Go 系统调用与 C 系统调用

Question

Go, and C both involve system calls directly (Technically, C will call a stub).

Technically, write is both a system call and a C function (at least on many systems). However, the C function is just a stub which invokes the system call. Go does not call this stub, it invokes the system call directly, which means that C is not involved here

From Differences between C write call and Go syscall.Write

My benchmark shows, pure C system call is 15.82% faster than pure Go system call in the latest release (go1.11).

What did I miss? What could be a reason and how to optimize them?

Benchmarks:

Go:

package main_test

import (
    "syscall"
    "testing"
)

func writeAll(fd int, buf []byte) error {
    for len(buf) > 0 {
        n, err := syscall.Write(fd, buf)
        if n < 0 {
            return err
        }
        buf = buf[n:]
    }
    return nil
}

func BenchmarkReadWriteGoCalls(b *testing.B) {
    fds, _ := syscall.Socketpair(syscall.AF_UNIX, syscall.SOCK_STREAM, 0)
    message := "hello, world!"
    buffer := make([]byte, 13)
    for i := 0; i < b.N; i++ {
        writeAll(fds[0], []byte(message))
        syscall.Read(fds[1], buffer)
    }
}

C:

#include <time.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/socket.h>

int write_all(int fd, void* buffer, size_t length) {
    while (length > 0) {
        int written = write(fd, buffer, length);
        if (written < 0)
            return -1;
        length -= written;
        buffer += written;
    }
    return length;
}

int read_call(int fd, void *buffer, size_t length) {
    return read(fd, buffer, length);
}

struct timespec timer_start(){
    struct timespec start_time;
    clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start_time);
    return start_time;
}

long timer_end(struct timespec start_time){
    struct timespec end_time;
    clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &end_time);
    long diffInNanos = (end_time.tv_sec - start_time.tv_sec) * (long)1e9 + (end_time.tv_nsec - start_time.tv_nsec);
    return diffInNanos;
}

int main() {
    int i = 0;
    int N = 500000;
    int fds[2];
    char message[14] = "hello, world!\0";
    char buffer[14] = {0};

    socketpair(AF_UNIX, SOCK_STREAM, 0, fds);
    struct timespec vartime = timer_start();
    for(i = 0; i < N; i++) {
        write_all(fds[0], message, sizeof(message));
        read_call(fds[1], buffer, 14);
    }
    long time_elapsed_nanos = timer_end(vartime);
    printf("BenchmarkReadWritePureCCalls\t%d\t%.2ld ns/op\n", N, time_elapsed_nanos/N);
}

340 different running, each C running contains 500000 executions, and each Go running contains bN executions (mostly 500000, few times executed in 1000000 times):

T-Test for 2 Independent Means: The t-value is -22.45426. The p-value is < .00001. The result is significant at p < .05.

T-Test Calculator for 2 Dependent Means: The value of t is 15.902782. The value of p is < 0.00001. The result is significant at p ≤ 0.05.

---

Update: I managed the proposal in the answers and wrote another benchmark, it shows the proposed approach significantly drops the performance of massive I/O calls, its performance close to CGO calls.

Benchmark:

func BenchmarkReadWriteNetCalls(b *testing.B) {
    cs, _ := socketpair()
    message := "hello, world!"
    buffer := make([]byte, 13)
    for i := 0; i < b.N; i++ {
        cs[0].Write([]byte(message))
        cs[1].Read(buffer)
    }
}

func socketpair() (conns [2]net.Conn, err error) {
    fds, err := syscall.Socketpair(syscall.AF_LOCAL, syscall.SOCK_STREAM, 0)
    if err != nil {
        return
    }
    conns[0], err = fdToFileConn(fds[0])
    if err != nil {
        return
    }
    conns[1], err = fdToFileConn(fds[1])
    if err != nil {
        conns[0].Close()
        return
    }
    return
}

func fdToFileConn(fd int) (net.Conn, error) {
    f := os.NewFile(uintptr(fd), "")
    defer f.Close()
    return net.FileConn(f)
}

The above figure shows, 100 different running, each C running contains 500000 executions, and each Go running contains bN executions (mostly 500000, few times executed in 1000000 times)

Answer 1

My benchmark shows, pure C system call is 15.82% faster than pure Go system call in the latest release (go1.11).

What did I miss? What could be a reason and how to optimize them?

The reason is that while both C and Go (on a typical platform Go supports—such as Linux or *BSD or Windows) are compiled down to machine code, Go-native code runs in an environment quite different from that of C.

The two chief differences to C are:

• Go code runs in the context of so-called goroutines which are freely scheduled by the Go runtime on different OS threads.
• Goroutines use their own (growable and reallocatable) lightweight stacks which have nothing to do with the OS-supplied stack C code uses.

So, when Go code wants to make a syscall, quite a lot should happen:

1. The goroutine which is about to enter a syscall must be "pinned" to the OS thread on which it's currently running.
2. The execution must be switched to use the OS-supplied C stack.
3. The necessary preparation in the Go runtime's scheduler are made.
4. The goroutine enters the syscall.
5. Upon exiting the execution of the goroutine has to be resumed, which is a relatively involved process in itself which may be additionaly hampered if the goroutine was in the syscall for too long and the scheduler removed the so-called "processor" from under that goroutine, spawned another OS thread and made that processor run another goroutine ("processors", or P s are thingies which run goroutines on OS threads).

---

Update to answer the OP's comment

<…> Thus there is no way to optimize and I must suffer that if I make massive IO calls, mustn't I?

It heavily depends on the nature of the "massive I/O" you're after.

If your example (with socketpair(2) ) is not toy, there is simply no reason to use syscalls directly: the FDs returned by socketpair(2) are "pollable" and hence the Go runtime may use its native "netpoller" machinery to perform I/O on them. Here is a working code from one of my projects which properly "wraps" FDs produced by socketpair(2) so that they can be used as "regular" sockets (produced by functions from the net standard package):

func socketpair() (net.Conn, net.Conn, error) {
       fds, err := syscall.Socketpair(syscall.AF_LOCAL, syscall.SOCK_STREAM, 0)
       if err != nil {
               return nil, nil, err
       }

       c1, err := fdToFileConn(fds[0])
       if err != nil {
               return nil, nil, err
       }

       c2, err := fdToFileConn(fds[1])
       if err != nil {
               c1.Close()
               return nil, nil, err
       }

       return c1, c2, nil
}

func fdToFileConn(fd int) (net.Conn, error) {
       f := os.NewFile(uintptr(fd), "")
       defer f.Close()
       return net.FileConn(f)
}

If you're talking about some other sort of I/O, the answer is that yes, syscalls are not really cheap and if you must do lots of them, there are ways to work around their cost (such as offloading to some C code—linked in or hooked up as an external process—which would somehow batch them so that each call to that C code would result in several syscalls done by the C side).