From one blocking accept() to epoll: a C TCP server up the I/O ladder, measured

I connected one client to a blocking TCP server and held the socket open without sending a single byte. Then I connected a second client and sent it a line of text. The second client sat there for 1.51 seconds with no reply. It got its echo back one millisecond after I closed the first connection. That 1.51 seconds is the reason the other six versions of this server exist. Last week I wrote up why I rebuilt this server seven times : framework knowledge resets every few years, the layer underneath it compounds. That piece stayed at the level of outcomes. This one goes the other way, down into the code and the numbers. The claims that matter here are the kind you can read a hundred times without being able to derive them. "select is O(n)." "epoll only hands you the ready fds." I had read both for years. I wanted to make my own machine say them out loud. The target the whole exercise is built around is Dan Kegel's old C10K problem : how do you serve ten thousand clients at once on one server? Each of the seven versions hits a wall, and the wall is what names the next one. The whole thing is one echo server written seven times, no libraries beyond libc, on GitHub . Every number below is from running it on macOS (Apple clang 21, darwin 25.4) on 2026-06-29. The binaries are built with AddressSanitizer and UBSan on, so read the absolute microseconds loosely. The structure is what holds. Phase 01: blocking, and the 1.5 second stall The first server is the one everybody writes first. Accept a connection, talk to it, close it, accept the next. for (;;) { int client_fd = accept ( server_fd , NULL , NULL ); if ( client_fd == - 1 ) { perror ( "accept" ); continue ; } handle_client ( client_fd ); close ( client_fd ); } handle_client loops on read until the client hangs up. Both accept and read block: when there is nothing to do, the thread sleeps in the kernel. That is good for idle cost and fatal for everything else. While the server is parked in read waiting on client A, client