Instrumenting Redis to evaluate the efficiency of COW

gistfile1.c

/* 
This function makes use of /proc to evaluate the ratio between shared and private memory blocks.
It is expressed as a percentage: 90 means 90% of the blocks are shared, and 10% have been duplicated.
Call this function at the very end of the child process.
For Redis, it is supposed to be called at the end of rdbSave (file rdb.c)

Ugly but effective. 
Please note it does not work if huge pages are used
*/

void rdbDisplayCOW() {

    char smaps[256];
    char tmp[256];
    sprintf( smaps, "awk '/^Shared_Dirty:/ { x += $2 } /^Rss:/ { y += $2 } END { printf \"%%.3f\\n\", x/y*100 }' < /proc/%d/smaps", getpid() );
    FILE *f = popen( smaps, "r" );
    if ( !f )
       return;
    if ( fscanf( f, "%s", tmp ) != 1 ) {
        pclose( f );
        return;
    }
    pclose( f );
    redisLog(REDIS_NOTICE,"Background saving COW ratio: %s",tmp);
}