[PERFORMANCE]fs: sendfile suffer performance degradation when buffer size have performance impact on underling IO

[David Wang]

Hi, 

I was trying to confirm the performance improvement via replacing read/write sequences with sendfile, 
But I got quite a surprising result:

$ gcc -DUSE_SENDFILE cp.cpp
$ time ./a.out 

real	0m56.121s
user	0m0.000s
sys	0m4.844s

$ gcc  cp.cpp
$ time ./a.out 

real	0m27.363s
user	0m0.014s
sys	0m4.443s

The result show that, in my test scenario,  the read/write sequences only use half of the time by sendfile.
My guess is that sendfile using a default pipe with buffer size 1<<16 (16 pages), which is not tuned for the underling IO, 
hence a read/write sequences with buffer size 1<<17 is much faster than sendfile.

But the problem with sendfile is that there is no parameter to tune the buffer size from userspace...Any chance to fix this?

The test code is as following:

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/sendfile.h>
#include <fcntl.h>

char buf[1<<17];   // much better than 1<<16
int main() {
	int i, fin, fout, n, m;
	for (i=0; i<128; i++) {
		// dd if=/dev/urandom of=./bigfile bs=131072 count=256
		fin  = open("./bigfile", O_RDONLY);
		fout = open("./target", O_WRONLY | O_CREAT | O_DSYNC, S_IWUSR);
#ifndef USE_SENDFILE 
		while(1) {
			n = read(fin, buf, sizeof(buf));
			if (n==0) break;
			m = write(fout, buf, n);
			if (n != m) {
				printf("fail to write, expect %d, actual %d\n", n, m);
				perror(":");
				return 1;
			}
		}
#else
		off_t offset = 0;
		struct stat st;
		if (fstat(fin, &st) != 0) {
			perror("fail to fstat\n");
			return 1;
		}
		sendfile(fout, fin, &offset, st.st_size);

#endif
		close(fin);
		close(fout);

	}
	return 0;
}

FYI
David