Re: Adding column "mem_usage" to view pg_prepared_statements

From: Andres Freund <andres(at)anarazel(dot)de>
To: Daniel Migowski <dmigowski(at)ikoffice(dot)de>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "pgsql-hackers(at)lists(dot)postgresql(dot)org" <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Adding column "mem_usage" to view pg_prepared_statements
Date: 2019-08-05 17:16:08
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On 2019-07-28 06:20:40 +0000, Daniel Migowski wrote:
> how do you want to generalize it? Are you thinking about a view solely
> for the display of the memory usage of different objects?

I'm not quite sure. I'm just not sure that adding separate
infrastructure for various objects is a sutainable approach. We'd likely
want to have this for prepared statements, for cursors, for the current
statement, for various caches, ...

I think an approach would be to add an 'owning_object' field to memory
contexts, which has to point to a Node type if set. A table returning reporting
function could recursively walk through the memory contexts, starting at
TopMemoryContext. Whenever it encounters a context with owning_object
set, it prints a string version of nodeTag(owning_object). For some node
types it knows about (e.g. PreparedStatement, Portal, perhaps some of
the caches), it prints additional metadata specific to the type (so for
prepared statement it'd be something like 'prepared statement', '[name
of prepared statement]'), and prints information about the associated
context and all its children.

The general context information probably should be something like:
context_name, context_ident,
context_total_bytes, context_total_blocks, context_total_freespace, context_total_freechunks, context_total_used, context_total_children
context_self_bytes, context_self_blocks, context_self_freespace, context_self_freechunks, context_self_used, context_self_children,

It might make sense to have said function return a row for the contexts
it encounters that do not have an owner set too (that way we'd e.g. get
CacheMemoryContext handled), but then still recurse.

Arguably the proposed owning_object field would be a bit redundant with
the already existing ident/MemoryContextSetIdentifier field, which
e.g. already associates the query string with the contexts used for a
prepared statement. But I'm not convinced that's going to be enough
context in a lot of cases, because e.g. for prepared statements it could
be interesting to have access to both the prepared statement name, and
the statement.

The reason I like something like this is that we wouldn't add new
columns to a number of views, and lack views to associate such
information to for some objects. And it'd be disproportional to add all
the information to numerous places anyway.

One counter-argument is that it'd be more expensive to get information
specific to prepared statements (or other object types) that way. I'm
not sure I buy that that's a problem - this isn't something that's
likely going to be used at a high frequency. But if it becomes a
problem, we can add a function that starts that process at a distinct
memory context (e.g. a function that does this just for a single
prepared statement, identified by name) - but I'd not start there.

> While being interesting I still believe monitoring the mem usage of
> prepared statements is a bit more important than that of other objects
> because of how they change memory consumption of the server without
> using any DDL or configuration options and I am not aware of other
> objects with the same properties, or are there some? And for the other
> volatile objects like tables and indexes and their contents PostgreSQL
> already has it's information functions.

Plenty other objects have that property. E.g. cursors. And for the
catalog/relation/... caches it's even more pernicious - the client might
have closed all its "handles", but we still use memory (and it's
absolutely crucial for performance).


Andres Freund

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