On Mon, 3 Dec 2007, Lachlan Andrew wrote:

On 03/12/2007, Stephen Hemminger [off-list ref] wrote:

quoted

On Mon, 3 Dec 2007 15:59:23 -0800
"Shao Liu" [off-list ref] wrote:

quoted

And also a question, why the samples when SACK is active are outliers?

Any sample with SACK is going to mean a loss or reordering has occurred.
So shouldn't the SACK values be useful, but RTT values from retransmits
are not useful.

When SACK is active, the per-packet processing becomes more involved,
tracking the list of lost/SACKed packets.  This causes a CPU spike
just after a loss, which increases the RTTs, at least in my
experience.

I suspect that as long as old code was able to use hint, it wasn't doing 
that bad. But it was seriously lacking ability to take advantage of sack 
processing hint when e.g., a new hole appeared, or cumulative ACK arrived.

...Code available in net-2.6.25 might cure those.


-- 
 i.