On 2017-02-06 16:06:25 -0500, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > On 2017-02-06 15:39:10 -0500, Peter Eisentraut wrote:
> >> On 2/6/17 6:28 AM, Konstantin Knizhnik wrote:
> >>> I wonder why do we prohibit now configuration of Postgres without mmap?
>
> >> It's not really prohibited, but it's not something that people generally
> >> need, and we want to keep the number of configuration variations low.
>
> > I think that was a fairly bad call. Making it hard to use anything but
> > mmap (on mmap supporting platforms) caused a fair bit of trouble and
> > performance regressions on several platforms by now (freebsd reported it
> > fairly quickly, and now aix), all to avoid a trivial amount of code and
> > one guc.
>
> > FWIW, there's a patch somewhere in the archive making it configurable.
>
> Clearly we should do something, but I'm not sure that a GUC is the right
> answer; far too few people would set it correctly. I think it might be
> better to have the per-platform "template" files decide whether to set
> USE_ANONYMOUS_SHMEM or not.
Well, sysv shmem will be less "comfortable" to use on those platforms
too. And you'll usually only hit the performance problems on bigger
installations. I don't think it'll be an improvement if after an upgrade
postgres doesn't work anymore because people have gotten used to not
having to configure sys shmem.
I suspect a better solution would be to have a list GUC with a platform
dependant default (i.e. sysv, anonymous on freebsd/aix; the other way
round on linux). At startup we'd then try those in order.
Regards,
Andres