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jumbo frames are slower, why?

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Hi, I connected a D-Link DGE-530T on both my workstation (A64 3000+ - MSI K8T Neo FISR2 - OCZ 1GB DDR - Raptor HD - win XP) and my server (PIII 866 - Asus CUSL2C - Crucial 256 SDRAM - 200 GB Seagate 8mb HD - win 2k Adv Server).

When I enable jumbo frames, transfers between computers are slower (using a crossover cable 5e).

Jumbo frames ON:

CPU utilization = ~50%
MAX transfer speed = 23.33 MBs
AVG transfer speed = 15.33 MBs
Time to transfer 2 or so gigs (avg of 300 MBs each) = 3min 15 sec

----

Jumbo frames OFF:

CPU utilization = ~50%
MAX transfer speed = 22.23 MBs
AVG transfer speed = 20.33 MBs
Time to transfer 2 or so gigs (avg of 300 MBs each) = 2min 30 sec

I just purchased these NICs and have a SMC switch on the way 🙁, and i have no idea what to do 🙁

please help, i'm confused
 
This is just a guess, but do you maybe need to increase your MTU on the NICs? If its set to 1500 and the Jumbo Frames are 9000 bytes or so, it will need to fragment the frames before it sends it, causing the slower transfer rates.

Look at the DrTCP utility to tweak your MTU settings: http://www.dslreports.com/drtcp

-Ryan


EDIT: After i thought about it more, i'm not sure what i said above really makes sense, but you still might want to try playing with the MTU settings and stuff to see how it effects your transfer rates.
 
i tried increasing the MTU to 9000, it doesn't seem to be makign a difference 🙁
 
MTU is the maximum size that can be placed in a data frame without fragmentation. For 10 or 100BaseT Ethernet, the MTU is 1500. Gigabit Ethernet supports jumbo frames which allow you to use a MTU of 9000. This means that you get 1/6th the frame overhead, leading to more efficient communications at layer 2. This should also mean less CPU utilization as there are 1/6th as many interrupts to handle a frame and a lot less time is spent re-assembling frames to deliver to L3.

So, to answer your question Kamper, regular Ethernet is 1500 byte MTU - Gig is 9000 byte MTU.

- G
 
Originally posted by: Garion
MTU is the maximum size that can be placed in a data frame without fragmentation. For 10 or 100BaseT Ethernet, the MTU is 1500. Gigabit Ethernet supports jumbo frames which allow you to use a MTU of 9000. This means that you get 1/6th the frame overhead, leading to more efficient communications at layer 2. This should also mean less CPU utilization as there are 1/6th as many interrupts to handle a frame and a lot less time is spent re-assembling frames to deliver to L3.

So, to answer your question Kamper, regular Ethernet is 1500 byte MTU - Gig is 9000 byte MTU.

- G
Click to expand...

i tried changing the MTU to 9000 with the application from dslreports (Look at the DrTCP utility to tweak your MTU settings: http://www.dslreports.com/drtcp - ryan) but it doesn't seem to be making a difference.
 
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