Re: Retain dynamic shared memory segments for postmaster lifetime

On Sun, Jan 12, 2014 at 12:41 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> Currently there is no way user can keep the dsm
> segments if he wants for postmaster lifetime, so I
> have exposed a new API dsm_keep_segment()
> to implement the same.
>
> The specs and need for this API is already discussed
> in thread:
> http://www.postgresql.org/message-id/CA+TgmoaKoGuJQbEdGeYKYSXud9EAidqx77J2_HXzRgFo3Hr46A@mail.gmail.com

We have decided to bump reference count for segment
and call DuplicateHandle for Windows, but I think it should
also do what dsm_keep_mapping() does that is
ResourceOwnerForgetDSM(), else it will give
Warning: dynamic shared memory leak at transaction end.

> I had used dsm_demo (hacked it a bit) module used
> during initial tests for dsm API's to verify the working on
> Windows. So one idea could be that I can extend
> that module to use this new API, so that it can be tested
> by others as well or if you have any other better way, please
> do let me know.

I have extended test (contrib) module dsm_demo such that now user
can specify during dsm_demo_create the lifespan of segment.
The values it can accept are 0 or 1. Default value is 0.
0 -- means segment will be accessible for session life time
1 -- means segment will be accessible for postmaster life time

The behaviour is as below:
Test -1 (Session life time)
Session - 1
-- here it will create segment for session lifetime
select dsm_demo_create('this message is from session-1', 0);
dsm_demo_create
-----------------
827121111

Session - 2
-----------------
select dsm_demo_read(827121111);
dsm_demo_read
----------------------------
this message is from session-1
(1 row)

Session-1
\q

Session-2
postgres=# select dsm_demo_read(827121111);
dsm_demo_read
---------------

(1 row)

Conclusion of Test-1 : As soon as session which has created segment finished,
the segment becomes non-accessible.

Test -2 (Postmaster life time)
Session - 1
-- here it will create segment for postmaster lifetime
select dsm_demo_create('this message is from session-1', 1);
dsm_demo_create
-----------------
827121111

Session - 2
-----------------
select dsm_demo_read(827121111);
dsm_demo_read
----------------------------
this message is from session-1
(1 row)

Session-1
\q

Session-2
postgres=# select dsm_demo_read(827121111);
dsm_demo_read
---------------
this message is from session-1
(1 row)

Conclusion of Test-2 : a. Segment is accessible for postmaster lifetime.
b. if user restart server, segment is
not accessible.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com