Re: pg_shmem_allocations view

On Mon, May 5, 2014 at 3:09 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Sun, May 4, 2014 at 7:50 AM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
>>> Thinking about this, I think it was a mistake to not add a 'name' field
>>> to dynamic shared memory's dsm_control_item.
>
>> Well, right now a dsm_control_item is 8 bytes. If we add a name field
>> of our usual 64 bytes, they'll each be 9 times bigger.
>
> And the controlled shared segment is likely to be how big exactly? It's
> probably not even possible for it to be smaller than a page size, 4K or
> so depending on the OS. I agree with Andres that a name would be a good
> idea; complaining about the space needed to hold it is penny-wise and
> pound-foolish.

The control segment is sized to support a number of dynamic shared
memory segments not exceeding 64 + 2 *MaxBackends. With default
settings, that currently works out to 288 segments, or 2306 bytes.
So, adding a 64-byte name to each of those structures would increase
the size from 2k to about 20k.

So, sure, that's not a lot of memory. But I'm still not convinced
that's it's very useful. What I think is going to happen is that (1)
most people won't be used dynamic shared memory at all, so they won't
have any use for this; (2) those people who do run an extension that
uses dynamic shared memory will most likely only be running one such
extension, so they won't need a name to know what the segments are
being used for; and (3) if and when we eventually get parallel query,
dynamic shared memory segments will be widely used, but a bunch of
segments that are all named "parallel_query" or "parallel_query.$PID"
isn't going to be too informative.

Now, all that having been said, I recognize that human-readable names
are a generally useful thing, so I'm not going to hold my breath until
I turn blue if other people really want this, and it may turn out to
be useful someday. But if anyone is curious whether I'm *confident*
that it will be useful someday: at this point, no.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company