Andrus wrote:
> Richard,
>
>> At a quick glance, the plans look the same to me. The overall costs are
>> certainly identical. That means whatever is affecting the query times it
>> isn't the query plan.
>>
>> So - what other activity is happening on this machine? Either other
>> queries are taking up noticeable resources, or some other process is (it
>> might be disk activity from checkpointing, logging some other
>> application).
>
> Thank you.
> This is dedicated server running only PostgreSql which serves approx 6
> point of sales at this time.
>
> Maybe those other clients make queries which invalidate lot of data
> loaded into server cache.
> In next time server must read it again from disk which causes those
> perfomance differences.
In addition to "top" below, you'll probably find "vmstat 5" useful.
> top output is currently:
>
> top - 13:13:10 up 22 days, 18:25, 1 user, load average: 0.19, 0.12, 0.19
> Tasks: 53 total, 2 running, 51 sleeping, 0 stopped, 0 zombie
> Cpu(s): 13.7% us, 2.0% sy, 0.0% ni, 78.3% id, 6.0% wa, 0.0% hi,
> 0.0% si
> Mem: 2075828k total, 2022808k used, 53020k free, 0k buffers
> Swap: 3911816k total, 88k used, 3911728k free, 1908536k cached
>
> PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
> 5382 postgres 15 0 144m 43m 40m S 15.0 2.2 0:00.45 postmaster
> 5358 postgres 15 0 152m 87m 75m S 0.3 4.3 0:00.97 postmaster
> 1 root 16 0 1480 508 444 S 0.0 0.0 0:01.35 init
Looks pretty quiet.
> in few seconds later:
>
> top - 13:14:01 up 22 days, 18:26, 1 user, load average: 1.72, 0.53, 0.32
> Tasks: 52 total, 2 running, 50 sleeping, 0 stopped, 0 zombie
> Cpu(s): 5.3% us, 3.0% sy, 0.0% ni, 0.0% id, 91.0% wa, 0.0% hi,
> 0.7% si
> Mem: 2075828k total, 2022692k used, 53136k free, 0k buffers
> Swap: 3911816k total, 88k used, 3911728k free, 1905028k cached
>
> PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
> 1179 postgres 18 0 155m 136m 122m D 6.7 6.7 1:32.52 postmaster
> 4748 postgres 15 0 145m 126m 122m D 1.3 6.2 0:14.38 postmaster
> 5358 postgres 16 0 160m 98m 81m D 0.7 4.9 0:01.21 postmaster
> 1 root 16 0 1480 508 444 S 0.0 0.0 0:01.35 init
Here you're stuck waiting for disks (91.0% wa). Check out vmstat and
iostat to see what's happening.
--
Richard Huxton
Archonet Ltd
