Re: Performance degradation in commit ac1d794

On Sun, May 29, 2016 at 1:40 AM, Noah Misch <noah(at)leadboat(dot)com> wrote:
> On Fri, Dec 25, 2015 at 08:08:15PM +0300, Васильев Дмитрий wrote:
>> I suddenly found commit ac1d794 gives up to 3 times performance degradation.
>>
>> I tried to run pgbench -s 1000 -j 48 -c 48 -S -M prepared on 70 CPU-core
>> machine:
>> commit ac1d794 gives me 363,474 tps
>> and previous commit a05dc4d gives me 956,146
>> and master( 3d0c50f ) with revert ac1d794 gives me 969,265
>
> [This is a generic notification.]
>
> The above-described topic is currently a PostgreSQL 9.6 open item. Robert,
> since you committed the patch believed to have created it, you own this open
> item. If some other commit is more relevant or if this does not belong as a
> 9.6 open item, please let us know. Otherwise, please observe the policy on
> open item ownership[1] and send a status update within 72 hours of this
> message. Include a date for your subsequent status update. Testers may
> discover new open items at any time, and I want to plan to get them all fixed
> well in advance of shipping 9.6rc1. Consequently, I will appreciate your
> efforts toward speedy resolution. Thanks.

So, the reason this is back on the open items list is that Mithun Cy
re-reported this problem in:

https://www.postgresql.org/message-id/CAD__OuhPmc6XH=wYRm_+Q657yQE88DakN4=Ybh2oveFasHkoeA@mail.gmail.com

When I saw that, I moved this from CLOSE_WAIT back to open. However,
subsequently, Ashutosh Sharma posted this, which suggests (not
conclusively) that in fact the problem has been fixed:

https://www.postgresql.org/message-id/CAE9k0PkFEhVq-Zg4MH0bZ-zt_oE5PAS6dAuxRCXwX9kEVWceag@mail.gmail.com

What I *think* is going on here is:

- ac1d794 lowered performance
- backend_flush_after with a non-zero default lowered performance with
a vengeance
- 98a64d0 repaired the damage done by ac1d794, or much of it, but
Mithun couldn't see it in his benchmarks because backend_flush_after>0
is so bad

That could be wrong, but I haven't seen any evidence that it's wrong.
So I'm inclined to say we should just move this open item back to the
CLOSE_WAIT list (adding a link to this email to explain why we did
so). Does that work for you?

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company