[VDD] Client-side latency

[Dridi Boukelmoune]

Hello everyone,

I'd like to not put this item on the agenda for next Monday so I'll
briefly present it here since nobody at Varnish Software thinks it
will be controversial. Attendees outside of Varnish Software, please
ack that this should have a de-facto "send patches" green light or we
can discuss this further next Monday.

I have recently run into simple latency problems that were tricky to
observe in Varnish.

First, I ran into a case where we didn't have enough workers and
sessions were queued. The client would report response times that
wouldn't match what we observed in varnishlog because Timestamp
record don't account for queuing (and it becomes tricky with h2).

Second, we then realized that the client was also too eager on
pipelining and many requests would pile up a "long" time before
Varnish would process them. Once again everything looks good in
varnishlog.

Proposed feature:

A new %{Varnish:date}x variable in varnishncsa that converts
req.http.date or beresp.http.date into a Timestamp-like format. This
should help spot "significant" differences between what the peer
claims and what the Timestamp:Start or Timestamp:Beresp records
say.

Dridi