Network speed mysteries

[Matthew Seaman]

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.

	Cheers,

	Matthew

-- 
Dr Matthew J Seaman MA, D.Phil.                       26 The Paddocks
                                                      Savill Way
PGP: http://www.infracaninophile.co.uk/pgpkey         Marlow
Tel: +44 1628 476614                                  Bucks., SL7 1TH UK
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