[dpdk-dev] [PATCH 07/13] mbuf: use macros only to access the mbuf metadata

[Richardson, Bruce]

> -----Original Message-----
> From: Ramia, Kannan Babu
> Sent: Tuesday, September 16, 2014 11:06 PM
> To: Dumitrescu, Cristian; Olivier MATZ; Richardson, Bruce; dev at dpdk.org
> Subject: RE: [dpdk-dev] [PATCH 07/13] mbuf: use macros only to access the
> mbuf metadata
> 
> I completely agree with Cristian here, instead of leaving to applications where to
> place their meta data, we can provide a guidance by having this field about
> placement of application meta while maintaining transparency on the contents
> of application meta information.
> 
My opinion on this is that this is better served via documentation or a comment in the code. The reason is that this approach is not going to be suitable for all applications. The mbuf headroom being used by the metadata is actually designed to be used for any additional headers to be added to the packet - though other things can obviously be stored in it too. Therefore the amount of metadata that can be stored in it will depend from application to application, as any apps doing e.g. tunnelling will need the headroom for tunnelling headers and may only be able to store a small amount of metadata - potentially none. For larger amounts of metadata - I would feel that anything over 64-bytes or so - I have proposed adding in a separate userdata pointer in the mbuf structure so that apps have the option of storing the metadata externally e.g. pointing to a flow table entry or similar. [Please see mbuf rework patch set 3 proposal].
Because of this, I think it's better to put in a comment in the code indicating that metadata can go in the headroom, document this properly - including caveats and limitations - in the documentation, and provide an example of doing such - something we already have in the packet framework.

All that being said, and while I think this is a good patch, I don't feel too strongly about it. I'm happy enough if this particular patch does not get merged in for 1.8, as it's incidental to the overall mbuf changes.

Regards,
/Bruce


> Regards
> Kannan Babu Ramia
> Sr.System Architect
> Communication Storage Infrastructure Group
> DCG
> -----Original Message-----
> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Dumitrescu, Cristian
> Sent: Wednesday, September 17, 2014 1:37 AM
> To: Olivier MATZ; Richardson, Bruce; dev at dpdk.org
> Subject: Re: [dpdk-dev] [PATCH 07/13] mbuf: use macros only to access the
> mbuf metadata
> 
> Hi Olivier,
> 
> I agree that your suggested approach for application-dependent metadata
> makes sense, in fact the two approaches work in exactly the same way (packet
> metadata immediately after the regular mbuf), there is only a subtle difference,
> which is related to defining consistent DPDK usage guidelines.
> 
> 1. Advertising the presence of application-dependent meta-data as supported
> mechanism If we explicitly have a metadata zero-size field at the end of the
> mbuf, we basically tell people that adding their own application meta-data at
> the end of the mandatory meta-data (mbuf structure) is a mechanism that DPDK
> allows and supports, and will continue to do so for the foreseeable future. In
> other words, we guarantee that an application doing so will continue to build
> successfully with future releases of DPDK, and we will not introduce changes in
> DPDK that could potentially break this mechanism. It is also a hint to people of
> where to put their application dependent meta-data.
> 
> 2. Defining a standard base address for the application-dependent metadata
> - There are also libraries in DPDK that work with application dependent meta-
> data, currently these are the Packet Framework libraries: librte_port,
> librte_table, librte_pipeline. Of course, the library does not have the knowledge
> of the application dependent meta-data format, so they treat it as opaque array
> of bytes, with the offset and size of the array given as arguments. In my opinion,
> it is safer (and more elegant) if these libraries (and others) can rely on an mbuf
> API to access the application dependent meta-data (in an opaque way) rather
> than make an assumption about the mbuf (i.e. the location of custom metadata
> relative to the mbuf) that is not clearly supported/defined by the mbuf library.
> - By having this API, we basically say: we define the custom meta-data base
> address (first location where custom metadata _could_ be placed) immediately
> after the mbuf, so libraries and apps accessing custom meta-data should do so
> by using a relative offset from this base rather than each application defining its
> own base: immediately after mbuf, or 128 bytes after mbuf, or 64 bytes before
> the end of the buffer, or other.
> 
> More (minor) comments inline below.
> 
> Thanks,
> Cristian
> 
> -----Original Message-----
> From: Olivier MATZ [mailto:olivier.matz at 6wind.com]
> Sent: Friday, September 12, 2014 10:02 PM
> To: Dumitrescu, Cristian; Richardson, Bruce; dev at dpdk.org
> Subject: Re: [dpdk-dev] [PATCH 07/13] mbuf: use macros only to access the
> mbuf metadata
> 
> Hello Cristian,
> 
> > What is the reason to remove this field? Please explain the rationale
> > of removing this field.
> 
> The rationale is explained in
> http://dpdk.org/ml/archives/dev/2014-September/005232.html
> 
> "The format of the metadata is up to the application".
> 
> The type of data the application stores after the mbuf has not to be defined in
> the mbuf. These macros limits the types of metadata to uint8, uint16, uint32,
> uint64? What should I do if I need a void*, a struct foo ? Should we add a macro
> for each possible type?
> 
> [Cristian] Actually, this is not correct, as macros to access metadata through
> pointers (to void or scalar types) are provided as well. This pointer can be
> converted by the application to the format is defines.
> 
> > We previously agreed we need to provide an easy and standard mechanism
> > for applications to extend the mandatory per buffer metadata (mbuf)
> > with optional application-dependent metadata.
> 
> Defining a structure in the application which does not pollute the rte_mbuf
> structure is "easy and standard(TM)" too.
> 
> [Cristian] I agree, both approaches work the same really, it is just the difference
> in advertising the presence of meta-data as supported mechanism and defining a
> standard base address for it.
> 
> > This field just provides a clean way for the apps to know where is the
> > end of the mandatory metadata, i.e. the first location in the packet
> > buffer where the app can add its own metadata (of course, the app has
> > to manage the headroom space before the first byte of packet data). A
> > zero-size field is the standard mechanism that DPDK uses extensively
> > in pretty much every library to access memory immediately after a
> > header structure.
> 
> Having the following is clean too:
> 
> struct metadata {
>      ...
> };
> 
> struct app_mbuf {
>      struct rte_mbuf mbuf;
>      struct metadata metadata;
> };
> 
> There is no need to define anything in the mbuf structure.
> 
> [Cristian] I agree, both approaches work the same really, it is just the difference
> in advertising the presence of meta-data as supported mechanism and defining a
> standard base address for it.
> 
> >
> > The impact of removing this field is that there is no standard way to
> > identify where the end of the mandatory metadata is, so each
> > application will have to reinvent this. With no clear convention, we
> > will end up with a lot of non-standard ways. Every time the format of
> > the mbuf structure is going to be changed, this can potentially break
> > applications that use custom metadata, while using this simple
> > standard mechanism would prevent this. So why remove this?
> 
> Waow. Five occurences of "standard" until now.
> [Cristian] I am sorry :)
> 
> Could you give a
> reference to the standard you're refering to? :)
> 
> [Cristian] See the IEFT Service Function Chaining link below, the environment is
> different (data center pipeline vs. CPU core-level pipeline), but the concepts are
> very similar.
> 
> Our application defines private metadata in mbufs in the way described above,
> we never changed that since we're supporting the dpdk. So I don't understand
> when you say that each time mbuf is reformatted it breaks the application.
> 
> > Having applications define their optional meta-data is a real need.
> 
> Sure. This patch does not prevent this at all. You can continue to do exactly the
> same, but in the concerned application, not in the generic mbuf structure.
> 
> 
> > Please take a look at the Service Chaining IEFT emerging protocols
> > (https://datatracker.ietf.org/wg/sfc/documents/), which provide
> > standard mechanisms for applications to define their own
> 
> Six :)
> 
> I'm not sure these documents define the way to extend a packet structure with
> metadata in a C program. Again, Bruce's patch does not prevent to do what you
> need, it just moves it at the proper place.
> 
> > packet meta-data and share it between the elements of the processing
> > pipeline (for Service Chaining, these are typically virtual machines
> > scattered amongst the data center).
> >
> > And, in my opinion, there is no negative impact/cost associated with
> > keeping this field.
> 
> To summarize what I think:
> 
> - this patch does not prevent to do what you want to do
> - removing the macros help to have a shorter and more comprehensible
>   mbuf structure
> - the previous approach does not scale because it would require
>   a macro per type
> 
> 
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> 
> Regards,
> Olivier
> 
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