[PATCH] memcg: doc the pagefault count

[PATCH] memcg: doc the pagefault count

[Wanlong Gao]

Pagefault count has added to memcg stats since commit 456f998e,
but forgot to doc it.

Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
---
 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
 		LRU list.
 active_anon	- # of bytes of anonymous and swap cache memory on active
-- 
1.7.8.rc0

Re: [PATCH] memcg: doc the pagefault count

[KAMEZAWA Hiroyuki]

On Fri, 4 Nov 2011 17:15:21 +0800
Wanlong Gao <gaowanlong@cn.fujitsu.com> wrote:

> Pagefault count has added to memcg stats since commit 456f998e,
> but forgot to doc it.
> 
> Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>

Thanks,
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

> ---
>  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.
> +

Re: [PATCH] memcg: doc the pagefault count

[Andrew Morton]

On Fri, 4 Nov 2011 17:15:21 +0800
Wanlong Gao <gaowanlong@cn.fujitsu.com> wrote:

> Pagefault count has added to memcg stats since commit 456f998e,
> but forgot to doc it.
> 
> Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
> ---
>  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
setups.


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?

Re: [PATCH] memcg: doc the pagefault count

[Ying Han]

On Wed, Nov 9, 2011 at 2:44 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Fri, 4 Nov 2011 17:15:21 +0800
> Wanlong Gao <gaowanlong@cn.fujitsu.com> wrote:
>
> > Pagefault count has added to memcg stats since commit 456f998e,
> > but forgot to doc it.
> >
> > Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
> > ---
> >  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!)

 Andrew,
I am afraid those two stats in memcg world have different perspective
than the ones in vmstat. They are only incremented/decremented during
charge/uncharge, which is regardless of having IO involved. The
existing documentation isn't that clear i agree. How about:

 pgpgin         - # of 4k page charging events to the memory cgroup
 pgpgout       - # of 4k page uncharging events to the memory cgroup

--Ying


>
> 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
> setups.
>
>
> 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?
>

Re: [PATCH] memcg: doc the pagefault count

[Andrew Morton]

On Wed, 9 Nov 2011 16:05:36 -0800
Ying Han <yinghan@google.com> wrote:

> On Wed, Nov 9, 2011 at 2:44 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > On Fri, 4 Nov 2011 17:15:21 +0800
> > Wanlong Gao <gaowanlong@cn.fujitsu.com> wrote:
> >
> > > Pagefault count has added to memcg stats since commit 456f998e,
> > > but forgot to doc it.
> > >
> > > Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
> > > ---
> > >  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!)
> 
>  Andrew,
> I am afraid those two stats in memcg world have different perspective
> than the ones in vmstat.

Wow.  Ouch.

> They are only incremented/decremented during
> charge/uncharge, which is regardless of having IO involved. The
> existing documentation isn't that clear i agree. How about:
> 
>  pgpgin         - # of 4k page charging events to the memory cgroup
>  pgpgout       - # of 4k page uncharging events to the memory cgroup

Did we tell readers what a "charging event" is?

What do these numbers represent if the machine is using 16k pages?

And what do pgfault/pgmajfault mean within memcg?  I now fear to ask -
given the pgpgin/pgpgout situation, these are probably related to my
shampoo viscosity or something :(