Re: track_planning causing performance regression

On 2020/06/29 18:56, Fujii Masao wrote:
> 
> 
> On 2020/06/29 18:53, Julien Rouhaud wrote:
>> On Mon, Jun 29, 2020 at 11:38 AM Fujii Masao
>> <masao.fujii@oss.nttdata.com> wrote:
>>>
>>>>> Your benchmark result seems to suggest that the cause of the problem is
>>>>> the contention of per-query spinlock in pgss_store(). Right?
>>>>> This lock contention is likely to happen when multiple sessions run
>>>>> the same queries.
>>>>>
>>>>> One idea to reduce that lock contention is to separate per-query spinlock
>>>>> into two; one is for planning, and the other is for execution. pgss_store()
>>>>> determines which lock to use based on the given "kind" argument.
>>>>> To make this idea work, also every pgss counters like shared_blks_hit
>>>>> need to be separated into two, i.e., for planning and execution.
>>>>
>>>> This can probably remove some overhead, but won't it eventually hit
>>>> the same issue when multiple connections try to plan the same query,
>>>> given the number of different queries and very low execution runtime?
>>>
>>> Yes. But maybe we can expect that the idea would improve
>>> the performance to the near same level as v12?
>>
>> A POC patch should be easy to do and see how much it solves this
>> problem.  However I'm not able to reproduce the issue, and IMHO unless
>> we specifically want to be able to distinguish planner-time counters
>> from execution-time counters, I'd prefer to disable track_planning by
>> default than going this way, so that users with a sane usage won't
>> have to suffer from a memory increase.
> 
> Agreed. +1 to change that default to off.

Attached patch does this.

I also add the following into the description about each *_plan_time column
in the docs. IMO this is helpful for users when they see that those columns
report zero by default and try to understand why.

(if <varname>pg_stat_statements.track_planning</varname> is enabled, otherwise zero)

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION