Subject: Re: Luca Deri's paper: Improving Passive Packet Capture: Beyond Device Polling

From: Luca Deri <[email-masked]>
Date: Tue, 06 Apr 2004 14:25:49 +0200

Hi all,
the problem with libpcap-mmap is that:
- it does not reduce the journey of the packet from NIC to userland 
beside the last part of it (syscall replaced with mmap). This has a 
negative impact on the overall performance.
- it does not feature things like kernel packet sampling that pushes 
people to fetch all the packets out of a NIC then discard most of them 
(i.e. CPU cycles not very well spent). Somehow this is a limitation of 
pcap that does not feature a pcap_sample call.

In addition if you do care about performance, I believe you're willing 
to turn off packet transmission and only do packet receive. 
Unfortunately I have no access to a "real" traffic generator (I use a PC 
as traffic generator). However if you read my paper you can see that 
with a Pentium IV 1.7 you can capture over 500'000 pkt/sec, so in your 
setup (Xeon + Spirent) you can have better figures.

IRQ: Linux has far too much latency, in particular at high speeds. I'm 
not the right person who can say "this is the way to go", however I 
believe that we need some sort of interrupt prioritization like RTIRQ does.

FYI, I've just polished the code and added kernel packet filtering to 
PF_RING. As soon as I have completed my tests I will release a new version.

Finally It would be nice to have in the standard Linux core some packet 
capture improvements. It could either be based on my work or on somebody 
else's work. It doesn't really matter as long as Linux gets faster.

Cheers, Luca

[email-masked] wrote:

> Jason Lunz wrote:
>
>> [email-masked] said:
>>
>>> Jason Lunz actually seemed to have been doing more work on this and
>>> e1000 - he could provide better perfomance numbers.
>>
>>
>>
>> Well, not really. What I have is still available at:
>>
>> http://gtf.org/lunz/linux/net/perf/
>>
>> ...but those are mainly measurements of very outdated versions of the
>> e1000 napi driver backported to 2.4, running on 1.8Ghz Xeon systems.
>> That work hasn't really been kept up to date, I'm afraid.
>>
>>
>>> It should also be noted that infact packet mmap already uses rings.
>>
>>
>>
>> Yes, I read the paper (but not his code). What stood out to me is that
>> the description of his custom socket implementation matches exactly what
>> packet-mmap already is.
>>
>> I noticed he only mentioned testing of libpcap-mmap, but did not use
>> mmap packet sockets directly -- maybe there's something about libpcap
>> that limits performance? I haven't looked.
>
>
> That's my experience. I'm thinking of redoing libpcap-mmap completely
> as it has huge amounts of statistics messing in the fast path.
> Also the ring gets corrupted if packets are being received while
> the ring buffer is being setup.
>
> I've a patch for http://public.lanl.gov/cpw/libpcap-0.8.030808.tar.gz
> here: http://www.pixelbeat.org/patches/libpcap-0.8.030808-pb.diff
> (you need to compile with PB defined)
> Note this only addresses the speed issue.
> Also there are newer versions of libpcap-mmap available which I
> haven't looked at yet.
>
>> What I can say for sure is that the napi + packet-mmap performance with
>> many small packets is almost surely limited by problems with irq/softirq
>> load. There was an excellent thread last week about this with Andrea
>> Arcangeli, Robert Olsson and others about the balancing of softirq and
>> userspace load; they eventually were beginning to agree that running
>> softirqs on return from hardirq and bh was a bigger load than expected
>> when there was lots of napi work to do. So despite NAPI, too much kernel
>> time is spent handling (soft)irq load with many small packets.
>
>
> agreed.
>
>> It appears this problem became worse in 2.6 with HZ=1000, because now
>> the napi rx softirq work is being done 10X as much on return from the
>> timer interrupt.  I'm not sure if a solution was reached.
>
>
> Pádraig
> .

-- 
Luca Deri <[email-masked]>	http://luca.ntop.org/
Hacker: someone who loves to program and enjoys being
clever about it - Richard Stallman