Network speed mysteries

[Andrew P.]

Matthew Seaman wrote:

>On Tue, Oct 26, 2004 at 11:46:02PM +0400, Andrew P. wrote:
>
>  
>
>>Just an hour ago I decided to rebuild the file-server kernel - and it 
>>takes time to build it there, as it's an old Celeron box with little 
>>RAM. By coincidence, some files were being uploaded just when I entered 
>>"make buildkernel ...". I looked at FileZilla windows, expecting to see 
>>the speed drop - but WOW! - the speed was at 7Mbytes/s!!! It then 
>>hovered around 6.5-9Mbytes/s while the kernel was being built! I waited 
>>for some minutes until the kernel was finally built - and the upload 
>>speed dropped back to 2.5-3.5Mb/s. I couldn't believe it - and I still 
>>can't - so I waited and built a kernel once more - with all the same 
>>effects on speed! It's worth to mention, that when I was installing the 
>>built kernel, the speed didn't change from usual 3Mb/s.
>>
>>Please let me know what the heck is going on - or just what you think 
>>about it.
>>    
>>
>
>So, when the system is under load, the network throughput goes up?
>That must be a timing issue to do with dealing with ACKs -- perhaps
>the FreeBSD box just responds too fast for the other end, and loading
>down the system delays things just long enough to get both ends into
>sync.
>
>What would be useful would be to use tcpdump(1) or ethereal(1) to
>capture a sample of the network traffic in either situation and bring
>this up on the freebsd-net at ... list.  Note that running tcpdump itself
>might affect the traffic in a similar way -- there's a thread on
>freebsd-net which you might find interesting, starting here:
>
>    http://lists.freebsd.org/pipermail/freebsd-net/2004-October/005368.html
>
>I you do post to freebsd-net, including the output of 'ifconfig -a'
>would be a good move.
>  
>
Thanks a lot! When I rebooted after installing my custom kernel, I was 
delighted to find out that the speed grew to 9.7Mb/s. That's another 
mystery, cuz I've switched from GENERIC kernel to one with ipfw, 
ipdivert and some other networking code. So I think I'll just leave it 
at that. Besides, I don't want to spoil my first impression. I just want 
to keep wondering - how building a kernel itself can speed up network 
thruoghput. Real magic :-)

Thanks again!

Best regards,
Andrew P.