IP Networking

Hi,

Off late I had been looking at the differences b/w select() & poll() system calls. The requirement is to reduce the overhead, processor power in waiting for the data. In the kind of connections under consideration there would be very frequent data arriving on the sockets, so poll() fares well.

But!!! Even in the few millisec when the data is not arriving, the application can do something better! So will select() be a better function to go for?

In that case will there be any performance improvement at all?

Instead, is there a scope for some kind of asynchronous call back functions where the appliction will just be initimated in case data arrives.

On the whole, the requirement is that unnecessary waiting time should be cut & perfromance improved!

If this is not the right forum, can somebody suggest the right one.

Thanks in advance!!!

In many cases the choice is made for you.

If the system is BSD derived, then use select. In this case poll is merely a wrapper around select.

If the system is System V derived, then use poll. In this case select is merely a wrapper around poll.

The main advantage of poll is the number of file descriptors is not limited by the size of fd_set, other than that there is no real difference.

Just because you can specify the time in microseconds or nanoseconds does not mean the OS will honour those, they are still limited by the system/heartbeat clock frequency.

Hi,
I dont know which platform your are working on. But of the query,
I guess for solaris 10 (Sun OS 5.10), there is a new amenity called "Completion Ports". I wonder if this may be useful for you. I dont have much background on this feature. But on quick glance at it, I guess it is a type of asynchronous request handler.

Excuse me if the information is misleading/away from the topic...

Thanks
Srini

Hi,

Srinivasan: Thanks for the information. That was exactly something I was looking out for. Do you by any chance have any idea about how much performance improvement can be achieved by replacing poll with Asynchronous I/O framework.

Porter: How do I check if my system is BSD derived or V derived?

/smanu.

There are a number of ways but here we are trying to find out if select or poll is the more fundamental call. A suggestion is to look at sys/syscall.h and see if SYS_select or SYS_poll exist. If both exist then *maybe* both are native, but for instance not in the case of Darwin. In that case you could check to see which is the "earlier" call, ie if SYS_select is less than SYS_poll.

Ok! My syscall.h has got SYS_poll only.
So poll() seems to be the primitive one.

But could you tell me what are those other number of ways in which we can check if our system is BSD derived or V derived?

Off the top of my head the three main differences for a programmer are:

1. STREAMS

if has #include <sys/stream.h> it's SYS V

2. signals

if it has sigblock & sigsetmask it's BSD, if sighold & sigrelse it's SYS V

3. terminal handling

check prototype for "setpgrp()", if takes two args it's BSD


But as systems have all adopted BSD sockets and merging to standardisation on posix compatibility it is more important to check the actual feature you are trying to use.