Add function to log the memory contexts of specified backend process

Enterprise / PostgreSQL - Fujii Masao [postgresql.org] - 6 April 2021 04:44 UTC

Commit 3e98c0bafb added pg_backend_memory_contexts view to display the memory contexts of the backend process. However its target process is limited to the backend that is accessing to the view. So this is not so convenient when investigating the local memory bloat of other backend process. To improve this situation, this commit adds pg_log_backend_memory_contexts() function that requests to log the memory contexts of the specified backend process.

This information can be also collected by calling MemoryContextStats(TopMemoryContext) via a debugger. But this technique cannot be used in some environments because no debugger is available there. So, pg_log_backend_memory_contexts() allows us to see the memory contexts of specified backend more easily.

Only superusers are allowed to request to log the memory contexts because allowing any users to issue this request at an unbounded rate would cause lots of log messages and which can lead to denial of service.

On receipt of the request, at the next CHECK_FOR_INTERRUPTS(), the target backend logs its memory contexts at LOG_SERVER_ONLY level, so that these memory contexts will appear in the server log but not be sent to the client. It logs one message per memory context. Because if it buffers all memory contexts into StringInfo to log them as one message, which may require the buffer to be enlarged very much and lead to OOM error since there can be a large number of memory contexts in a backend.

When a backend process is consuming huge memory, logging all its memory contexts might overrun available disk space. To prevent this, now this patch limits the number of child contexts to log per parent to 100. As with MemoryContextStats(), it supposes that practical cases where the log gets long will typically be huge numbers of siblings under the same parent context; while the additional debugging value from seeing details about individual siblings beyond 100 will not be large.

There was another proposed patch to add the function to return the memory contexts of specified backend as the result sets, instead of logging them, in the discussion. However that patch is not included in this commit because it had several issues to address.

Thanks to Tatsuhito Kasahara, Andres Freund, Tom Lane, Tomas Vondra, Michael Paquier, Kyotaro Horiguchi and Zhihong Yu for the discussion.

Bump catalog version.

Author: Atsushi Torikoshi

43620e3286 Add function to log the memory contexts of specified backend process.
doc/src/sgml/func.sgml | 52 ++++++++
src/backend/storage/ipc/procsignal.c | 4 +
src/backend/tcop/postgres.c | 3 +
src/backend/utils/adt/mcxtfuncs.c | 60 ++++++++-
src/backend/utils/init/globals.c | 1 +
src/backend/utils/mmgr/aset.c | 8 +-
src/backend/utils/mmgr/generation.c | 8 +-
src/backend/utils/mmgr/mcxt.c | 180 ++++++++++++++++++++++-----
src/backend/utils/mmgr/slab.c | 9 +-
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.dat | 6 +
src/include/miscadmin.h | 1 +
src/include/nodes/memnodes.h | 6 +-
src/include/storage/procsignal.h | 1 +
src/include/utils/memutils.h | 5 +-
src/test/regress/expected/misc_functions.out | 13 ++
src/test/regress/sql/misc_functions.sql | 9 ++
17 files changed, 320 insertions(+), 48 deletions(-)

Upstream: git.postgresql.org


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