Re: pgbench vs. wait events

On Thu, Oct 6, 2016 at 11:38 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> Hi,
>
> I decided to do some testing on hydra (IBM-provided community
> resource, POWER, 16 cores/64 threads, kernel 3.2.6-3.fc16.ppc64) using
> the newly-enhanced wait event stuff to try to get an idea of what
> we're waiting for during pgbench. I did 30-minute pgbench runs with
> various configurations, but all had max_connections = 200,
> shared_buffers = 8GB, maintenance_work_mem = 4GB, synchronous_commit =
> off, checkpoint_timeout = 15min, checkpoint_completion_target = 0.9,
> log_line_prefix = '%t [%p] ', max_wal_size = 40GB, log_checkpoints =
> on. During each run, I ran this psql script in another window and
> captured the output:
>
> \t
> select wait_event_type, wait_event from pg_stat_activity where pid !=
> pg_backend_pid()
> \watch 0.5
>
> Then, I used a little shell-scripting to count up the number of times
> each wait event occurred in the output. First, I tried scale factor
> 3000 with 32 clients and got these results:
>

Scale factor 3000 obviously doesn't fit in shared_buffers. But does it fit
in RAM? That is, are the backends doing real IO, or they just doing fake
IO to the kernel's fs cache?

Cheers,

Jeff