High CPU usage - simple packet receiver on Linux - linux

Question

I'm writing simple application under Linux that gathers all packets from network. I'm using blocking receiving by calling "recvfrom()" function. When I generate big network load with hping3 (~100k raw frames per second, 130 bytes each) "top" tool shows high CPU usage for my process - it is about 37-38%. It is big value for me. When I decrease number of packets, usage is lower - for example top shows 3% for 4k frames per second.
I've check DC++ when it downloads ~10MB/s and its process doesn't use 38% of CPU but 5%. Is there any programmable way in C to reduce CPU usage and still receive a lot of frames?
My CPU:
Intel i5-2400 CPU # 3.10Ghz
My system:
Ubuntu 11.04 kernel 3.6.6 with PREEMPT-RT patch
And here is my code:
#include <stdlib.h>
#include <stdio.h>
#include <sys/mman.h>
#include <string.h>
#include <sys/socket.h>
#include <linux/if_packet.h>
#include <linux/if_ether.h>
#include <linux/if_arp.h>
#include <arpa/inet.h>
/* Socket descriptor. */
int mainSocket;
/* Buffer for frame. */
unsigned char* buffer;
int main(int argc, char* argv[])
{
/** Create socket. **/
mainSocket = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
if (mainSocket == -1) {
printf("Error: cannot create socket!\n");
}
/** Create buffer for frame **/
buffer = malloc(ETH_FRAME_LEN);
printf("Listing...");
while(1) {
// Length of received packet
int length = recvfrom(mainSocket, buffer, ETH_FRAME_LEN, 0, NULL, NULL);
if(length > 0)
{
// ... do something ...
}
}

Answer 1

I don't know if this will help, but looking on Google I see that:
Raw socket, Packet socket and Zero copy networking in Linux as well as http://lxr.linux.no/linux+v2.6.36/Documentation/networking/packet_mmap.txt talk about using PACKET_MMAP and mmap() to improve the performance of raw sockets
The Overview of Packet Reception suggests setting your process's affinity to match the CPU to which you bind the NIC using RPS.
Does DC++ do a promiscuous receive? I wouldn't have guessed so. So instead of comparing your performance to DC++, perhaps you should compare your performance to the performance of a utility like libpcap.

Answer 2

May be because, TCP/IP stack running on NIC and DC++ is getting stream of data directly from NIC, so your processor is not doing any TCP/IP work. But in your case I think you are directly trying to get data from NIC, so it will not be processed by NIC but by your processor, and as you have infinite loop to fetch data, you doing lot of processing... so CPU usage spiked.