efficiency of fwrite for massive numbers of small writes

Question

I have a program that saves many large files >1GB using fwrite It works fine, but unfortunately due to the nature of the data each call to fwrite only writes 1-4bytes. with the result that the write can take over an hour, with most of this time seemingly due to the syscall overhead (or at least in the library function of fwrite). I have a similar problem with fread.

Does anyone know of any existing / library functions that will buffer these writes and reads with an inline function, or is this another roll your own?

Answer 1

First of all, fwrite() is a library and not a system call. Secondly, it already buffers the data.

You might want to experiment with increasing the size of the buffer. This is done by using setvbuf(). On my system this only helps a tiny bit, but YMMV.

If setvbuf() does not help, you could do your own buffering and only call fwrite() once you've accumulated enough data. This involves more work, but will almost certainly speed up the writing as your own buffering can be made much more lightweight that fwrite()'s.

edit: If anyone tells you that it's the sheer number of fwrite() calls that is the problem, demand to see evidence. Better still, do your own performance tests. On my computer, 500,000,000 two-byte writes using fwrite() take 11 seconds. This equates to throughput of about 90MB/s.

Last but not least, the huge discrepancy between 11 seconds in my test and one hour mentioned in your question hints at the possibility that there's something else going on in your code that's causing the very poor performance.

Answer 2

First and foremost: small fwrites() are slower, because each fwrite has to test the validity of its parameters, do the equivalent of flockfile(), possibly fflush(), append the data, return success: this overhead adds up -- not so much as tiny calls to write(2), but it's still noticeable.

Proof:

#include <stdio.h>
#include <stdlib.h>

static void w(const void *buf, size_t nbytes)
{
    size_t n;
    if(!nbytes)
        return;
    n = fwrite(buf, 1, nbytes, stdout);
    if(n >= nbytes)
        return;
    if(!n) {
        perror("stdout");
        exit(111);
    }
    w(buf+n, nbytes-n);
}

/* Usage: time $0 <$bigfile >/dev/null */
int main(int argc, char *argv[])
{
    char buf[32*1024];
    size_t sz;

    sz = atoi(argv[1]);
    if(sz > sizeof(buf))
        return 111;
    if(sz == 0)
        sz = sizeof(buf);
    for(;;) {
        size_t r = fread(buf, 1, sz, stdin);
        if(r < 1)
            break;
        w(buf, r);
    }
    return 0;
}

That being said, you could do what many commenters suggested, ie add your own buffering before fwrite: it's very trivial code, but you should test if it really gives you any benefit.

If you don't want to roll your own, you can use eg the buffer interface in skalibs, but you'll probably take longer to read the docs than to write it yourself (imho).