Re: [HACKERS] Should we cacheline align PGXACT? - Mailing list pgsql-hackers

From Alexander Korotkov
Subject Re: [HACKERS] Should we cacheline align PGXACT?
Date
Msg-id CAPpHfdvoCVFQo_x9hthisdV-JfkK9_GjhSoRzck4nbScByW1Ng@mail.gmail.com
Whole thread Raw
In response to Re: [HACKERS] Should we cacheline align PGXACT?  (Alvaro Herrera <alvherre@2ndquadrant.com>)
Responses Re: [HACKERS] Should we cacheline align PGXACT?  (Alvaro Herrera <alvherre@2ndquadrant.com>)
List pgsql-hackers
On Mon, Feb 13, 2017 at 7:07 PM, Alvaro Herrera <alvherre@2ndquadrant.com> wrote:
Alexander Korotkov wrote:

> Yes, influence seems to be low.  But nevertheless it's important to insure
> that there is no regression here.
> Despite pg_prewarm'ing and running tests 300s, there is still significant
> variation.
> For instance, with clients count = 80:
>  * pgxact-result-2.txt – 474704
>  * pgxact-results.txt – 574844
> Could some background processes influence the tests?  Or could it be
> another virtual machine?
> Also, I wonder why I can't see this variation on the graphs.
> Another issue with graphs is that we can't see details of read and write
> TPS variation on the same scale, because write TPS values are too low.  I
> think you should draw write benchmark on the separate graph.

So, I'm reading that on PPC64 there is no effect, and on the "lesser"
machine Tomas tested on, there is no effect either; this patch only
seems to benefit Alexander's 72 core x86_64 machine.

It seems to me that Andres comments here were largely ignored:
https://www.postgresql.org/message-id/20160822021747.u5bqx2xwwjzac5u5@alap3.anarazel.de
He was suggesting to increase the struct size to 16 bytes rather than
going all the way up to 128.  Did anybody test this?

Thank you for pointing.  I'll provide such version of patch and test it on 72 core x86_64 machine. 
 
Re the coding of the padding computation, seems it'd be better to use
our standard "offsetof(last-struct-member) + sizeof(last-struct-member)"
rather than adding each of the members' sizes individually.

It was done so in order to evade extra level of nesting for PGXACT.  See discussion with Tom Lane in [1] and upthread.
Do you think we should introduce extra level of nesting or have better ideas about how to evade it?


------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company 

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