Re: Get memory contexts of an arbitrary backend process

Hi,

On Mon, Aug 31, 2020 at 8:22 PM torikoshia <torikoshia@oss.nttdata.com> wrote:
> As discussed in the thread[1], it'll be useful to make it
> possible to get the memory contexts of an arbitrary backend
> process.
+1

> Attached PoC patch makes pg_get_backend_memory_contexts()
> display meory contexts of the specified PID of the process.
Thanks, it's a very good patch for discussion.

> It doesn't display contexts of all the backends but only
> the contexts of specified process.
or we can  "SELECT (pg_get_backend_memory_contexts(pid)).* FROM
pg_stat_activity WHERE ...",
so I don't think it's a big deal.

> The rough idea of implementation is like below:
>
>    1. send  a signal to the specified process
>    2. signaled process dumps its memory contexts to a file
>    3. read the dumped file and display it to the user
I agree with the overview of the idea.
Here are some comments and questions.

- Currently, "the signal transmission for dumping memory information"
and "the read & output of dump information "
  are on the same interface, but I think it would be better to separate them.
  How about providing the following three types of functions for users?
  - send a signal to specified pid
  - check the status of the signal sent and received
  - read the dumped information
- How about managing the status of signal send/receive and dump
operations on a shared hash or others ?
  Sending and receiving signals, dumping memory information, and
referencing dump information all work asynchronously.
  Therefore, it would be good to have management information to check
the status of each process.
  A simple idea is that ..
   - send a signal to dump to a PID, it first record following
information into the shared hash.
     pid (specified pid)
     loc (dump location, currently might be ASAP)
     recv (did the pid process receive a signal? first false)
     dumped (did the pid process dump a mem information?  first false)
   - specified process receive the signal, update the status in the
shared hash, then dumped at specified location.
   - specified process finish dump mem information,  update the status
in the shared hash.
- Does it allow one process to output multiple dump files?
  It appears to be a specification to overwrite at present, but I
thought it would be good to be able to generate
  multiple dump files in different phases (e.g., planning phase and
execution phase) in the future.
- How is the dump file cleaned up?

Best regards,

--
Tatsuhito Kasahara
kasahara.tatsuhito _at_ gmail.com