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Re: [Xen-devel] [PATCH] Network Checksum Removal

Jon Mason wrote:

I'm surprised there's much benefit to csum offload on the tx side at all
as its almost always done as part of a copy.

Why?  The tx checksumming is just as expensive as the rx checksumming.

Normally (i.e. non sendfile() case), on the transmit side, you
have to copy the data from user space to kernel space, and
usually, during this step, you perform the checksum operation
for a few extra instructions - you have to take the hit of
pulling in each byte of data in any case.

So checksum offload on the transmit path _normally_ buys you
no throughput gain, and very slight reduction in CPU utilization.
Of course, for every segment sent out (or bunches thereof),
we get an ack back. But checksumming a TCP header (pure
ack case) is again, fairly trivial (20 bytes).


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