Re: [PATCH 1/2] rcfs core patch

Herbert Poetzl
2007-03-10T01:19:55Z

On Fri, Mar 09, 2007 at 11:27:07PM +0530, Srivatsa Vaddagiri wrote:
> On Fri, Mar 09, 2007 at 01:38:19AM +0100, Herbert Poetzl wrote:
> > > 2) you allow a task to selectively reshare namespaces/subsystems with
> > > another task, i.e. you can update current->task_proxy to point to
> > > a proxy that matches your existing task_proxy in some ways and the
> > > task_proxy of your destination in others. In that case a trivial
> > > implementation would be to allocate a new task_proxy and copy some
> > > pointers from the old task_proxy and some from the new. But then
> > > whenever a task moves between different groupings it acquires a
> > > new unique task_proxy. So moving a bunch of tasks between two
> > > groupings, they'd all end up with unique task_proxy objects with
> > > identical contents.

> > this is exactly what Linux-VServer does right now, and I'm
> > still not convinced that the nsproxy really buys us anything
> > compared to a number of different pointers to various spaces
> > (located in the task struct)

> Are you saying that the current scheme of storing pointers to
> different spaces (uts_ns, ipc_ns etc) in nsproxy doesn't buy
> anything?

> Or are you referring to storage of pointers to resource
> (name)spaces in nsproxy doesn't buy anything?

> In either case, doesn't it buy speed and storage space?

let's do a few examples here, just to illustrate the
advantages and disadvantages of nsproxy as separate
structure over nsproxy as part of the task_struct

1) typical setup, 100 guests as shell servers, 5
tasks each when unused, 10 tasks when used 10%
used in average

a) separate nsproxy, we need at least 100
structs to handle that (saves some space)

we might end up with ~500 nsproxies, if
the shell clones a new namespace (so might
not save that much space)

we do a single inc/dec when the nsproxy
is reused, but do the full N inc/dec when
we have to copy an nsproxy (might save
some refcounting)

we need to do the indirection step, from
task to nsproxy to space (and data)

b) we have ~600 tasks with 600 times the
nsproxy data (uses up some more space)

we have to do the full N inc/dev when
we create a new task (more refcounting)

we do not need to do the indirection, we
access spaces directly from the 'hot'
task struct (makes hot pathes quite fast)

so basically we trade a little more space and
overhead on task creation for having no
indirection to the data accessed quite often
throughout the tasks life (hopefully)

2) context migration: for whatever reason, we decide
to migrate a task into a subset (space mix) of a
context 1000 times

a) separate nsproxy, we need to create a new one
consisting of the 'new' mix, which will

- allocate the nsproxy struct
- inc refcounts to all copied spaces
- inc refcount nsproxy and assign to task
- dec refcount existing task nsproxy

after task completion
- dec nsproxy refcount
- dec refcounts for all spaces
- free up nsproxy struct

b) nsproxy data in task struct

- inc/dec refcounts to changed spaces

after task completion
- dec refcounts to spaces

so here we gain nothing with the nsproxy, unless
the chosen subset is identical to the one already
used, where we end up with a single refcount
instead of N

> > I'd prefer to do accounting (and limits) in a very simple
> > and especially performant way, and the reason for doing
> > so is quite simple:

> Can you elaborate on the relationship between data structures
> used to store those limits to the task_struct?

sure it is one to many, i.e. each task points to
exactly one context struct, while a context can
consist of zero, one or many tasks (no back-
pointers there)

> Does task_struct store pointers to those objects directly?

it contains a single pointer to the context struct,
and that contains (as a substruct) the accounting
and limit information

HTC,
Herbert

> --
> Regards,
> vatsa
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