Re: Perfomance degradation 9.3 (vs 9.2) for FreeBSD

On 4/21/14, 9:52 AM, Alvaro Herrera wrote:
> Alfred Perlstein wrote:
>
>> I am unsure of the true overhead of making this a runtime tunable so
>> pardon if I'm asking for "a lot". From the perspective of both an
>> OS developer and postgresql user (I am both) it really makes more
>> sense to have it a runtime tunable for the following reasons:
>>
>> From an OS developer making this a runtime allows us to much more
>> easily do the testing (instead of needing two compiled versions).
>> From a sysadmin perspective it makes switching to/from a LOT easier
>> in case the new mmap code exposes a stability or performance bug.
> In this case, AFAICS the only overhead of a runtime option (what we call
> a GUC) is the added potential for user confusion, and the necessary
> documentation. If we instead go for a compile-time option, both items
> become smaller.
>
> In any case, I don't see that there's much need for a runtime option,
> really; you already know that the mmap code path is slower in FreeBSD.
> You only need to benchmark both options once the FreeBSD vm code itself
> is fixed, right?
>
> In fact, it might not even need to be a configure option; I would
> suggest a pg_config_manual.h setting instead, and perhaps tweaks to the
> src/template/freebsd file to enable it automatically on the "broken"
> FreeBSD releases. We could then, in the future, have the template
> itself turn the option off for the future FreeBSD release that fixes the
> problem.
>
That is correct, until you're in prod and suddenly one option becomes
unstable, or you want to try a quick kernel patch without rebooting.

Look, this is an argument I've lost time and time again in open source
software communities, the idea of a software option as opposed to
compile time really seems to hit people the wrong way.

I think from now on it just makes sense to sit back and let whatever
happens happen.

-Alfred