Understand V4L2 request buffer number - c

Question

I'm newbie in using V4L2 on embedded device and I have two questions related to V4L2 library usage:
First is the buffer allocation for v4l2_requestbuffers:
struct v4l2_requestbuffers req;
CLEAR(req);
req.count = 4;
req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
req.memory = V4L2_MEMORY_MMAP;
and later it is used by:
if (req.count < 2) {
fprintf(stderr, "Insufficient buffer memory on %s\n",
dev_name);
exit(EXIT_FAILURE);
}
buffers = calloc(req.count, sizeof(*buffers));
if (!buffers) {
fprintf(stderr, "Out of memory\n");
exit(EXIT_FAILURE);
}
for (n_buffers = 0; n_buffers < req.count; ++n_buffers) {
struct v4l2_buffer buf;
CLEAR(buf);
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP;
buf.index = n_buffers;
if (-1 == xioctl(fd, VIDIOC_QUERYBUF, &buf))
errno_exit("VIDIOC_QUERYBUF");
buffers[n_buffers].length = buf.length;
buffers[n_buffers].start =
mmap(NULL /* start anywhere */,
buf.length,
PROT_READ | PROT_WRITE /* required */,
MAP_SHARED /* recommended */,
fd, buf.m.offset);
if (MAP_FAILED == buffers[n_buffers].start)
errno_exit("mmap");
}
For capturing still image, req.count is 1. For capturing video, do we need to tune the req.count number to a number that we get good fps? That's mean we have to change the req.count from 2 (in this example) to a suitable number to get acceptable fps?
Second question is the timeout setting, as used in the mainloop:
static void mainloop(void)
{
unsigned int count;
count = frame_count;
while (count-- > 0) {
for (;;) {
fd_set fds;
struct timeval tv;
int r;
FD_ZERO(&fds);
FD_SET(fd, &fds);
/* Timeout. */
tv.tv_sec = 2;
tv.tv_usec = 0;
r = select(fd + 1, &fds, NULL, NULL, &tv);
if (-1 == r) {
if (EINTR == errno)
continue;
errno_exit("select");
}
if (0 == r) {
fprintf(stderr, "select timeout\n");
exit(EXIT_FAILURE);
}
if (read_frame())
break;
/* EAGAIN - continue select loop. */
}
}
}
If I set
/* Timeout. */
tv.tv_sec = 0;
tv.tv_usec = 0;
r = select(fd + 1, &fds, NULL, NULL, &tv);
My CPU load is 100%, I have to put it is 1 or 2 to make decrease the CPU load. If I remove timeout select() function, my CPU load is 100% again.
From my reading, the select() operation cost more CPU cycles but how can we remove select() and timeout setting in the mainloop()? Because when I look into other V4L2 capturing code from senior software engineer, I don't see they use timeout function as in the example above.
So my second question is how to avoid using timeout select() but still keep low CPU load .