Re: Slow Connectivity via Gigabit
- From: Rick Jones <rick.jones2@xxxxxx>
- Date: Fri, 21 Jul 2006 00:17:23 GMT
Recently I upgraded to gigabit, however the increase in speed I got was
far less than I thought. Copying a 20G tar file using scp transfers at
around 6.5 mb/sec, which while greater than the 4.5 mb/sec I used to
get using 100 mbps connection, is far less than the 15-20 mb/sec I
though I'd get. The gigabit lights are displayed correctly on both PCI
cards, as well as the switch. I put in new 3 ft cat 6 cables for both
machines. There are other computers connected to the switch which are
running at 100 mbps, including a separate 100mbps card on comp1. MTU is
set to 7200 on both machines. Does anyone have any idea how to speed it
7200 byte MTU? Where did that come from? "Proper" JumboFrame support
in GbE is a 9000 byte MTU. What happens if you keep the MTU like that
FWIW, you now have experienced firsthand that changing just one
component of a multicomponent system may not have as large an effect
as the difference in the changed components themselves. File transfer
scp isn't "just" networking.
Things you need to investigate:
*) How much idle CPU did you have when transferring over 100BT? In
and of itself, gigabit ethernet does _nothing_ to make transfer any
easier on a host than 100BT. Just as 100BT did nothing in and of
itself to make transfer any easier on the host than 10BT. In
broadhandwaving terms, for "ethernet" links it takes just as many
CPU cycles to transfer a KB of data over 10BT as it does for 100BT,
or Gigabit, or even 10 Gigabit Ethernet.
Now, there can be differences in how NICs _implement_ those things
that will alter the level of effort required to transfer data, but
none of them are fundamental to the underlying "Ethernet" specs. Eg
ChecKsum Offload, Transport Segmentation Offload, non-standard MTUs.
So, if you were at 60ish percent CPU util for your 4.5 mb/s (did
you really mean to say 'B' for bytes instead of 'b' for bits?)
transfer, and the network were the bottleneck, it would stand to
reason that switching from 100BT to 1Gbit would not let you go more
than 50% faster than you did before.
*) How busy were your discs when you were transferring over 100BT?
The same sort of logic applies there.
*) What is the bandwidth-delay product of your connection? One limit
to the performance of a TCP connection (and IIRC scp uses TCP) is
the window size divided by the Round-Trip-Time (RTT) of the
connction. Going from 100BT to 1Gig does not mean the RTT will be
1/10th what it was before, so perhaps a larger TCP window is
required to achieve higher throughput over the Gigabit link. This
can be true even over a LAN.
*) What manner of I/O bus(ses) do you have in the system and what was
their level of utilization when transferring over 100BT?
If you want to see the network-level differences in isolation from the
rest of the components in the system you need to run a networking
benchmark - eg netperf - http://www.netperf.org/
firebug n, the idiot who tosses a lit cigarette out his car window
these opinions are mine, all mine; HP might not want them anyway... :)
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...
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