Why competitive programmers should join ISUCON for tremendous growth

ISUCON is a programming contest about making web applications faster. Competitive programming is the best possible material for learning algorithms, but outside of research positions it is often hard to see it as a directly applicable skill. ISUCON is packed with the foundational knowledge needed in the web industry, and I wrote this article believing that if many competitive programmers join ISUCON, both the competitive programming community and the web industry can leap forward. I also hope that if competitive programmers win prizes, the latent value of competitive programming will be recognized and its visibility will rise. When it comes to bottlenecks in web development, anything that can be solved with knowledge has, in most cases, already been solved by well-established software if you use standard tools; instead, many problems arise because time complexity was not considered—for example, a full scan is used where adding an index would make it fast—so I believe the experience gained in competitive programming can be put to good use.

Introduction

This article mainly explores how to win at ISUCON using C++. Because few people use C++ in the Japanese web industry, it is not provided as a reference implementation at ISUCON. However, as its wide use in competitive programming shows, C++ is the fastest language. In fact, Google develops web applications in C++ and keeps CPU costs low. To show that, once you set up the environment and get used to it, C++ lets you optimize efficiently in web application development without sacrificing development speed, Team Anago entered ISUCON6 with a custom C++ server and successfully advanced to the finals. Being able to wield C++ freely is one of the strengths of competitive programmers, so in this article, based on the experience gained through ISUCON6, I introduce how to take on ISUCON efficiently in a language that is not provided as a reference implementation.

• Filter style … You replace the heavy processing parts of the web application with a special string (for example, <!--hoge-->), process that part quickly in the C++ server, and substitute it back. It has the advantage of not requiring deep knowledge of the reference implementation language, but the disadvantage that you must rewrite in fairly large units. This is mainly what we used in ISUCON6.
• Callback style … The web application calls the custom server. It has the advantage of allowing fine-grained, incremental development at the function level, but the disadvantages that you must implement callback handling for the reference implementation language and that latency tends to grow.

• Disabling network isolation … Network isolation makes the Docker process consume a lot of CPU for forwarding, so we configure it to use the host network.
• Mounting some file systems … The file systems Docker provides automatically are slow, so we mount as needed. For example, nginx temporarily writes a file when it receives a POST request, and if this write destination is on the Docker file system it can be slow.
• (Optional) Disabling process isolation … On some OSes, if you disable network isolation without also disabling process isolation, you may get permission-related errors, so we optionally disable process isolation as well.

• HTTP connection library … You need to be able to make HTTP connections to the reference implementation. Since something mimicking an external service may be given and speak HTTP2, if you have time it may be good to be able to connect over HTTP2 as well.
• Database libraries … Connecting to MySQL is a must. Problems using Redis, PostgreSQL, MongoDB, memcached, and so on are possible, so if you have time it may be wise to prepare them.
• String libraries … Being able to process strings easily gives you room to breathe during development. Regular expressions are not essential since they can be substituted by other means, but they appear frequently in web development, so it may be good to be able to use them.
• Encoding libraries … BASE64 encoding, URL encoding, JSON encoding, and the like appear frequently in web development.
• Hashing libraries … MD5, SHA1, SHA256, and the like are also commonly used.
• Date function libraries … You may need to process dates given in HTTP headers or dates stored in MySQL.

worker_processes auto;

events {
  worker_connections 8096;
  multi_accept on;
  use epoll;
}

http {
  sendfile on;
  tcp_nopush on;
  tcp_nodelay on;
  keepalive_timeout 15;

  upstream app {
    server 127.0.0.1:8000;
    keepalive 16;
  }

  server {
    listen 80;

    location / {
      proxy_pass http://app;
      proxy_redirect off;
      proxy_set_header X-Real-IP $remote_addr;
      proxy_buffering off;
      proxy_buffer_size 128k;
      proxy_buffers 100 128k;
      proxy_http_version 1.1;
      proxy_set_header Connection "";
    }
  }
}

cat <<'EOM' > /etc/sysctl.conf
net.core.netdev_max_backlog=32768
net.core.rmem_max = 16777216
net.core.somaxconn=32768
net.core.wmem_max = 16777216
net.ipv4.ip_local_port_range= 10000 65535
net.ipv4.tcp_fin_timeout=10
net.ipv4.tcp_max_syn_backlog=32768
net.ipv4.tcp_rmem = 4096 349520 16777216
net.ipv4.tcp_timestamps = 0
net.ipv4.tcp_tw_recycle=1
net.ipv4.tcp_tw_reuse=1
net.ipv4.tcp_wmem = 4096 65536 16777216
net.ipv4.tcp_rfc1337=1
net.ipv4.tcp_keepalive_probes=5
net.ipv4.tcp_slow_start_after_idle=0
net.core.somaxconn=65535
EOM
sysctl -p