On Wednesday 28 February 2007 14:37, John wrote: > John wrote: > > I know it's possible to have Linux timestamp incoming datagrams as soon > > as they are received, then for one to retrieve this timestamp later with > > an ioctl command or a recvmsg call. > > Has it ever been proposed to modify struct skb_timeval to hold > nanosecond stamps instead of just microsecond stamps? Then make the > improved precision somehow available to user space. John, Most modern NICS are able to delay packet delivery, in order to reduce number of interrupts and benefit from better cache hits. tg3 for example are able to delay up to 1024 us. Then kernel is not realtime and some delays can occur between the hardware interrupt and the very moment we timestamp the packet. If CPU caches are cold, even the instruction fetches could easily add some us. Enabling nanosecond stamps would be a lie to users, because real accuracy is not nanosecond, but in the order of 10 us (at least) If you depend on a < 50 us precision, then linux might be the wrong OS for your application. Or maybe you need a NIC that is able to provide a timestamp in the packet itself (well... along with the packet...) , so that kernel latencies are not a problem. Eric - To unsubscribe from this list: send the line "unsubscribe linux-net" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
