Re: Regarding Postgres Dynamic Shared Memory (DSA)

On Thu, Jun 22, 2017 at 10:59 PM, Mahendranath Gurram
<mahendranath(at)zohocorp(dot)com> wrote:
> I'm implementing the In-Memory index as per your suggestion. So far it's
> good.

Great news.

> As of now, only one thing is unclear for me. How could i detach the
> dsa(dsa_detach() call) in backend (typically during backend quit).
>
> Of-course when process quits, all it's associated memory will be
> cleared/destroyed. But we have to decrement dsm reference counts as part of
> the potgres dsa framework implementation. which can not be done now.
>
> I have gone through the postgres source code. But unfortunately, i couldn't
> see anything in handling this case.

When you create or attach to a DSA area, a detach callback is
automatically registered on the control segment (or containing
segment, for an in-place DSA area). See the code like this in dsa.c:

/* Clean up when the control segment detaches. */
on_dsm_detach(segment, &dsa_on_dsm_detach_release_in_place,

PointerGetDatum(dsm_segment_address(segment)));

So the reference counting is automatically looked after and you don't
need to do anything. There are four possibilities: (1) you explicitly
detach from the area, (2) the ResourceManager active when you created
or attached goes out of scope, detaching you automatically (you
probably want to disable that with dsa_pin_mapping(area)), (3) your
backend exits normally and it's automatically detached (4) you crash
and the postmaster restarts the whole cluster on the basis that shared
memory could be arbitrarily corrupted.

Note that if you call dsa_pin(area) after creating the DSA area the
reference count cannot reach zero until you either call
dsa_unpin(area) or the cluster exits. That's the right thing to do
for a case where backends might come and go and it's possible for no
one to be attached for periods of time, but you want the DSA area to
continue to exist. I think that's probably what you need for a
DSA-backed in-memory index.

--
Thomas Munro
http://www.enterprisedb.com