Re: Get memory contexts of an arbitrary backend process

On 2021/03/26 12:26, Fujii Masao wrote:
> 
> 
> On 2021/03/26 12:01, Kyotaro Horiguchi wrote:
>> At Thu, 25 Mar 2021 23:45:17 +0900, torikoshia <torikoshia@oss.nttdata.com> wrote in
>>> Attached new one.
> 
> Regarding the argument max_children, isn't it better to set its default value,
> e.g., 100 like MemoryContextStats() uses?

+    mcxtLogData->maxChildrenPerContext = max_children;
+
+    if(!SendProcSignal(pid, PROCSIG_LOG_MEMORY_CONTEXT, proc->backendId))
+        PG_RETURN_BOOL(true);

Do we need memory barrier here? Otherwise, there is a race condition
where maxChildrenPerContext has not been set yet when the target backend
that received signal read that shared variable. No?

+        Note that when multiple
+        <function>pg_get_backend_memory_contexts</function> called in
+        succession or simultaneously, <parameter>max_children</parameter> can
+        be the one of another
+        <function>pg_get_backend_memory_contexts</function>.
+       </para></entry>

This might not be an issue in practice as Horiguchi-san said upthread.
But this looks a bit ugly and might be footgun later. The current approach
using shared memory to pass this information to the target backends
might be overkill, and too complicated to polish the design at the stage
just before feature freeze. So I'd withdraw my previous comment and
am OK to use the hard-coded value as max_children at the first version
of this feature. Thought?

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION