[PATCH v1 0/1] Report perpage metadata information.

Sourav Panda
2023-09-13T17:30:07Z

Hi!

This patch adds a new per-node PageMetadata field to
/sys/devices/system/node/nodeN/meminfo and a global PageMetadata field
to /proc/meminfo. This information can be used by users to see how much
memory is being used by per-page metadata, which can vary depending on
build configuration, machine architecture, and system use.

Per-page metadata is the amount of memory that Linux needs in order to
manage memory at the page granularity. The majority of such memory is
used by "struct page" and "page_ext" data structures.

Background
----------

Kernel overhead observability is missing some of the largest
allocations during runtime, including vmemmap (struct pages) and
page_ext. This patch aims to address this problem by exporting a
new metric PageMetadata.

On the contrary, the kernel does provide observibility for boot memory
allocations. For example, the metric reserved_pages depicts the pages
allocated by the bootmem allocator. This can be simply calculated as
present_pages - managed_pages, which are both exported in /proc/zoneinfo.
The metric reserved_pages is primarily composed of struct pages and
page_ext.

What about the struct pages (allocated by bootmem allocator) that are
free'd during hugetlbfs allocations and then allocated by buddy-allocator
once hugtlbfs pages are free'd?

/proc/meminfo MemTotal changes: MemTotal does not include memblock
allocations but includes buddy allocations. However, during runtime
memblock allocations can be shifted into buddy allocations, and therefore
become part of MemTotal.

Once the struct pages get allocated by buddy allocator, we lose track of
these struct page allocations overhead accounting. Therefore, we must
export a new metric that we shall refer to as PageMetadata (exported by
node). This shall also comprise the struct page and page_ext allocations
made during runtime.

Results and analysis
--------------------

Memory model: Sparsemem-vmemmap
$ echo 1 > /proc/sys/vm/hugetlb_optimize_vmemmap

$ cat /proc/meminfo | grep MemTotal
MemTotal: 32918196 kB
$ cat /proc/meminfo | grep Meta
PageMetadata: 589824 kB
$ cat /sys/devices/system/node/node0/meminfo | grep Meta
Node 0 PageMetadata: 294912 kB
$ cat /sys/devices/system/node/node1/meminfo | grep Meta
Node 1 PageMetadata: 294912 kB

AFTER HUGTLBFS RESERVATION
$ echo 512 > /proc/sys/vm/nr_hugepages

$ cat /proc/meminfo | grep MemTotal

MemTotal: 32934580 kB
$ cat /proc/meminfo | grep Meta
PageMetadata: 575488 kB
$ cat /sys/devices/system/node/node0/meminfo | grep Meta
Node 0 PageMetadata: 287744 kB
$ cat /sys/devices/system/node/node1/meminfo | grep Meta
Node 1 PageMetadata: 287744 kB

AFTER FREEING HUGTLBFS RESERVATION
$ echo 0 > /proc/sys/vm/nr_hugepages
$ cat /proc/meminfo | grep MemTotal
MemTotal: 32934580 kB
$ cat /proc/meminfo | grep Meta
PageMetadata: 589824 kB
$ cat /sys/devices/system/node/node0/meminfo | grep Meta
Node 0 PageMetadata: 294912 kB
$ cat /sys/devices/system/node/node1/meminfo | grep Meta
Node 1 PageMetadata: 294912 kB

Sourav Panda (1):
mm: report per-page metadata information

Documentation/filesystems/proc.rst | 3 +++
drivers/base/node.c | 2 ++
fs/proc/meminfo.c | 7 +++++++
include/linux/mmzone.h | 3 +++
include/linux/vmstat.h | 4 ++++
mm/hugetlb.c | 8 +++++++-
mm/hugetlb_vmemmap.c | 9 ++++++++-
mm/mm_init.c | 3 +++
mm/page_alloc.c | 1 +
mm/page_ext.c | 17 +++++++++++++----
mm/sparse-vmemmap.c | 3 +++
mm/sparse.c | 7 ++++++-
mm/vmstat.c | 21 +++++++++++++++++++++
13 files changed, 81 insertions(+), 7 deletions(-)

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2.42.0.283.g2d96d420d3-goog