Jumbo Packet vs Transmit Buffers

Question

I want to increase the transfer speed between two network adapters:

1) Atheros AR8152 PCI-E Fast Network Adapter.

2) Marvell Yukon 88E8053 PCI-E Gigabit Ethernet Controller.

Marvell has a "Jumbo Packet" size up to 9014 Bytes.

Atheros doesn't have anything related to "Jumbo Packet" but it has "Transmit Buffers" with a default value of 256.

If they are both the same, what "Transmit Buffers" value should be used to match Marvell's Bytes?

Thank you.

Solution

Jumbo frames and transmit buffers are different things.

A traditional ethernet frame has a data payload of 1500 bytes - so this is the largest individual unit of data that can be transmitted. Some additional headers get attached to this.

Jumbo frames allows this to be extended as you have seen to 9000 bytes (plus headers) in this case, but anything over 1500 would be considered a jumbo frame, though most of the time it is 9000 bytes. Putting more data into each packet means less overheads, and less CPU usage.

However, unless all devices that communicate with this machine have jumbo frames enabled, then it can actually have a negative effect on performance. The jumbo frame machine will try and send full size packets, and they will either be fragmented down to 1500 packets, or negotiated down to a size both ends can cope with. This takes time, and so can slow things down (a little).

Transmit buffers is basically a portion of memory put aside to hold data that is to be sent. This won't speed up data transfers much, as the wire speed is the limiting factor, but it can help with performance of applications where they don't have to wait for the network card to send the data before getting on with the next thing. Experimentation will help you find out whether it helps in your case.