Eli Bendersky: Concurrent Servers: Part 2 – Threads

Sequential client-handling flow

This is part 2 of a series on writing concurrent network servers. Part 1
presented the protocol implemented by the server, as well as the code for a
simple sequential server, as a baseline for the series.

In this part, we’re going to look at multi-threading as one approach to
concurrency, with a bare-bones threaded server implementation in C, as well as a
thread pool based implementation in Python.

All posts in the series:

The multi-threaded approach to concurrent server design

When discussing the performance of the sequential server in part 1, it was
immediately obvious that a lot of compute resources are wasted while the server
processes a client connection. Even assuming a client that sends messages
immediately and doesn’t do any waiting, network communication is still involved;
networks tend to be millions (or more) times slower than a modern CPU, so the
CPU running the sequential server will spend the vast majority of time in
gloriuos boredom waiting for new socket traffic to arrive.

Here’s a chart showing how sequential client processing happens over time:

The diagrams shows 3 clients. The diamond shapes denote the client’s “arrival
time” (the time at which the client attempted to connect to the server). The
black lines denote “wait time” (the time clients spent waiting for the server to
actually accept their connection), and the colored bars denote actual
“processing time” (the time server and client are interacting using the
protocol). At the end of the colored bar, the client disconnects.

In the diagram above, even though the green and orange clients arrived shortly
after the blue one, they have to wait for a while until the server is done with
the blue client. At this point the green client is accepted, while the orange
one has to wait even longer.

A multi-threaded server would launch multiple control threads, letting the OS
manage concurrency on the CPU (and across multiple CPU cores). When a client
connects, a thread is created to serve it, while the server is ready to accept
more clients in the main thread. The time chart for this mode looks like the

Concurrent client-handling flow

One thread per client, in C using pthreads

Our first code sample
in this post is a simple “one thread per client” server, written in C using the
foundational pthreads API
for multi-threading. Here’s the main loop:

while (1) {
  struct sockaddr_in peer_addr;
  socklen_t peer_addr_len = sizeof(peer_addr);

  int newsockfd =
      accept(sockfd, (struct sockaddr*)&peer_addr, &peer_addr_len);

  if (newsockfd  0) {
    perror_die("ERROR on accept");

  report_peer_connected(&peer_addr, peer_addr_len);
  pthread_t the_thread;
  thread_config_t config = {.sockfd = newsockfd};
  pthread_create(&the_thread, NULL, server_thread, &config);

And this is the server_thread function:

void* server_thread(void* arg) {
  thread_config_t* config = (thread_config_t*)arg;
  int sockfd = config->sockfd;
  unsigned id = (unsigned)pthread_self();
  printf("Thread %u created to handle connection with socket %dn", id, sockfd);
  printf("Thread %u donen", id);
  return 0;

The thread “configuration” is passed as a thread_config_t structure:

typedef struct { int sockfd; } thread_config_t;

The pthread_create call in the main loop launches a new thread that runs the
server_thread function. This thread terminates when server_thread
returns. In turn, server_thread returns when serve_connection returns.
serve_connection is exactly the same function from part 1 .

In part 1 we used a script to launch multiple clients concurrently and observe
how the server handles them. Let’s do the same with the multithreaded server:

$ python3.6 simple-client.py  -n 3 localhost 9090
INFO:2017-09-20 06:31:56,632:conn1 connected...
INFO:2017-09-20 06:31:56,632:conn2 connected...
INFO:2017-09-20 06:31:56,632:conn0 connected...
INFO:2017-09-20 06:31:56,632:conn1 sending b'^abc$de^abte$f'
INFO:2017-09-20 06:31:56,632:conn2 sending b'^abc$de^abte$f'
INFO:2017-09-20 06:31:56,632:conn0 sending b'^abc$de^abte$f'
INFO:2017-09-20 06:31:56,633:conn1 received b'b'
INFO:2017-09-20 06:31:56,633:conn2 received b'b'
INFO:2017-09-20 06:31:56,633:conn0 received b'b'
INFO:2017-09-20 06:31:56,670:conn1 received b'cdbcuf'
INFO:2017-09-20 06:31:56,671:conn0 received b'cdbcuf'
INFO:2017-09-20 06:31:56,671:conn2 received b'cdbcuf'
INFO:2017-09-20 06:31:57,634:conn1 sending b'xyz^123'
INFO:2017-09-20 06:31:57,634:conn2 sending b'xyz^123'
INFO:2017-09-20 06:31:57,634:conn1 received b'234'
INFO:2017-09-20 06:31:57,634:conn0 sending b'xyz^123'
INFO:2017-09-20 06:31:57,634:conn2 received b'234'
INFO:2017-09-20 06:31:57,634:conn0 received b'234'
INFO:2017-09-20 06:31:58,635:conn1 sending b'25$^ab0000$abab'
INFO:2017-09-20 06:31:58,635:conn2 sending b'25$^ab0000$abab'
INFO:2017-09-20 06:31:58,636:conn1 received b'36bc1111'
INFO:2017-09-20 06:31:58,636:conn2 received b'36bc1111'
INFO:2017-09-20 06:31:58,637:conn0 sending b'25$^ab0000$abab'
INFO:2017-09-20 06:31:58,637:conn0 received b'36bc1111'
INFO:2017-09-20 06:31:58,836:conn2 disconnecting
INFO:2017-09-20 06:31:58,836:conn1 disconnecting
INFO:2017-09-20 06:31:58,837:conn0 disconnecting

Indeed, all clients connected at the same time, and their communication with
the server occurs concurrently.

Challenges with one thread per client

Even though threads are fairly efficient in terms of resource usage on modern
OSes, the approach outlined in the previous section can still present challenges
with some workloads.

Imagine a scenario where many clients are connecting simultaneously, and some
of the sessions are long-lived. This means that many threads may be active at
the same time in the server. Too many threads can consume a large amount of
memory and CPU time just for the context switching . An alternative way to
look at it is as a security problem: this design makes it the server an easy
target for a DoS attack – connect a few
100,000s of clients at the same time and let them all sit idle – this will
likely kill the server due to excessive resource usage.

A larger problem occurs when there’s a non-trivial amount of CPU-bound
computation the server has to do for each client. In this case, swamping the
server is considerably easier – just a few dozen clients can bring a server to
its knees.

For these reasons, it’s prudent the do some rate-limiting on the number of
concurrent clients handled by a multi-threaded server. There’s a number of ways
to do this. The simplest that comes to mind is simply count the number of
clients currently connected and restrict that number to some quantity (that was
determined by careful benchmarking, hopefully). A variation on this approach
that’s very popular in concurrent application design is using a thread pool.

Thread pools

The idea of a thread pool is
simple, yet powerful. The server creates a number of working threads that all
expect to get tasks from some queue. This is the “pool”. Then, each client
connection is dispatched as a task to the pool. As long as there’s an idle
thread in the pool, it’s handed the task. If all the threads in the pool are
currently busy, the server blocks until the pool accepts the task (which happens
after one of the busy threads finished processing its current task and went back
to an idle state).

Here’s a diagram showing a pool of 4 threads, each processing a task. Tasks
(client connections in our case) are waiting until one of the threads in the
pool is ready to accept new tasks.

It should be fairly obvious that the thread pool approach provides a
rate-limiting mechanism in its very definition. We can decide ahead of time how
many threads we want our server to have. Then, this is the maximal number of
clients processed concurrently – the rest are waiting until one of the threads
becomes free. If we have 8 threads in the pool, 8 is the maximal number of
concurrent clients the server handles – even if thousands are attempting to
connect simultaneously.

How do we decide how many threads should be in the pool? By a careful analysis
of the problem domain, benchmarking, experimentation and also by the HW we have.
If we have a single-core cloud instance that’s one answer, if we have a 100-core
dual socket server available, the answer is different. Picking the
thread pool size can also be done dynamically at runtime based on load – I’ll
touch upon this topic in future posts in this series.

Servers that use thread pools manifest graceful degradation in the face of
high load – clients are accepted at some steady rate, potentially slower than
their rate of arrival for some periods of time; that said, no matter how many
clients are trying to connect simultaneously, the server will remain responsive
and will just churn through the backlog of clients to its best ability. Contrast
this with the one-thread-per-client server which can merrily accept a large
number of clients until it gets overloaded, at which point it’s likely to either
crash or start working very slowly for all processed clients due to resource
exhaustion (such as virtual memory thrashing).

Using a thread pool for our network server

For this variation of the server
I’ve switched to Python, which comes with a robust implementation of a thread
pool in the standard library (ThreadPoolExecutor from the
concurrent.futures module) .

This server creates a thread pool, then loops to accept new clients on the main
listening socket. Each connected client is dispatched into the pool with

pool = ThreadPoolExecutor(args.n)
sockobj = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sockobj.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
sockobj.bind(('localhost', args.port))

    while True:
        client_socket, client_address = sockobj.accept()
        pool.submit(serve_connection, client_socket, client_address)
except KeyboardInterrupt as e:

The serve_connection function is very similar to its C counterpart, serving
a single client until the client disconnects, while following our protocol:

ProcessingState = Enum('ProcessingState', 'WAIT_FOR_MSG IN_MSG')

def serve_connection(sockobj, client_address):
    print('{0} connected'.format(client_address))
    state = ProcessingState.WAIT_FOR_MSG

    while True:
            buf = sockobj.recv(1024)
            if not buf:
        except IOError as e:
        for b in buf:
            if state == ProcessingState.WAIT_FOR_MSG:
                if b == ord(b'^'):
                    state = ProcessingState.IN_MSG
            elif state == ProcessingState.IN_MSG:
                if b == ord(b'$'):
                    state = ProcessingState.WAIT_FOR_MSG
                    sockobj.send(bytes([b + 1]))
                assert False

    print('{0} done'.format(client_address))

Let’s see how the thread pool size affects the blocking behavior for multiple
concurrent clients. For demonstration purposes, I’ll run the threadpool server
with a pool size of 2 (only two threads are created to service clients):

$ python3.6 threadpool-server.py -n 2

And in a separate terminal, let’s run the client simulator again, with 3
concurrent clients:

$ python3.6 simple-client.py  -n 3 localhost 9090
INFO:2017-09-22 05:58:52,815:conn1 connected...
INFO:2017-09-22 05:58:52,827:conn0 connected...
INFO:2017-09-22 05:58:52,828:conn1 sending b'^abc$de^abte$f'
INFO:2017-09-22 05:58:52,828:conn0 sending b'^abc$de^abte$f'
INFO:2017-09-22 05:58:52,828:conn1 received b'b'
INFO:2017-09-22 05:58:52,828:conn0 received b'b'
INFO:2017-09-22 05:58:52,867:conn1 received b'cdbcuf'
INFO:2017-09-22 05:58:52,867:conn0 received b'cdbcuf'
INFO:2017-09-22 05:58:53,829:conn1 sending b'xyz^123'
INFO:2017-09-22 05:58:53,829:conn0 sending b'xyz^123'
INFO:2017-09-22 05:58:53,830:conn1 received b'234'
INFO:2017-09-22 05:58:53,831:conn0 received b'2'
INFO:2017-09-22 05:58:53,831:conn0 received b'34'
INFO:2017-09-22 05:58:54,831:conn1 sending b'25$^ab0000$abab'
INFO:2017-09-22 05:58:54,832:conn1 received b'36bc1111'
INFO:2017-09-22 05:58:54,832:conn0 sending b'25$^ab0000$abab'
INFO:2017-09-22 05:58:54,833:conn0 received b'36bc1111'
INFO:2017-09-22 05:58:55,032:conn1 disconnecting
INFO:2017-09-22 05:58:55,032:conn2 connected...
INFO:2017-09-22 05:58:55,033:conn2 sending b'^abc$de^abte$f'
INFO:2017-09-22 05:58:55,033:conn0 disconnecting
INFO:2017-09-22 05:58:55,034:conn2 received b'b'
INFO:2017-09-22 05:58:55,071:conn2 received b'cdbcuf'
INFO:2017-09-22 05:58:56,036:conn2 sending b'xyz^123'
INFO:2017-09-22 05:58:56,036:conn2 received b'234'
INFO:2017-09-22 05:58:57,037:conn2 sending b'25$^ab0000$abab'
INFO:2017-09-22 05:58:57,038:conn2 received b'36bc1111'
INFO:2017-09-22 05:58:57,238:conn2 disconnecting

Recall the behavior of previously discussed servers:

  1. In the sequential server, all connections were serialized. One finished, and
    only then the next started.
  2. In the thread-per-client server earlier in this post, all connections wer
    accepted and serviced concurrently.

Here we see another possibility: two connections are serviced concurrently, and
only when one of them is done the third is admitted. This is a direct result of
the thread pool size set to 2. For a more realistic use case we’d set the thread
pool size to much higher, depending on the machine and the exact protocol. This
buffering behavior of thread pools is well understood – I’ve written about it
more in detail just a few months ago
in the context of Clojure’s core.async module.

Summary and next steps

This post discusses multi-threading as a means of concurrency in network
servers. The one-thread-per-client approach is presented for an initial
discussion, but this method is not common in practice since it’s a security

Thread pools are much more common, and most popular programming languages have
solid implementations (for some, like Python, it’s in the standard library). The
thread pool server presented here doesn’t suffer from the problems of

However, threads are not the only way to handle multiple clients concurrently.
In the next post we’re going to look at some solutions using asynchronous, or
event-driven programming.

Also found at : Eli Bendersky: Concurrent Servers: Part 2 – Threads

Post author

Dustin Gurley is an Designer, Developer, Artist, Instructor, Critical Theorist and Systems Engineer. He has an extensive background working professionally with 2D/2.5D/3D Motion Graphics, Compositing, Film, Video, Photography and client-side performance techniques as it pertains to web development. Dustin recently completed work on his Master of Fine Art degree in Motion Media Design (Motion Graphics) from the Savannah College of Art and Design. Prior to beginning his graduate work, Dustin obtained a Bachelor of Art degree in Communication Studies with a concentration in Broadcast and Emerging Media from the University of North Carolina at Wilmington. In addition to design and modeling, Dustin enjoys toying with his view camera, working with scratch film, authoring media related material and contributing to various industry conferences. When not in front of a computer, Dustin can be found with his wife, Regina Everett Gurley. The couple enjoys dividing their time between their home just outside of Raleigh, North Carolina and the beautiful North Carolina coast. Currently, Dustin serves as the Lead Instructor of Internet Technologies for Wake Technical Community College in Raleigh, North Carolina.