Why is there a threshold of concurrent HTTP requests I can make using `HttpClient`?

Question

I noticed that there exists some kind of threshold for concurrent HTTP requests I can making using .NET core's HttpClient ie it seems to work fine when I have <= 1,000 requests, but nearing 10,000 is problematic. Here is the relevant failing code:

using System;
using System.Net.Http;
using System.Threading.Tasks;

namespace RequestsGalore
{
    class Program
    {
        static HttpClient Client { get; set; } = new HttpClient();

        static void Main(string[] args)
        {
            var url = "http://example.com";

            int requests = 10_000;
            var tasks = new Task<HttpResponseMessage>[requests];

            for (int i = 0; i < requests; i++)
            {
                tasks[i] = Client.GetAsync(url, HttpCompletionOption.ResponseHeadersRead);
            }

            Task.WaitAll(tasks);

            for (int i = 0; i < requests; i++)
            {
                Console.WriteLine(tasks[i].Result.StatusCode);
            }
        }
    }
}

, and the exception:

Unhandled Exception: System.AggregateException: One or more errors occurred.
(An error occurred while sending the request.)
(A task was canceled.)
.
. [MANY OF THE ABOVE TWO MESSAGES]
.
---> System.Net.Http.HttpRequestException: An error occurred while sending the request.
---> System.IO.IOException: Unable to read data from the transport connection: Connection reset by peer.
---> System.Net.Sockets.SocketException: Connection reset by peer
   --- End of inner exception stack trace ---
   at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.ThrowException(SocketError error)
   at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.GetResult(Int16 token)
   at System.Net.Http.HttpConnection.FillAsync()
   at System.Net.Http.HttpConnection.ReadNextResponseHeaderLineAsync(Boolean foldedHeadersAllowed)
   at System.Net.Http.HttpConnection.SendAsyncCore(HttpRequestMessage request, CancellationToken cancellationToken)
   --- End of inner exception stack trace ---
   at System.Net.Http.HttpConnection.SendAsyncCore(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.SendWithNtConnectionAuthAsync(HttpConnection connection, HttpRequestMessage request, Boolean doRequestAuth, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.SendWithRetryAsync(HttpRequestMessage request, Boolean doRequestAuth, CancellationToken cancellationToken)
   at System.Net.Http.RedirectHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.HttpClient.FinishSendAsyncUnbuffered(Task`1 sendTask, HttpRequestMessage request, CancellationTokenSource cts, Boolean disposeCts)
   --- End of inner exception stack trace ---
   at System.Threading.Tasks.Task.WaitAllCore(Task[] tasks, Int32 millisecondsTimeout, CancellationToken cancellationToken)
   at RequestsGalore.Program.Main(String[] args) in /home/[REDACTED]/Downloads/RequestsGalore/Program.cs:line 27

And some information about my machine:

$ dotnet --info
.NET Core SDK (reflecting any global.json):
 Version:   2.2.401
 Commit:    729b316c13

Runtime Environment:
 OS Name:     ubuntu
 OS Version:  19.04
 OS Platform: Linux
 RID:         ubuntu.19.04-x64
 Base Path:   /usr/share/dotnet/sdk/2.2.401/

Host (useful for support):
  Version: 2.2.6
  Commit:  7dac9b1b51

.NET Core SDKs installed:
  2.2.401 [/usr/share/dotnet/sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.2.6 [/usr/share/dotnet/shared/Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.2.6 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.2.6 [/usr/share/dotnet/shared/Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
  https://aka.ms/dotnet-download

Answer 1

“对等连接重置”指向另一端，而不是您的代码，从而删除了连接。

Answer 2

This code opens too many concurrent connections to the target web server, probably triggering anti-denial-of-service protections. There's a simple way to limit the concurrent requests to any target in .NET Framework the ServicePoint , and there are default limits in place.

In .NET Core ServicePoint is not used. And you set the limit using the HttpClientHandler :

        var url = "http://example.com";
        HttpClientHandler handler = new HttpClientHandler();
        handler.MaxConnectionsPerServer = 10;
        Client = new HttpClient(handler);