msg
  {
+	/** New URI to bind to. */
  	const char *uri;
+	bool need_update_uri;
+
+	/** New IProto max message count in fly. */
+	int iproto_msg_max;
+	bool need_update_msg_max;
  };
  
+static struct iproto_cfg_msg cfg_msg;
+
  static int
-iproto_do_bind(struct cbus_call_msg *m)
+iproto_do_cfg(struct cbus_call_msg *m)
  {
-	const char *uri  = ((struct iproto_bind_msg *) m)->uri;
+	assert(m == &cfg_msg);
  	try {
-		if (evio_service_is_active(&binary))
-			evio_service_stop(&binary);
-		if (uri != NULL)
-			evio_service_bind(&binary, uri);
+		if (cfg_msg.need_update_uri) {
+			if (evio_service_is_active(&binary))
+				evio_service_stop(&binary);
+			if (cfg_msg.uri != NULL)
+				evio_service_bind(&binary, cfg_msg.uri);
+		}
+		if (cfg_msg.need_update_msg_max) {
+			cpipe_set_max_input(&tx_pipe,
+					    cfg_msg.iproto_msg_max / 2);
+			int old = iproto_msg_max;
+			iproto_msg_max = cfg_msg.iproto_msg_max;
+			if (old < iproto_msg_max)
+				iproto_resume();
+		}
  	} catch (Exception *e) {
  		return -1;
  	}
@@ -1705,9 +1728,10 @@ iproto_do_listen(struct cbus_call_msg *m)
  void
  iproto_bind(const char *uri)
  {
-	static struct iproto_bind_msg m;
-	m.uri = uri;
-	if (cbus_call(&net_pipe, &tx_pipe, &m, iproto_do_bind,
+	memset(&cfg_msg, 0, sizeof(cfg_msg));
+	cfg_msg.need_update_uri = true;
+	cfg_msg.uri = uri;
+	if (cbus_call(&net_pipe, &tx_pipe, &cfg_msg, iproto_do_cfg,
  		      NULL, TIMEOUT_INFINITY))
  		diag_raise();
  }
@@ -1733,3 +1757,20 @@ iproto_reset_stat(void)
  {
  	rmean_cleanup(rmean_net);
  }
+
+void
+iproto_set_msg_max(int new_iproto_msg_max)
+{
+	if (new_iproto_msg_max < IPROTO_MSG_MAX_MIN) {
+		tnt_raise(ClientError, ER_CFG, "iproto_msg_max",
+			  tt_sprintf("minimal value is %d",
+				     IPROTO_MSG_MAX_MIN));
+	}
+	memset(&cfg_msg, 0, sizeof(cfg_msg));
+	cfg_msg.need_update_msg_max = true;
+	cfg_msg.iproto_msg_max = new_iproto_msg_max;
+	if (cbus_call(&net_pipe, &tx_pipe, &cfg_msg, iproto_do_cfg,
+		      NULL, TIMEOUT_INFINITY))
+		diag_raise();
+	cpipe_set_max_input(&net_pipe, new_iproto_msg_max / 2);
+}
diff --git a/src/box/iproto.h b/src/box/iproto.h
index 0268000da..a1dddc405 100644
--- a/src/box/iproto.h
+++ b/src/box/iproto.h
@@ -63,6 +63,9 @@ iproto_bind(const char *uri);
  void
  iproto_listen();
  
+void
+iproto_set_msg_max(int iproto_msg_max);
+
  #endif /* defined(__cplusplus) */
  
  #endif
diff --git a/src/box/lua/cfg.cc b/src/box/lua/cfg.cc
index 5e88ca348..1fd953011 100644
--- a/src/box/lua/cfg.cc
+++ b/src/box/lua/cfg.cc
@@ -220,6 +220,17 @@ lbox_cfg_set_vinyl_timeout(struct lua_State *L)
  	return 0;
  }
  
+static int
+lbox_cfg_set_iproto_msg_max(struct lua_State *L)
+{
+	try {
+		box_set_iproto_msg_max();
+	} catch (Exception *) {
+		luaT_error(L);
+	}
+	return 0;
+}
+
  static int
  lbox_cfg_set_worker_pool_threads(struct lua_State *L)
  {
@@ -275,6 +286,7 @@ box_lua_cfg_init(struct lua_State *L)
  		{"cfg_set_replication_timeout", lbox_cfg_set_replication_timeout},
  		{"cfg_set_replication_connect_quorum",
  			lbox_cfg_set_replication_connect_quorum},
+		{"cfg_set_iproto_msg_max", lbox_cfg_set_iproto_msg_max},
  		{NULL, NULL}
  	};
  
diff --git a/src/box/lua/load_cfg.lua b/src/box/lua/load_cfg.lua
index 3a5a6d46a..6ed30e016 100644
--- a/src/box/lua/load_cfg.lua
+++ b/src/box/lua/load_cfg.lua
@@ -63,6 +63,7 @@ local default_cfg = {
      feedback_enabled      = true,
      feedback_host         = "https://feedback.tarantool.io",
      feedback_interval     = 3600,
+    iproto_msg_max        = 768,
  }
  
  -- types of available options
@@ -123,6 +124,7 @@ local template_cfg = {
      feedback_enabled      = 'boolean',
      feedback_host         = 'string',
      feedback_interval     = 'number',
+    iproto_msg_max        = 'number',
  }
  
  local function normalize_uri(port)
@@ -195,6 +197,7 @@ local dynamic_cfg = {
      replication_timeout     = private.cfg_set_replication_timeout,
      replication_connect_quorum = private.cfg_set_replication_connect_quorum,
      replication_skip_conflict = function() end,
+    iproto_msg_max          = private.cfg_set_iproto_msg_max,
  }
  
  local dynamic_cfg_skip_at_load = {
diff --git a/src/fiber_pool.c b/src/fiber_pool.c
index aa8b19510..3b9718ad3 100644
--- a/src/fiber_pool.c
+++ b/src/fiber_pool.c
@@ -136,6 +136,20 @@ fiber_pool_cb(ev_loop *loop, struct ev_watcher *watcher, int events)
  	}
  }
  
+void
+fiber_pool_set_max_size(struct fiber_pool *pool, int new_max_size)
+{
+	if (new_max_size < FIBER_POOL_SIZE_DEFAULT)
+		new_max_size = FIBER_POOL_SIZE_DEFAULT;
+
+	if (new_max_size > pool->max_size) {
+		pool->max_size = new_max_size;
+		cbus_process(&pool->endpoint);
+	} else {
+		pool->max_size = new_max_size;
+	}
+}
+
  void
  fiber_pool_create(struct fiber_pool *pool, const char *name, int max_pool_size,
  		  float idle_timeout)
diff --git a/src/fiber_pool.h b/src/fiber_pool.h
index d6a95105b..2a04d6063 100644
--- a/src/fiber_pool.h
+++ b/src/fiber_pool.h
@@ -41,7 +41,10 @@
  extern "C" {
  #endif /* defined(__cplusplus) */
  
-enum { FIBER_POOL_SIZE = 4096, FIBER_POOL_IDLE_TIMEOUT = 1 };
+enum {
+	FIBER_POOL_SIZE_DEFAULT = 4096,
+	FIBER_POOL_IDLE_TIMEOUT = 1
+};
  
  /**
   * A pool of worker fibers to handle messages,
@@ -83,6 +86,15 @@ void
  fiber_pool_create(struct fiber_pool *pool, const char *name, int max_pool_size,
  		  float idle_timeout);
  
+/**
+ * Set maximal fiber pool size. If a new size is bigger than the
+ * current, then pending requests processing is started.
+ * @param pool Fiber pool to set size.
+ * @param new_max_size New maximal size.
+ */
+void
+fiber_pool_set_max_size(struct fiber_pool *pool, int new_max_size);
+
  /**
   * Destroy a fiber pool
   */
diff --git a/test/app-tap/init_script.result b/test/app-tap/init_script.result
index 5625f1466..741f764af 100644
--- a/test/app-tap/init_script.result
+++ b/test/app-tap/init_script.result
@@ -12,42 +12,43 @@ box.cfg
  7	feedback_interval:3600
  8	force_recovery:false
  9	hot_standby:false
-10	listen:port
-11	log:tarantool.log
-12	log_format:plain
-13	log_level:5
-14	log_nonblock:true
-15	memtx_dir:.
-16	memtx_max_tuple_size:1048576
-17	memtx_memory:107374182
-18	memtx_min_tuple_size:16
-19	pid_file:box.pid
-20	read_only:false
-21	readahead:16320
-22	replication_connect_timeout:4
-23	replication_skip_conflict:false
-24	replication_sync_lag:10
-25	replication_timeout:1
-26	rows_per_wal:500000
-27	slab_alloc_factor:1.05
-28	too_long_threshold:0.5
-29	vinyl_bloom_fpr:0.05
-30	vinyl_cache:134217728
-31	vinyl_dir:.
-32	vinyl_max_tuple_size:1048576
-33	vinyl_memory:134217728
-34	vinyl_page_size:8192
-35	vinyl_range_size:1073741824
-36	vinyl_read_threads:1
-37	vinyl_run_count_per_level:2
-38	vinyl_run_size_ratio:3.5
-39	vinyl_timeout:60
-40	vinyl_write_threads:2
-41	wal_dir:.
-42	wal_dir_rescan_delay:2
-43	wal_max_size:268435456
-44	wal_mode:write
-45	worker_pool_threads:4
+10	iproto_msg_max:768
+11	listen:port
+12	log:tarantool.log
+13	log_format:plain
+14	log_level:5
+15	log_nonblock:true
+16	memtx_dir:.
+17	memtx_max_tuple_size:1048576
+18	memtx_memory:107374182
+19	memtx_min_tuple_size:16
+20	pid_file:box.pid
+21	read_only:false
+22	readahead:16320
+23	replication_connect_timeout:4
+24	replication_skip_conflict:false
+25	replication_sync_lag:10
+26	replication_timeout:1
+27	rows_per_wal:500000
+28	slab_alloc_factor:1.05
+29	too_long_threshold:0.5
+30	vinyl_bloom_fpr:0.05
+31	vinyl_cache:134217728
+32	vinyl_dir:.
+33	vinyl_max_tuple_size:1048576
+34	vinyl_memory:134217728
+35	vinyl_page_size:8192
+36	vinyl_range_size:1073741824
+37	vinyl_read_threads:1
+38	vinyl_run_count_per_level:2
+39	vinyl_run_size_ratio:3.5
+40	vinyl_timeout:60
+41	vinyl_write_threads:2
+42	wal_dir:.
+43	wal_dir_rescan_delay:2
+44	wal_max_size:268435456
+45	wal_mode:write
+46	worker_pool_threads:4
  --
  -- Test insert from detached fiber
  --
diff --git a/test/box/admin.result b/test/box/admin.result
index 2168c3adb..7a1432a76 100644
--- a/test/box/admin.result
+++ b/test/box/admin.result
@@ -36,6 +36,8 @@ cfg_filter(box.cfg)
      - false
    - - hot_standby
      - false
+  - - iproto_msg_max
+    - 768
    - - listen
      - <hidden>
    - - log
diff --git a/test/box/cfg.result b/test/box/cfg.result
index 28449d9cc..c309847ef 100644
--- a/test/box/cfg.result
+++ b/test/box/cfg.result
@@ -32,6 +32,8 @@ cfg_filter(box.cfg)
      - false
    - - hot_standby
      - false
+  - - iproto_msg_max
+    - 768
    - - listen
      - <hidden>
    - - log
@@ -129,6 +131,8 @@ cfg_filter(box.cfg)
      - false
    - - hot_standby
      - false
+  - - iproto_msg_max
+    - 768
    - - listen
      - <hidden>
    - - log
@@ -411,6 +415,29 @@ test_run:cmd("cleanup server cfg_tester")
  ---
  - true
  ...
+--
+-- gh-3320: box.cfg{iproto_msg_max}.
+--
+box.cfg{iproto_msg_max = 'invalid'}
+---
+- error: 'Incorrect value for option ''iproto_msg_max'': should be of type number'
+...
+box.cfg{iproto_msg_max = 0}
+---
+- error: 'Incorrect value for option ''iproto_msg_max'': minimal value is 2'
+...
+old = box.cfg.iproto_msg_max
+---
+...
+box.cfg{iproto_msg_max = 2}
+---
+...
+box.cfg{iproto_msg_max = old + 1000}
+---
+...
+box.cfg{iproto_msg_max = old}
+---
+...
  test_run:cmd("clear filter")
  ---
  - true
diff --git a/test/box/cfg.test.lua b/test/box/cfg.test.lua
index a73ae395b..2c40346fe 100644
--- a/test/box/cfg.test.lua
+++ b/test/box/cfg.test.lua
@@ -81,4 +81,14 @@ test_run:cmd("switch default")
  test_run:cmd("stop server cfg_tester")
  test_run:cmd("cleanup server cfg_tester")
  
+--
+-- gh-3320: box.cfg{iproto_msg_max}.
+--
+box.cfg{iproto_msg_max = 'invalid'}
+box.cfg{iproto_msg_max = 0}
+old = box.cfg.iproto_msg_max
+box.cfg{iproto_msg_max = 2}
+box.cfg{iproto_msg_max = old + 1000}
+box.cfg{iproto_msg_max = old}
+
  test_run:cmd("clear filter")
diff --git a/test/box/request_limit.result b/test/box/request_limit.result
index 2691aa329..5d8ba4e9a 100644
--- a/test/box/request_limit.result
+++ b/test/box/request_limit.result
@@ -25,7 +25,7 @@ finished = 0
  continue = false
  ---
  ...
-limit = 768
+limit = box.cfg.iproto_msg_max
  ---
  ...
  run_max = (limit - 100) / 2
@@ -128,6 +128,143 @@ active
  wait_finished(run_max)
  ---
  ...
+--
+-- gh-3320: allow to change maximal count of messages.
+--
+--
+-- Test minimal iproto msg count.
+--
+box.cfg{iproto_msg_max = 2}
+---
+...
+conn:ping()
+---
+- true
+...
+#conn.space._space:select{} > 0
+---
+- true
+...
+run_max = 15
+---
+...
+run_workers(conn)
+---
+...
+wait_block()
+---
+...
+active
+---
+- 3
+...
+wait_finished(run_max)
+---
+...
+--
+-- Increate maximal message count when nothing is blocked.
+--
+box.cfg{iproto_msg_max = limit * 2}
+---
+...
+run_max = limit * 2 - 100
+---
+...
+run_workers(conn)
+---
+...
+wait_block()
+---
+...
+active == run_max
+---
+- true
+...
+-- Max can be decreased back even if now the limit is violated.
+-- But a new input is blocked in such a case.
+box.cfg{iproto_msg_max = limit}
+---
+...
+old_active = active
+---
+...
+for i = 1, 300 do fiber.create(do_long, conn) end
+---
+...
+-- Afer time active count is not changed - the input is blocked.
+wait_block()
+---
+...
+active == old_active
+---
+- true
+...
+wait_finished(active + 300)
+---
+...
+--
+-- Check that changing iproto_msg_max can resume stopped
+-- connections.
+--
+run_max = limit / 2 + 100
+---
+...
+run_workers(conn)
+---
+...
+run_workers(conn2)
+---
+...
+wait_block()
+---
+...
+active >= limit
+---
+- true
+...
+active < run_max * 2
+---
+- true
+...
+box.cfg{iproto_msg_max = limit * 2}
+---
+...
+wait_block()
+---
+...
+active == run_max * 2
+---
+- true
+...
+wait_finished(active)
+---
+...
+--
+-- Test TX fiber pool size limit. It is increased together with
+-- iproto msg max.
+--
+run_max = 2500
+---
+...
+box.cfg{iproto_msg_max = 5000}
+---
+...
+run_workers(conn)
+---
+...
+run_workers(conn2)
+---
+...
+wait_block()
+---
+...
+active
+---
+- 5000
+...
+wait_finished(run_max * 2)
+---
+...
  conn2:close()
  ---
  ...
@@ -137,6 +274,6 @@ conn:close()
  box.schema.user.revoke('guest', 'read,write,execute', 'universe')
  ---
  ...
-box.cfg{readahead = old_readahead}
+box.cfg{readahead = old_readahead, iproto_msg_max = limit}
  ---
  ...
diff --git a/test/box/request_limit.test.lua b/test/box/request_limit.test.lua
index bff7b5282..3e7ddf79d 100644
--- a/test/box/request_limit.test.lua
+++ b/test/box/request_limit.test.lua
@@ -9,7 +9,7 @@ conn2 = net_box.connect(box.cfg.listen)
  active = 0
  finished = 0
  continue = false
-limit = 768
+limit = box.cfg.iproto_msg_max
  run_max = (limit - 100) / 2
  
  old_readahead = box.cfg.readahead
@@ -73,8 +73,69 @@ wait_block()
  active
  wait_finished(run_max)
  
+--
+-- gh-3320: allow to change maximal count of messages.
+--
+
+--
+-- Test minimal iproto msg count.
+--
+box.cfg{iproto_msg_max = 2}
+conn:ping()
+#conn.space._space:select{} > 0
+run_max = 15
+run_workers(conn)
+wait_block()
+active
+wait_finished(run_max)
+
+--
+-- Increate maximal message count when nothing is blocked.
+--
+box.cfg{iproto_msg_max = limit * 2}
+run_max = limit * 2 - 100
+run_workers(conn)
+wait_block()
+active == run_max
+-- Max can be decreased back even if now the limit is violated.
+-- But a new input is blocked in such a case.
+box.cfg{iproto_msg_max = limit}
+old_active = active
+for i = 1, 300 do fiber.create(do_long, conn) end
+-- Afer time active count is not changed - the input is blocked.
+wait_block()
+active == old_active
+wait_finished(active + 300)
+
+--
+-- Check that changing iproto_msg_max can resume stopped
+-- connections.
+--
+run_max = limit / 2 + 100
+run_workers(conn)
+run_workers(conn2)
+wait_block()
+active >= limit
+active < run_max * 2
+box.cfg{iproto_msg_max = limit * 2}
+wait_block()
+active == run_max * 2
+wait_finished(active)
+
+--
+-- Test TX fiber pool size limit. It is increased together with
+-- iproto msg max.
+--
+run_max = 2500
+box.cfg{iproto_msg_max = 5000}
+run_workers(conn)
+run_workers(conn2)
+wait_block()
+active
+wait_finished(run_max * 2)
+
  conn2:close()
  conn:close()
  
  box.schema.user.revoke('guest', 'read,write,execute', 'universe')
-box.cfg{readahead = old_readahead}
+box.cfg{readahead = old_readahead, iproto_msg_max = limit}