[realtek] What throughput can be expected

[Donald Becker]

On Mon, 7 Jan 2002, Jeff Garzik wrote:
> Donald Becker wrote:
> > The extra copying copy of packets to and from the memory used by the
> > rtl8139, and various overheads depending on the kernel version.  The 2.0
> > kernels were more efficient that the 2.2 and 2.4 kernels.
> 
> I do not see how this could possibly be true:  8139too in 2.4 kernels
> copies buffers half as much as other 8139 drivers / kernels.  See
> skb_copy_and_csum_dev and its use.  A P166 should fare far better on 2.4
> than either kernels due to the dramatically lowered packet buffer copy
> costs.

You parsed that differently than it was written.  The comma was
intentional: the 2.0 kernel has lower protocol processing overhead than
the 2.2 and 2.4 kernels.  This is independent of the driver data copy
path.

The rtl8139 driver has always used eth_copy_and_sum() in the Rx path.
It is unique in being the only driver that takes maximum advantage of
this function -- other PCI NICs are descriptor-based and don't require
the extra Rx copy.

Computing the Tx checksum in the copy-align step is a less-important issue.
The Tx data is usually fresh in the cache when the checksum is computed,
and thus the operation is usually inexpensive.

Donald Becker				becker@scyld.com
Scyld Computing Corporation		http://www.scyld.com
410 Severn Ave. Suite 210		Second Generation Beowulf Clusters
Annapolis MD 21403			410-990-9993