Re: [PATCH 1/2] rcfs core patch

[Herbert Poetzl <herbert@13thfloor.at>]

On Sun, Mar 11, 2007 at 11:36:04AM -0500, Serge E. Hallyn wrote:
> Quoting Herbert Poetzl (herbert@13thfloor.at):
> > 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
> 
> But you're forgetting the *common* case, which is hundreds or
> thousands of tasks with just one nsproxy. That's case for 
> which we have to optimize.

yes, I agree here, maybe we should do something
I suggested (and submitted a patch for some time
ago) and add some kind of accounting for the various
spaces (and the nsproxy) so that we can get a feeling
how many of them are there and how many create/destroy
cycles really happen ...

those things will definitely be accounted in the
Linux-VServer devel versions, don't know about OVZ

> When that case is no longer the common case, we can yank the 
> nsproxy.  As I keep saying, it *is* just an optimization.

yes, fine with me, just wanted to paint a picture ...

best,
Herbert

> -serge
> 
> > 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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