procps and new kernel fields

procps and new kernel fields

[Dave Hansen]

There was a very brief conversation about this a year ago or so.  We've
got a ton of newfangled output in /proc/meminfo, but procps ignores most
of it.  There's also a bunch of types of memory that don't get shown by
the various procps commands these days.

	http://marc.info/?l=linux-mm&m=120496901605830&w=2

Novell has integrated that patch into procps which has the side-effect
that now reclaimable slab and unstable NFS pages are included in the
'cached' output of vmstat and free.  

	https://bugzilla.novell.com/show_bug.cgi?id=405246

The most worrisome side-effect of this change to me is that we can no
longer run vmstat or free on two machines and compare their output.

At the same time, we have machines that have dozens of GB of slab
objects that are mostly reclaimable.  Yet, 'free' and 'vmstat' basically
ignore slab.  Surely we need to find some way to report on those,
especially since we can now break out {un,}reclaimable slab.

We also have "new" memory use like unstable NFS pages.  How should we
account for those?

I'd love to see an --extended output from things like vmstat. It could
include wider output since fitting in 80 columns just isn't that
important any more, and my 256GB machine's output really screws up the
column alignment.  We could also add some information which is in
addition to what we already provide in order to account for things like
slab more precisely.

-- Dave

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Re: [feedback] procps and new kernel fields

[Albert Cahalan]

On Mon, Apr 6, 2009 at 5:55 PM, Dave Hansen <dave@linux.vnet.ibm.com> wrote:

> Novell has integrated that patch into procps
...
> The most worrisome side-effect of this change to me is that we can no
> longer run vmstat or free on two machines and compare their output.

Right. Vendors never consier that. They then expect upstream
to accept and support their hack until the end of time.

> At the same time, we have machines that have dozens of GB of slab
> objects that are mostly reclaimable.  Yet, 'free' and 'vmstat' basically
> ignore slab.  Surely we need to find some way to report on those,
> especially since we can now break out {un,}reclaimable slab.
>
> We also have "new" memory use like unstable NFS pages.  How should we
> account for those?

How should I know? :-)

I've seen memstat fields go away before. This makes me
reluctant to bother with data that few people will need or
even understand. Why should I believe that these fields
are now securely part of the kernel ABI?

> I'd love to see an --extended output from things like vmstat. It could
> include wider output since fitting in 80 columns just isn't that
> important any more, and my 256GB machine's output really screws up the
> column alignment.

80 is still the default xterm width, the default console
width, and sometimes the only available width.

I certainly like the idea of allowing options for extra columns
and for wider columns.

The current code is too simplistic to handle such changes
without becoming an unreadable mess. It really needs to be
rewritten in a more serious style, much like ps.

> We could also add some information which is in
> addition to what we already provide in order to account for things like
> slab more precisely.

How do I even explain a slab? What about a slob or slub?
A few years from now, will this allocator even exist?

Remember that I need something for the man page, and most
of my audience knows almost nothing about programming.

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Re: [feedback] procps and new kernel fields

[Bart Van Assche]

On Tue, Apr 7, 2009 at 6:40 AM, Albert Cahalan <acahalan@cs.uml.edu> wrote:
> On Mon, Apr 6, 2009 at 5:55 PM, Dave Hansen <dave@linux.vnet.ibm.com> wrote:
>> Novell has integrated that patch into procps
>...
>> The most worrisome side-effect of this change to me is that we can no
>> longer run vmstat or free on two machines and compare their output.
>
> Right. Vendors never consier that. They then expect upstream
> to accept and support their hack until the end of time.

The patch that was integrated by this vendor in their procps package
was posted on a public mailing list more than a year ago. It would
have helped if someone would have commented earlier on that patch.

>> We could also add some information which is in
>> addition to what we already provide in order to account for things like
>> slab more precisely.
>
> How do I even explain a slab? What about a slob or slub?
> A few years from now, will this allocator even exist?
>
> Remember that I need something for the man page, and most
> of my audience knows almost nothing about programming.

It's not the difference between SLAB, SLOB and SLUB that matters here,
but the fact that some of the memory allocated by these kernel
allocators can be reclaimed. The procps tools currently count
reclaimable SLAB / SLOB / SLUB memory as used memory, which is
misleading. How can this be explained to someone who is not a
programmer ?

Bart.

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Re: [feedback] procps and new kernel fields

[Dave Hansen]

On Tue, 2009-04-07 at 15:02 +0200, Bart Van Assche wrote:
> On Tue, Apr 7, 2009 at 6:40 AM, Albert Cahalan <acahalan@cs.uml.edu> wrote:
> > On Mon, Apr 6, 2009 at 5:55 PM, Dave Hansen <dave@linux.vnet.ibm.com> wrote:
> >> Novell has integrated that patch into procps
> >...
> >> The most worrisome side-effect of this change to me is that we can no
> >> longer run vmstat or free on two machines and compare their output.
> >
> > Right. Vendors never consier that. They then expect upstream
> > to accept and support their hack until the end of time.
> 
> The patch that was integrated by this vendor in their procps package
> was posted on a public mailing list more than a year ago. It would
> have helped if someone would have commented earlier on that patch.

We suck. :)

> >> We could also add some information which is in
> >> addition to what we already provide in order to account for things like
> >> slab more precisely.
> >
> > How do I even explain a slab? What about a slob or slub?
> > A few years from now, will this allocator even exist?
> >
> > Remember that I need something for the man page, and most
> > of my audience knows almost nothing about programming.
> 
> It's not the difference between SLAB, SLOB and SLUB that matters here,
> but the fact that some of the memory allocated by these kernel
> allocators can be reclaimed. The procps tools currently count
> reclaimable SLAB / SLOB / SLUB memory as used memory, which is
> misleading. How can this be explained to someone who is not a
> programmer ?

I actually think it is probably OK to call them "cache".  They *are*
quite similar to the page cache from a user's perspective.  I just have
a problem with changing it *now*, though.

Page cache is fundamentally user data.  It is verbatim in memory exactly
what came off or is going to the filesystem, and gets exposed to
userspace directly.

The various sl*bs are fundamentally kernel data.  They're never seen by
users directly.  

So, if I were to write a tool that told users of both slab and page
cache, I'd probably say "file cache" and "kernel cache" or something to
that effect.  It's a bit over-simplified, but I think it gets the point
across sufficiently.  Personally, I'd probably rather see 'buffers' get
collapsed in with 'cache' and get a new column for sl*b.

-- Dave

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