Algorithm of Massively Parallel Networking in C++
Vladislav Shpilevoy
Tech
Internals
Conf
Meetup 2025 Spring
Plan
Networking in C++
Existing solutions
New scheduler
Examples
Benchmarks
Networking in C++
Networking in C++
Motivation
System is already C++
Super expertise in C++
Need ultra performance
Available inventory
Networking in C++
socket(), send(), recv(), connect(),
accept(), bind(), listen()
epoll
io_uring
IOCP
IoRing
kqueue
Existing solutions
Existing solutions
boost::asio
thrift, gRPC
libev, libuv, libevent
userver
seastar
New scheduler
Motivation
New scheduler
boost::asio
thrift, gRPC
- unreadable code, questionable perf
userver
- no Windows
seastar
- no Windows, no MacOS
- enforce their protocol
Your company lives in its own framework
Requirements
New scheduler
Fairness
Coroutines
Events
Task Scheduler
Scheduler Kingdom
The Battle For The Key
The Front
Tavern
The Waiting Prison
The Map
The Ready
Guild
The Many Tasks
The Strong Workers
Example of scheduling
Task Scheduler
Example of scheduling
Task Scheduler
Example of scheduling
Task Scheduler
Example of scheduling
Task Scheduler
Example of scheduling
Task Scheduler
Example of scheduling
Task Scheduler
Example of scheduling
Task Scheduler
Example of scheduling
Task Scheduler
Example of scheduling
Task Scheduler
Example of scheduling
Task Scheduler
Example of scheduling
Task Scheduler
Example of scheduling
Task Scheduler
Example of scheduling
Task Scheduler
Example of scheduling
Task Scheduler
Example of scheduling
Task Scheduler
Wakeup
Task Scheduler
Wakeup
Task Scheduler
😭
Wakeup
Task Scheduler
😴
Wakeup
Task Scheduler
😴
The King Calls To Battle
Wakeup
Task Scheduler
😳
The King Calls To Battle
Wakeup
Task Scheduler
🤔
The King Calls To Battle
Wakeup
Task Scheduler
🤔
The King Calls To Battle
Wakeup
Task Scheduler
The Kernel
Castle
The King Calls To Battle
Implementation
Task Scheduler
Implementation
Task Scheduler
Less than 2k lines*
Lock-free*
Low memory
Simple code
Formally verified in TLA+
clang, gcc, msvc
MacOS, Windows, Linux
epoll, io_uring, kqueue, IOCP
x86, ARM
>= C++17
Examples
Simple task
Examples
Simple task
int
main()
{
mg::sch::TaskScheduler sched("tst",
1, // Thread count.
5 // Subqueue size.
);
sched.Post(new mg::sch::Task([&](mg::sch::Task *self) {
std::cout << "Executed in scheduler!\n";
delete self;
}));
return 0;
}
Create the scheduler
Post a task
The body is a lambda function
Examples
Multistep task
Examples
Multistep task [1]
class MyTask : public mg::sch::Task
{
public:
MyTask() : Task([this](mg::sch::Task* aSelf) {
TaskSendRequest(aSelf);
}) {}
private:
// Step 1
void
TaskSendRequest(
mg::sch::Task* aSelf);
// Step 2
void
TaskRecvResponse(
mg::sch::Task* aSelf);
// Step 3
void
TaskFinish(
mg::sch::Task* aSelf);
};Yield between the steps.
Can inherit to add context
Examples
Multistep task [2]
void
TaskSendRequest(mg::sch::Task* aSelf)
{
std::cout << "Send\n";
aSelf->SetCallback([this](mg::sch::Task* aSelf) {
TaskRecvResponse(aSelf);
});
mg::sch::TaskScheduler::This().Post(aSelf);
}
void
TaskRecvResponse(mg::sch::Task* aSelf)
{
std::cout << "Receive\n";
aSelf->SetCallback([this](mg::sch::Task *aSelf) {
TaskFinish(aSelf);
});
mg::sch::TaskScheduler::This().Post(aSelf);
}
void
TaskFinish(mg::sch::Task* aSelf)
{
std::cout << "Finish\n";
}
1. Do something.
2. Prepare next step.
3. Post self.
1. Do something.
2. Prepare next step.
3. Post self.
1. Finish the work.
2. Delete/destroy/reuse.
Examples
Multistep task [3]
int
main()
{
MyTask task;
mg::sch::TaskScheduler scheduler("tst",
1, // Thread count.
5 // Subqueue size.
);
scheduler.Post(&task);
return 0;
}
Examples
Coroutine task
Examples
Coroutine task
int
main()
{
mg::sch::Task task;
task.SetCallback([](
mg::sch::Task& aSelf) -> mg::box::Coro {
std::cout << "Sending request ...\n";
co_await aSelf.AsyncYield();
std::cout << "Received response!\n";
co_await aSelf.AsyncYield();
std::cout << "Finish\n";
co_return;
}(task));
mg::sch::TaskScheduler scheduler("tst",
1, // Thread count.
5 // Subqueue size.
);
scheduler.Post(&task);
return 0;
}Coroutine enabler
Reschedule self. Let other tasks to work
Examples
Interacting tasks
Examples
Interacting tasks [1]
static void
TaskSubmitRequest(
mg::sch::Task& aSender)
{
mg::sch::Task* worker = new mg::sch::Task();
worker->SetCallback([](
mg::sch::Task& aSelf,
mg::sch::Task& aSender) -> mg::box::Coro {
aSelf.SetDelay(1000);
co_await aSelf.AsyncYield();
aSender.PostSignal();
co_await aSelf.AsyncExitDelete();
}(*worker, aSender));
mg::sch::TaskScheduler::This().Post(worker);
}Temporary worker task
"Work" for 1 sec, wakeup the owner
Examples
Interacting tasks [2]
int
main()
{
mg::sch::Task task;
mg::sch::TaskScheduler scheduler("tst",
1, // Thread count.
5 // Subqueue size.
);
task.SetCallback([](
mg::sch::Task& aSelf) -> mg::box::Coro {
TaskSubmitRequest(aSelf);
do {
aSelf.SetWait();
} while (!co_await aSelf.AsyncReceiveSignal());
co_return;
}(task));
scheduler.Post(&task);
return 0;
}Start work and wait for its comletion
Examples
Simple TCP
Examples
Simple TCP [1]
int
main()
{
mg::aio::IOCore core;
core.Start(3 /* threads */);
MyServer server(core);
uint16_t port = server.Bind();
server.Start();
for (int i = 0; i < theClientCount; ++i)
new MyClient(i + 1, core, port);
core.WaitEmpty();
return 0;
}IO task scheduler
Task as a server
Tasks as clients
Examples
Simple TCP [2]
class MyServer final
: private mg::aio::TCPServerSubscription
{
public:
MyServer(
mg::aio::IOCore& aCore)
: myServer(mg::aio::TCPServer::NewShared(aCore)) {}
uint16_t
Bind()
{
myServer->Bind(mg::net::HostMakeAllIPV4(0));
return myServer->GetPort();
}
void
Start()
{
myServer->Listen(this);
}
// ... Some methods ...
mg::aio::TCPServer::Ptr myServer;
}
Server receives task-events via the "subscription"
Server socket is created attached to IOCore
Bind + get the resulting port if it was random
After Listen(subscription) the socket is active, runs in IOCore, and delivers events
Examples
Simple TCP [3]
void
MyServer::OnAccept(
mg::net::Socket aSock,
const mg::net::Host& aPeerAddress) final
{
new MyPeer(myServer->GetCore(), aSock);
}
Invoked by IOCore workers
Spawn a new task to handle the peer socket
Examples
Simple TCP [4]
class MyClient final
: private mg::aio::TCPSocketSubscription
{
public:
MyClient(
mg::aio::IOCore& aCore,
uint16_t aPort)
: mySock(new mg::aio::TCPSocket(aCore))
{
mySock->Open({});
mg::aio::TCPSocketConnectParams connParams;
connParams.myEndpoint = mg::net::HostMakeLocalIPV4(aPort).ToString();
mySock->PostConnect(connParams, this);
}
// ... Some methods ...
mg::aio::TCPSocket* mySock;
};
Client receives task-events via the "subscription"
Will work in the given IOCore
Choose connection parameters
Enter the IOCore with the async connect, start receiving events
Examples
Simple TCP [5]
void
MyClient::OnConnect() final
{
const char* msg = "hello handshake";
mySock->SendRef(msg, mg::box::Strlen(msg) + 1);
mySock->Recv(1);
}
void
MyClient::OnRecv(
mg::net::BufferReadStream& aStream) final
{
MG_BOX_ASSERT(aStream.GetReadSize() > 0);
mySock->PostClose();
}
void
MyClient::OnClose() final
{
mySock->Delete();
delete this;
}Async send "handshake" on connect and read an ack
Async close on confirmation
Delete self, when close is finished
Examples
Pipeline
Examples
Pipeline [1]
class CalcClient final
: private mg::aio::TCPSocketSubscription
{
public:
void
Submit(
char aOp,
int64_t aArg1,
int64_t aArg2,
std::function<void(int64_t)>&& aOnComplete);
mg::aio::TCPSocket* mySock;
};Calculator frontend, operations executed in a "remote microservice"
Client for the remote calculator
Submit async operation
Examples
Pipeline [2]
class MyRequest
{
public:
MyRequest(
CalcClient& aCalcClient,
mg::sch::TaskScheduler& aSched)
: myCalcClient(aCalcClient)
, myTask(Execute(this))
{
aSched.Post(&myTask);
}
private:
mg::box::Coro
Execute(
MyRequest* aSelf);
CalcClient& myCalcClient;
mg::sch::Task myTask;
};User request
It has a client to the calculator host, and a scheduler to work in
Start execution
Body
Examples
Pipeline [3]
mg::box::Coro
MyRequest::Execute(
MyRequest* aSelf)
{
int64_t res = 0;
myCalcClient.Submit('+', 10, myID, [aSelf, &res](int64_t aRes) {
res = aRes;
aSelf->myTask.PostSignal();
});
while (!co_await aSelf->myTask.AsyncReceiveSignal())
aSelf->myTask.SetWait();
std::cout << "Result: " << res << std::end;
// ... Do whatever else or delete the request.
}Submit async request to the calculator client
Async wait for completion
On completion save the result and wake the request up
Benchmarks
Benchmarks
Task Scheduler
Thread Pool
vs
Debian 11, 2.5GHz, 32 cores
18.7 mln / sec
3.8 mln / sec
4.3 mln / sec
8.2 mln / sec
1 thread, empty tasks:
x5.0
2 threads, empty tasks:
x1.4
10 threads, empty tasks:
x7.7
50 threads, empty tasks:
x168.5
==
x1.8
1.8 mln / sec
x4.0
2.8 mln / sec
x118.8
7.5 mln / sec
<=3 threads, micro tasks:
5 threads, micro tasks:
10 threads, micro tasks:
50 threads, micro tasks:
Benchmarks
IOCore
boost::asio
vs
Linux, epoll
Ubuntu 22.04.4, 0.8-4.8GHz, 24 cores
5 threads, 100 clients, 128 b:
x1.54
962'630 msg / sec
1 thread, 100 clients, 128 b:
x1.45
330'200 msg / sec
5 threads, 200 clients, 100 KB:
x4.44
51'100 msg / sec
Benchmarks
IOCore
boost::asio
vs
3 threads, 100 clients, 128 b:
Linux, io_uring
x1.49
Ubuntu 24.04.1, 0.8-4.8GHz, 24 cores
218'500 msg / sec
1 thread, 100 clients, 128 b:
x1.27
245'470 msg / sec
3 threads, 200 clients, 100 KB:
x1.48
13'254 msg / sec
Benchmarks
IOCore
boost::asio
vs
3 threads, 100 clients, 128 b:
Windows, IOCP
x1.10
Windows 11 Home, 3.70GHz, 12 cores
195'430 msg / sec
1 thread, 100 clients, 128 b:
x1.20
80'510 msg / sec
3 threads, 200 clients, 100 KB:
x2.11
28'368 msg / sec
Benchmarks
IOCore
boost::asio
vs
3 threads, 100 clients, 128 b:
MacOS, kqueue
x1.50
macOS Big Sur 11.7.10, 2.5GHz, 4 cores
146'930 msg / sec
1 thread, 100 clients, 128 b:
x1.41
101'800 msg / sec
3 threads, 200 clients, 100 KB:
x1.45
12'750 msg / sec
Benchmarks
epoll
vs
io_uring
Get notified about events, then do operation on socket each individually.
Post many operations on many sockets, get notified when done.
Why x2 faster?
