Re: Creating a function for exposing memory usage of backend process

On 2020-08-22 21:18, Michael Paquier wrote:

Thanks for reviewing!

> On Fri, Aug 21, 2020 at 11:27:06PM +0900, torikoshia wrote:
>> OK. Added a regression test on sysviews.sql.
>> (0001-Added-a-regression-test-for-pg_backend_memory_contex.patch)
>> 
>> Fujii-san gave us an example, but I added more simple one considering
>> the simplicity of other tests on that.
> 
> What you have sent in 0001 looks fine to me.  A small test is much
> better than nothing.
> 
>> Added a patch for relocating the codes to mcxtfuncs.c.
>> (patches/0001-Rellocated-the-codes-for-pg_backend_memory_contexts-.patch)
> 
> The same code is moved around line-by-line.
> 
>> Of course, this restriction makes pg_backend_memory_contexts hard to 
>> use
>> when the user of the target session is not granted pg_monitor because 
>> the
>> scope of this view is session local.
>> 
>> In this case, I imagine additional operations something like 
>> temporarily
>> granting pg_monitor to that user.
> 
> Hmm.  I am not completely sure either that pg_monitor is the best fit
> here, because this view provides information about a bunch of internal
> structures.  Something that could easily be done though is to revoke
> the access from public, and then users could just set up GRANT
> permissions post-initdb, with pg_monitor as one possible choice.  This
> is the safest path by default, and this stuff is of a caliber similar
> to pg_shmem_allocations in terms of internal contents.

I think this is a better way than what I did in
0001-Rellocated-the-codes-for-pg_backend_memory_contexts-.patch.

Attached a patch.

> 
> It seems to me that you are missing one "REVOKE ALL on
> pg_backend_memory_contexts FROM PUBLIC" in patch 0003.
> 
> By the way, if that was just for me, I would remove used_bytes, which
> is just a computation from the total and free numbers.  I'll defer
> that point to Fujii-san.
> --
> Michael


On 2020/08/20 2:59, Kasahara Tatsuhito wrote:
>>> I totally agree that it's not *enough*, but in contrast to you I 
>>> think
>>> it's a good step. Subsequently we should add a way to get any 
>>> backends
>>> memory usage.
>>> It's not too hard to imagine how to serialize it in a way that can be
>>> easily deserialized by another backend. I am imagining something like
>>> sending a procsignal that triggers (probably at CFR() time) a backend 
>>> to
>>> write its own memory usage into pg_memusage/<pid> or something 
>>> roughly
>>> like that.
>> 
>> Sounds good. Maybe we can also provide the SQL-callable function
>> or view to read pg_memusage/<pid>, to make the analysis easier.
> +1


I'm thinking about starting a new thread to discuss exposing other
backend's memory context.


Regards,

--
Atsushi Torikoshi
NTT DATA CORPORATION