Re: pgbench cpu overhead (was Re: lazy vxid locks, v1)

Excerpts from Jeff Janes's message of lun jun 13 20:27:15 -0400 2011:
> On Mon, Jun 13, 2011 at 7:03 AM, Stefan Kaltenbrunner
> <stefan(at)kaltenbrunner(dot)cc> wrote:
> ...
> >
> >
> > so it seems that sysbench is actually significantly less overhead than
> > pgbench and the lower throughput at the higher conncurency seems to be
> > cause by sysbench being able to stress the backend even more than
> > pgbench can.
>
> Hi Stefan,
>
> pgbench sends each query (per connection) and waits for the reply
> before sending another.

I noticed that pgbench's doCustom (the function highest in the profile
posted) returns doing nothing if the connection is supposed to be
"sleeping"; seems an open door for busy waiting. I didn't check the
rest of the code to see if there's something avoiding that condition. I
also noticed that it seems to be very liberal about calling
INSTR_TIME_SET_CURRENT in the same function which perhaps could be
optimizing by calling it a single time at entry and reusing the value,
but I guess that would show up in the profile as a kernel call so it's
maybe not a problem.

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Álvaro Herrera <alvherre(at)commandprompt(dot)com>
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