Concurrent Server Model

August 16, 2014

Author：Eric
Source：http://blog.wjin.org/posts/concurrent-server-model.html
Declaration: this work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

Introduction

Develop large-scale, high concurrent networking service application is challenging. Here are a few models. PPC (process per connection), TPC (thread per connection), and I/O multiplexing, such as select, poll and epoll.

PPC and TPC

PPC and TPC create a new process/thread for each connection. Therefore, this mode has its inherent drawbacks：when connecting increases dramatically, the overhead of create a new process/thread cannot be ignored.

Even though process/thread pool may alleviate it, switch and resource contention between processes/threads can obviously slow down the system response. That is the famous problem of apache webserver avalanche.

I/O Multiplexing

Select and Poll

Select and poll allows a program to monitor multiple file descriptors, waiting until one or more of the file descriptors become “ready”. A file descriptor is considered ready if it is possible to perform the corresponding I/O operation without blocking.

There are some drawbacks for select:

The max number of concurrency is limited by FD (set by FD_SETSIZE)
O(n) time complexity when scanning FD
Memory Copy between kernel and user space

Linux Epoll

Epoll standards for event poll. It is a variant of poll that can be used either as an edge-triggered or a level-triggered interface and scales well to large numbers of watched file descriptors. It overcomes the disadvantages of select and poll.

No limitation of FD, the concurrent number is related to how many files this process can open
O(1) time complexity when scanning FD
Memory Sharing

To develop a concurrent and high performance service in linux environment, we should choose epoll with no doubt, and its related sytem calls are also easy to use.

epoll_create

int epoll_create(int size);

epoll_ctl

int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event);

epoll_wait

int epoll_wait(int epfd, struct epoll_event *events,
                      int maxevents, int timeout);

struct epoll_event {
    __uint32_t events; // Epoll events
    epoll_data_t data; // User data variable
};

typedef union epoll_data {
    void *ptr;
    int fd;
    __uint32_t u32;
    __uint64_t u64;
} epoll_data_t;