Reg: exact field for response time

[Geoff Simmons]

On 3/20/19 07:14, Hardik wrote:
> 
> what time should give to customer as a response time? Is it 2nd field(Total
> round trip time) or 3rd field(only response time after processed fetched
> content) ?
>        Timestamp Resp: 1501601912.806787* 0.048125* *0.000037*
> 
> Reason of asking this question is, Only first time request will go to
> origin. After that next all the request will be served from cache. Also
> generally we do not have control over other network (customer's network)
> when request goes to origin. As per this understanding, I should give 3rd
> field to customer as per response time. Please correct me if I am wrong.

The third field of Resp is not a useful measurement. It's the time taken
for Varnish userland code to complete network send operations -- when
the syscalls say they're done, there's essentially nothing left for
Varnish to do with the response, so Varnish writes the final timestamp.

But return from the syscalls for network send may mean nothing more
than: the data has been placed on queues in the TCP stack. It doesn't
tell you anything about the network send, or even if the data has been
sent on the network yet at all.

As a practical matter, if you tell your customer that "response time"
was 37 microseconds, they probably won't believe you. (I wouldn't.)

From what you've said, it sounds like you're looking for something like:
the time taken to process the request, but not including the time for a
fetch from the origin server. Is that about right?

For that, you'd need to do more than read one field from one Timestamp
entry -- you'll need to read at least two, maybe more, and then do some
arithmetic.

The best measure for the total time of request processing is the 2nd
field in Timestamp:Resp, 48ms in your example above.

The best measure for the fetch time is in the backend log, the 3rd field
of Timestamp:Beresp, maybe added to the 3rd field of
Timestamp:BerespBody. So you'd have to find the backend log
corresponding to client log. -g request can help you with that.

Then (if this is what you're after), subtract the fetch time from the
total request processing time.

There are some other ways to go about it, but it depends on what exactly
you want to measure as "response time". And since I may have
misunderstood what you're trying to measure, I'll stop there.


HTH,
Geoff
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