Re: POC: Sharing record typmods between backends

On Wed, May 31, 2017 at 10:57 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> Simplehash provides an option to provide your own allocator function
>> to it. So in the allocator function, you can allocate memory from DSA.
>> After it reaches some threshold it expands the size (double) and it
>> will again call the allocator function to allocate the bigger memory.
>> You can refer pagetable_allocate in tidbitmap.c.
>
> That only allows the pagetable to be shared, not the hash table itself.

I agree with you. But, if I understand the use case correctly we need
to store the TupleDesc for the RECORD in shared hash so that it can be
shared across multiple processes. I think this can be achieved with
the simplehash as well.

For getting this done, we need some fixed shared memory for holding
static members of SH_TYPE and the process which creates the simplehash
will be responsible for copying these static members to the shared
location so that other processes can access the SH_TYPE. And, the
dynamic part (the actual hash entries) can be allocated using DSA by
registering SH_ALLOCATE function.

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com