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SubjectRe: [PATCH] memcg: doc the pagefault count
On Fri, 4 Nov 2011 17:15:21 +0800
Wanlong Gao <> wrote:

> Pagefault count has added to memcg stats since commit 456f998e,
> but forgot to doc it.
> Signed-off-by: Wanlong Gao <>
> ---
> Documentation/cgroups/memory.txt | 7 +++++++
> 1 files changed, 7 insertions(+), 0 deletions(-)
> diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
> index cc0ebc5..081f4f44 100644
> --- a/Documentation/cgroups/memory.txt
> +++ b/Documentation/cgroups/memory.txt
> @@ -389,6 +389,13 @@ mapped_file - # of bytes of mapped file (includes tmpfs/shmem)
> pgpgin - # of pages paged in (equivalent to # of charging events).
> pgpgout - # of pages paged out (equivalent to # of uncharging events).
> swap - # of bytes of swap usage
> +
> +pgfault - # of page-faults
> +pgmajfault - # of major page-faults
> + These two are different from "pgpgin"/"pgpgout" stat which
> + count number of pages charged/discharged to the cgroup and
> + have no meaning of reading/writing page to disk.
> +
> inactive_anon - # of bytes of anonymous memory and swap cache memory on

OK, but the documentation was already rather poor.

For a start, what do pgpgin and pgpgout actually count? How about

pgpgin: the number of pages read from disk as a result of pagefault
activity within this memcg (swapin and mapped-file pagein). Does not
include other causes of disk I/O such as read().

pgpgout: the number of pages written to disk as a result of swap and
mapped-file activity within this memcg. Does not include other causes
of disk I/O such as write().

(I'm really not sure if this is correct!)

What are the units: is it "pages" or "multiples of 4k"? If the latter
then userspace must go off and work out what pagesize the kernel is,
which is a bit ugly and is just asking for people to develop
monitoring tools which report wrong results on other-than-4k-pagesize

Once we have that settled then we can move on to documenting pgfault
and pgmajfault. What are the units? Exactly which kernel actions lead
to these counters being altered?

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