Add "reactor" / "io_uring_ring_size" (unsigned int) config parameter.

This parameter specifies the number of entries used to size each
io_uring instance (passed to io_uring_queue_init). Defaults to 16384.
Specifying smaller values reduces the locked-memory footprint of each
ring.

Author: chriskohlhoff

include/asio/detail/impl/io_uring_service.ipp

@@ -83,6 +83,8 @@ io_uring_service::io_uring_service(asio::execution_context& ctx)
     iowait_(config(ctx).get("reactor", "io_uring_iowait", true)),
     submit_batch_size_(
         config(ctx).get("reactor", "io_uring_submit_batch_size", 128)),
+    ring_size_(config(ctx).get("reactor",
+          "io_uring_ring_size", static_cast<unsigned int>(default_ring_size))),
     unflushed_submits_(0),
     timeout_(),
     registration_mutex_(
@@ -797,6 +799,8 @@ void io_uring_service::interrupt()
 
 void io_uring_service::init_ring()
 {
+  const unsigned int entries = ring_size_ != 0
+    ? ring_size_ : static_cast<unsigned int>(default_ring_size);
   for (std::size_t i = 0; i < rings_.size(); ++i)
   {
     // All rings share the same kernel worker pool.
@@ -808,7 +812,7 @@ void io_uring_service::init_ring()
     }
 
     int result =
-      ::io_uring_queue_init_params(ring_size, &rings_[i].ring_, &params);
+      ::io_uring_queue_init_params(entries, &rings_[i].ring_, &params);
     if (result < 0)
     {
       rings_[i].ring_.ring_fd = -1;

include/asio/detail/io_uring_service.hpp

@@ -206,8 +206,10 @@ class io_uring_service
   ASIO_DECL void interrupt();
 
 private:
-  // The hint to pass to io_uring_queue_init to size its data structures.
-  enum { ring_size = 16384 };
+  // The default hint to pass to io_uring_queue_init to size its data
+  // structures, used when the "reactor" / "io_uring_ring_size" configuration
+  // value is not set.
+  enum { default_ring_size = 16384 };
 
   // The number of operations to complete in a batch.
   enum { complete_batch_size = 128 };
@@ -316,6 +318,9 @@ class io_uring_service
   // The number of operations to submit in a batch.
   const int submit_batch_size_;
 
+  // The number of entries used to size each io_uring.
+  const unsigned int ring_size_;
+
   // A count of SQE submissions that are yet to be flushed.
   atomic_count unflushed_submits_;
 

src/doc/overview/configuration.qbk

@@ -232,6 +232,24 @@ below.
       Values that are less than one `1` are treated as `1`.
     ]
   ]
+  [
+    [`reactor`]
+    [`io_uring_ring_size`]
+    [`unsigned int`]
+    [`16384`]
+    [
+      Linux [^io_uring] backend only.
+
+      The number of entries used to size each [^io_uring] instance, passed to
+      [^io_uring_queue_init]. Larger values allow more operations to be in
+      flight before submission queue entries must be flushed, at the cost of
+      more locked memory per ring. Reducing this value lowers the per-ring
+      memory footprint, which can matter in environments with a low
+      `RLIMIT_MEMLOCK` limit.
+
+      A value of `0` selects the default.
+    ]
+  ]
   [
     [`reactor`]
     [`reset_edge_on_partial_read`]

src/tests/unit/connect.cpp

@@ -17,7 +17,9 @@
 #include "asio/connect.hpp"
 
 #include <functional>
+#include <string>
 #include <vector>
+#include "asio/config.hpp"
 #include "asio/detail/thread.hpp"
 #include "asio/ip/tcp.hpp"
 #include "unit_test.hpp"
@@ -26,11 +28,15 @@ namespace bindns = std;
 using bindns::placeholders::_1;
 using bindns::placeholders::_2;
 
+asio::config_from_string io_context_config(
+    "reactor.io_uring_ring_size=64\n");
+
 class connection_sink
 {
 public:
   connection_sink()
-    : acceptor_(io_context_,
+    : io_context_(io_context_config),
+      acceptor_(io_context_,
         asio::ip::tcp::endpoint(
           asio::ip::address_v4::loopback(), 0)),
       target_endpoint_(acceptor_.local_endpoint()),
@@ -129,7 +135,7 @@ void iter_handler(const asio::error_code& ec,
 void test_connect_range()
 {
   connection_sink sink;
-  asio::io_context io_context;
+  asio::io_context io_context(io_context_config);
   asio::ip::tcp::socket socket(io_context);
   std::vector<asio::ip::tcp::endpoint> endpoints;
   asio::ip::tcp::endpoint result;
@@ -163,7 +169,7 @@ void test_connect_range()
 void test_connect_range_ec()
 {
   connection_sink sink;
-  asio::io_context io_context;
+  asio::io_context io_context(io_context_config);
   asio::ip::tcp::socket socket(io_context);
   std::vector<asio::ip::tcp::endpoint> endpoints;
   asio::ip::tcp::endpoint result;
@@ -195,7 +201,7 @@ void test_connect_range_ec()
 void test_connect_range_cond()
 {
   connection_sink sink;
-  asio::io_context io_context;
+  asio::io_context io_context(io_context_config);
   asio::ip::tcp::socket socket(io_context);
   std::vector<asio::ip::tcp::endpoint> endpoints;
   asio::ip::tcp::endpoint result;
@@ -326,7 +332,7 @@ void test_connect_range_cond()
 void test_connect_range_cond_ec()
 {
   connection_sink sink;
-  asio::io_context io_context;
+  asio::io_context io_context(io_context_config);
   asio::ip::tcp::socket socket(io_context);
   std::vector<asio::ip::tcp::endpoint> endpoints;
   asio::ip::tcp::endpoint result;
@@ -422,7 +428,7 @@ void test_connect_range_cond_ec()
 void test_connect_iter()
 {
   connection_sink sink;
-  asio::io_context io_context;
+  asio::io_context io_context(io_context_config);
   asio::ip::tcp::socket socket(io_context);
   std::vector<asio::ip::tcp::endpoint> endpoints;
   const std::vector<asio::ip::tcp::endpoint>& cendpoints = endpoints;
@@ -457,7 +463,7 @@ void test_connect_iter()
 void test_connect_iter_ec()
 {
   connection_sink sink;
-  asio::io_context io_context;
+  asio::io_context io_context(io_context_config);
   asio::ip::tcp::socket socket(io_context);
   std::vector<asio::ip::tcp::endpoint> endpoints;
   const std::vector<asio::ip::tcp::endpoint>& cendpoints = endpoints;
@@ -494,7 +500,7 @@ void test_connect_iter_ec()
 void test_connect_iter_cond()
 {
   connection_sink sink;
-  asio::io_context io_context;
+  asio::io_context io_context(io_context_config);
   asio::ip::tcp::socket socket(io_context);
   std::vector<asio::ip::tcp::endpoint> endpoints;
   const std::vector<asio::ip::tcp::endpoint>& cendpoints = endpoints;
@@ -646,7 +652,7 @@ void test_connect_iter_cond()
 void test_connect_iter_cond_ec()
 {
   connection_sink sink;
-  asio::io_context io_context;
+  asio::io_context io_context(io_context_config);
   asio::ip::tcp::socket socket(io_context);
   std::vector<asio::ip::tcp::endpoint> endpoints;
   const std::vector<asio::ip::tcp::endpoint>& cendpoints = endpoints;
@@ -763,7 +769,7 @@ void test_connect_iter_cond_ec()
 void test_async_connect_range()
 {
   connection_sink sink;
-  asio::io_context io_context;
+  asio::io_context io_context(io_context_config);
   asio::ip::tcp::socket socket(io_context);
   std::vector<asio::ip::tcp::endpoint> endpoints;
   asio::ip::tcp::endpoint result;
@@ -814,7 +820,7 @@ void test_async_connect_range()
 void test_async_connect_range_cond()
 {
   connection_sink sink;
-  asio::io_context io_context;
+  asio::io_context io_context(io_context_config);
   asio::ip::tcp::socket socket(io_context);
   std::vector<asio::ip::tcp::endpoint> endpoints;
   asio::ip::tcp::endpoint result;
@@ -977,7 +983,7 @@ void test_async_connect_range_cond()
 void test_async_connect_iter()
 {
   connection_sink sink;
-  asio::io_context io_context;
+  asio::io_context io_context(io_context_config);
   asio::ip::tcp::socket socket(io_context);
   std::vector<asio::ip::tcp::endpoint> endpoints;
   const std::vector<asio::ip::tcp::endpoint>& cendpoints = endpoints;
@@ -1029,7 +1035,7 @@ void test_async_connect_iter()
 void test_async_connect_iter_cond()
 {
   connection_sink sink;
-  asio::io_context io_context;
+  asio::io_context io_context(io_context_config);
   asio::ip::tcp::socket socket(io_context);
   std::vector<asio::ip::tcp::endpoint> endpoints;
   const std::vector<asio::ip::tcp::endpoint>& cendpoints = endpoints;