Re: pgbench throttling latency limit

>> That model might make some sense if you think e.g. of a web application,
>> where the web server has a timeout for how long it waits to get a
>> database connection from a pool, but once a query is started, the
>> transaction is considered a succeess no matter how long it takes. The
>> latency limit would be that timeout. But I think a more useful model is
>> that when the user clicks a button, he waits at most X seconds for the
>> result. If that deadline is exceeded, the web server will give a 404, or
>> the user will simply get bored and go away, and the transaction is
>> considered a failure.
>
> Correct, the whole TPC-B model better fits an application where client
> requests enter a queue at the specified TPS rate and that queue is processed.
>
> While we are at it,
>
> Note that in the original TPC-B specification the transaction duration
> measured is the time from receiving the client request (in current pgbench
> under throttling that is for when the transaction is scheduled) and when the
> request is answered. This is the client visible response time, which has
> nothing to do with the database latency.

Ok. This correspond to the definition used in the current patch. However
ISTM that the tpc-b bench is "as fast as possible", there is no underlying
schedule as with the throttled pgbench.

> As per TPC-B, the entire test is only valid if 90% of all client response
> times are within 2 seconds.
>
> It would be useful if pgbench would
>
> A) measure and report that client response time in the per transaction
> log files and

I never used the per transaction log file. I think that it may already be
the case that it contains this information when not throttled. When under
throttling, I do not know.

> B) report at the end what percentage of transactions finished within
> a specified response time constraint (default 2 seconds).

What is currently reported is the complement (% of transactions completed
over the time limit).

Note that despite pg appaling latency performance, in may stay well over
the 90% limit, or even 100%: when things are going well a lot of
transaction run in about ms, while when things are going bad transactions
would take a long time (although possibly under or about 1s anyway), *but*
very few transactions are passed, the throughput is very small. The fact
that during 15 seconds only 30 transactions are processed is a detail that
does not show up in the metric.

--
Fabien.