High-performance TCP Socket programming in .NET C#

Because this question gets a lot of views I decided to post an "answer", but technically this isn't an answer, but my final conclusion for now, so I will mark it as answer.

About the approaches:

The async/await functions tend to produce awaitable async Tasks assigned to the TaskScheduler of the dotnet runtime, so having thousands of simultaneous connections, therefore thousands or reading/writing operations will start up thousands of Tasks. As far as I know this creates thousands of StateMachines stored in ram and countless context switchings in the threads they are assigned to, resulting in very high CPU overhead. With a few connections/async calls it is better balanced, but as the awaitable Task count grows it gets slow exponentially.

The BeginReceive/EndReceive/BeginSend/EndSend socket methods are technically async methods with no awaitable Tasks, but with callbacks on the end of the call, which actually optimizes more the multithreading, but still the limitation of the dotnet design of these socket methods are poor in my opinion, but for simple solutions (or limited count of connections) it is the way to go.

The SocketAsyncEventArgs/ReceiveAsync/SendAsync type of socket implementation is the best on Windows for a reason. It utilizes the Windows IOCP in the background to achieve the fastest async socket calls and use the Overlapped I/O and a special socket mode. This solution is the "simplest" and fastest under Windows. But under mono/linux, it never will be that fast, because mono emulates the Windows IOCP by using linux epoll, which actually is much faster than IOCP, but it has to emulate the IOCP to achieve dotnet compatibility, this causes some overhead.

About buffer sizes:

There are countless ways to handle data on sockets. Reading is straightforward, data arrives, You know the length of it, You just copy bytes from the socket buffer to Your application and process it. Sending data is a bit different.

• You can pass Your complete data to the socket and it will cut it to chunks, copy the chucks to the socket buffer until there is no more to send and the sending method of the socket will return when all data is sent (or when error happens).
• You can take Your data, cut it to chunks and call the socket send method with a chunk, and when it returns then send the next chunk until there is no more.

In any cases You should consider what socket buffer size You should choose. If You are sending large amount of data, then the bigger the buffer is, the less chunks has to be sent, therefore less calls in Your (or in the socket's internal) loop has to be called, less memory copy, less overhead. But allocating large socket buffers and program data buffers will result in large memory usage, especially if You are having thousands of connections, and allocating (and freeing up) large memory multiple times is always expensive.

On sending side 1-2-4-8kB socket buffer size is ideal for most cases, but if You are preparing to send large files (over few MB) regularly then 16-32-64kB buffer size is the way to go. Over 64kB there is usually no point to go.

But this has only advantage if the receiver side has relatively large receiving buffers too.

Usually over the internet connections (not local network) no point to get over 32kB, even 16kB is ideal.

Going under 4-8kB can result in exponentially incremented call count in the reading/writing loop, causing large CPU load and slow data processing in the application.

Go under 4kB only if You know Your messages will usually be smaller than 4kB, or just very rarely over 4KB.

My conclusion:

Regarding my experiments built-in socket class/methods/solutions in dotnet are OK, but not efficient at all. My simple linux C test programs using non-blocking sockets could overperform the fastest and "high-performance" solution of dotnet sockets (SocketAsyncEventArgs).

This does not mean it is impossible to have fast socket programming in dotnet, but under Windows I had to make my own implementation of Windows IOCP by directly communicating with the Windows Kernel via InteropServices/Marshaling, directly calling Winsock2 methods, using a lot of unsafe codes to pass the context structs of my connections as pointers between my classes/calls, creating my own ThreadPool, creating IO event handler threads, creating my own TaskScheduler to limit the count of simultaneous async calls to avoid pointlessly much context switches.

This was a lot of job with a lot of research, experiment, and testing. If You want to do it on Your own, do it only if You really think it worth it. Mixing unsafe/unmanage code with managed code is a pain in the ass, but the end it worth it, because with this solution I could reach with my own http server about 36000 http request/sec on a 1gbit lan, on Windows 7, with an i7 4790.

This is such a high performance that I never could reach with dotnet built-in sockets.

When running my dotnet server on an i9 7900X on Windows 10, connected to a 4c/8t Intel Atom NAS on Linux, via 10gbit lan, I can use the complete bandwidth (therefore copying data with 1GB/s) no matter if I have only 1 or 10000 simultaneous connections.

My socket library also detects if the code is running on linux, and then instead of Windows IOCP (obviously) it is using linux kernel calls via InteropServices/Marshalling to create, use sockets, and handle the socket events directly with linux epoll, managed to max out the performance of the test machines.

Design tip:

As it turned out it is difficult to design a networking library from scatch, especially one, that is likely very universal for all purposes. You have to design it to have many settings, or especially to the task You need. This means finding the proper socket buffer sizes, the I/O processing thread count, the Worker thread count, the allowed async task count, these all has to be tuned to the machine the application running on and to the connection count, and data type You want to transfer through the network. This is why the built-in sockets are not performing that good, because they must be universal, and they do not let You set these parameters.

In my case assingning more than 2 dedicated threads to I/O event processing actually makes the overall performance worse, because using only 2 RSS Queues, and causing more context switching than what is ideal.

Choosing wrong buffer sizes will result in performance loss.

Always benchmark different implementations for the simulated task You need to find out which solution or setting is the best.

Different settings may produce different performance results on different machines and/or operating systems!

Mono vs Dotnet Core:

Since I've programmed my socket library in a FW/Core compatible way I could test them under linux with mono, and with core native compilation. Most interestingly I could not observe any remarkable performance differences, both were fast, but of course leaving mono and compiling in core should be the way to go.

Bonus performance tip:

If Your network card is capable of RSS (Receive Side Scaling) then enable it in Windows in the network device settings in the advanced properties, and set the RSS Queue from 1 to as high you can/as high is the best for your performance.

If it is supported by Your network card then it is usually set to 1, this assigns the network event to process only by one CPU core by the kernel. If You can increment this queue count to higher numbers then it will distribute the network events between more CPU cores, and will result in much better performance.

In linux it is also possible to set this up, but in different ways, better to search for Your linux distro/lan driver information.

I hope my experience will help some of You!

I had the same problem. You should take a look into: NetCoreServer

Every thread in the .NET clr threadpool can handle one task at one time. So to handle more async connects/reads etc., you have to change the threadpool size by using:

ThreadPool.SetMinThreads(Int32, Int32)

Using EAP (event based asynchronous pattern) is the way to go on Windows. I would use it on Linux too because of the problems you mentioned and take the performance plunge.

The best would be io completion ports on Windows, but they are not portable.

PS: when it comes to serialize objects, you are highly encouraged to use protobuf-net. It binary serializes objects up to 10x times faster than the .NET binary serializer and saves a little space too!