Linux: Is writeback related to number of files? - c++

Question

I am trying to do some experiments of pagecache and writeback mechanism on linux 5.4.81. I have set dirty_ratio to be 50, dirty_background_ratio 45, dirty_expire_centisecs 500000000, dirty_writeback_centisecs 500000000. When I write a big file of 250GB using write(), I can find that writeback begin to work when dirty_pages reach 45% of total memory, which is in line with expectation.
But when I write 125 small files of 2GB, the writeback begin to work when dirty_pages only reach about 25%. I wonder why this happen?
The total memory of my platform is 256G.
#include <stdio.h>
#include <string>
#include <sys/time.h>
#include <sstream>
#include <fstream>
#include <iostream>
#include <fcntl.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/stat.h>
using namespace std;
int main()
{
struct timeval start, end;
long long total_time = 0;
ofstream outfile("write_time.txt",ios::trunc);
int loop=1;
while(loop--){
for(int i=0; i<125; i++){
stringstream ss;
string num;
ss<<i;
ss>>num;
string a = "test";
string b = ".dat";
string fileName = a+num+b;
const char*tmp=fileName.c_str();
int fp = open(tmp,O_CREAT|O_RDWR,S_IRUSR|S_IWUSR);
// int fp = open(tmp,O_WRONLY|O_TRUNC);
int pos = 0;
char data[1024] = "ab\n";
gettimeofday( &start, NULL );
while (1)
{
write(fp, data, 1024);
pos++;
if (pos >= 2*1024*1024)
break;
}
gettimeofday( &end, NULL );
int timeuse = 1000000 * ( end.tv_sec - start.tv_sec ) + end.tv_usec - start.tv_usec;
printf("write_time: %d us\n", timeuse);
total_time +=timeuse;
outfile<<timeuse<<endl;
}
}
outfile<<total_time;
outfile.close();
return 0;
}