Subject: Re: pgbench results arent accurate
To: pgsql-performance@lists.postgresql.org
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From: Mark Kirkwood <mark.kirkwood@catalyst.net.nz>
Message-ID: <166c284a-6e06-8e8b-3b96-e292d3aac781@catalyst.net.nz>
Date: Sat, 15 Dec 2018 12:46:20 +1300
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If you have not amended any Postgres config parameters, then you'll get 
checkpoints approx every 5 min or so. Thus using a Pgbench run time of 
5min is going sometimes miss/sometimes hit a checkpoint in progress - 
which will hugely impact test results.

I tend to do Pgbench runs of about 2x checkpoint_timeout - (i.e 10 min 
for default configurations). Also for increased repeatability, I do a 
manually triggered checkpoint immediately before each run.

regards

Mark

On 13/12/18 1:53 AM, Mariel Cherkassky wrote:
> Hey,
> I installed a new postgres 9.6 on both of my machines. I'm trying to 
> measure the differences between the performances in each machine but 
> it seems that the results arent accurate.
> I did 2 tests :
>
> 1)In the first test the scale was set to 100 :
> pgbench -i -s 100 -U postgres -d bench -h machine_name
> pgbench -U postgres -d bench  -h machine_name -j 2 -c 16 -T 300
> RUN 	   TPS - machine1 	TPS-machine2
> 1 	697 	555
> 2 	732 	861
> 3 	784 	842
>
> 	
> 	
>
>
> 2)In this test the scale was set to 10000 :
> pgbench -i -s 10000 -U postgres -d bench -h machine_name
> pgbench -U postgres -d bench --progress=30 -h machine_name -j 2 -c 16 
> -T 300
> RUN 	TPS-MACHINE1 	TPS-MACHINE2
> 1 	103 	60
> 2 	63 	66
> 3 	74 	83
> 4 	56 	61
> 5 	75 	53
> 6 	73 	60
> 7 	62 	53
>
>
> In both cases after the initalization I restarted the database and 
> cleared the cashe(echo 1 > /proc/sys/vm/drop_caches) one time. During 
> all the runs I didnt shutdown the machine.
>
> Now, I was hopping the the tps will be almost the same in each machine 
> for all the runs. In other words, I wanted to see that the tps in 
> machine1 during all the tps are almost the same but I see that the 
> values arent accurate.
>
> Any idea what might cause the differences in every run ?