Re: [dpdk-dev] [PATCH] compressdev: implement API - mbuf alternative

["Trahe, Fiona" <fiona.trahe@intel.com>]

Hi Shally,

> -----Original Message-----
> From: Verma, Shally [mailto:Shally.Verma@cavium.com]
> Sent: Tuesday, March 13, 2018 8:15 AM
> To: Trahe, Fiona <fiona.trahe@intel.com>; Ahmed Mansour <ahmed.mansour@nxp.com>; dev@dpdk.org
> Cc: De Lara Guarch, Pablo <pablo.de.lara.guarch@intel.com>; Athreya, Narayana Prasad
> <NarayanaPrasad.Athreya@cavium.com>; Gupta, Ashish <Ashish.Gupta@cavium.com>; Sahu, Sunila
> <Sunila.Sahu@cavium.com>; Challa, Mahipal <Mahipal.Challa@cavium.com>; Jain, Deepak K
> <deepak.k.jain@intel.com>; Hemant Agrawal <hemant.agrawal@nxp.com>; Roy Pledge
> <roy.pledge@nxp.com>; Youri Querry <youri.querry_1@nxp.com>; fiona.trahe@gmail.com; Daly, Lee
> <lee.daly@intel.com>; Jozwiak, TomaszX <tomaszx.jozwiak@intel.com>
> Subject: RE: [dpdk-dev] [PATCH] compressdev: implement API - mbuf alternative
> 
> HI Fiona
> 
> So I understand we're moving away from mbufs because of its size limitation (64k) and cacheline overhead
> and their more suitability to n/w applications. Given that, I understand benefit of having another structure
> to input data but then what is proposal for ipcomp like application where mbuf usage may be a better
> option? Should we keep support for both (mbuf and this structure) so that apps can use appropriate data
> structure depending on their requirement.
[Fiona] An application can use pass buffers from an mbuf or mbuf chain to compressdev by filling in the 
compressdev struct fields with the mbuf meta-data, using rte_pktmbuf_data_len(), 
rte_pktmbuf_mtod(), rte_pktmbuf_mtophys(), etc
For simplicity I'd prefer to offer only 1 rather than 2 data formats on the API.
We see storage applications rather than IPComp as the main use-case for compressdev, so would prefer
to optimise for that.
Do you think otherwise?

> 
> Further comments, on github.
> 
> Thanks
> Shally
> 
> >-----Original Message-----
> >From: Trahe, Fiona [mailto:fiona.trahe@intel.com]
> >Sent: 12 March 2018 21:31
> >To: Ahmed Mansour <ahmed.mansour@nxp.com>; Verma, Shally <Shally.Verma@cavium.com>;
> dev@dpdk.org
> >Cc: De Lara Guarch, Pablo <pablo.de.lara.guarch@intel.com>; Athreya, Narayana Prasad
> <NarayanaPrasad.Athreya@cavium.com>;
> >Gupta, Ashish <Ashish.Gupta@cavium.com>; Sahu, Sunila <Sunila.Sahu@cavium.com>; Challa, Mahipal
> ><Mahipal.Challa@cavium.com>; Jain, Deepak K <deepak.k.jain@intel.com>; Hemant Agrawal
> <hemant.agrawal@nxp.com>; Roy
> >Pledge <roy.pledge@nxp.com>; Youri Querry <youri.querry_1@nxp.com>; fiona.trahe@gmail.com; Daly,
> Lee <lee.daly@intel.com>;
> >Jozwiak, TomaszX <tomaszx.jozwiak@intel.com>
> >Subject: RE: [dpdk-dev] [PATCH] compressdev: implement API - mbuf alternative
> >
> >Hi Shally, Ahmed, and anyone else interested in compressdev,
> >
> >I mentioned last week that we've been exploring using something other than mbufs to pass src/dst
> buffers to compressdev PMDs.
> >
> >Reasons:
> > - mbuf data is limited to 64k-1 in each segment of a chained mbuf. Data for compression
> >    can be greater and it would add cycles to have to break up into smaller segments.
> > - data may originate in mbufs, but is more likely, particularly for storage use-cases,  to
> >    originate in other data structures.
> > - There's a 2 cache-line overhead for every segment in a chain, most of this data
> >    is network-related, not needed by compressdev
> >So moving to a custom structure would minimise memory overhead, remove restriction on 64k-1 size and
> give more flexibility if
> >compressdev ever needs any comp-specific meta-data.
> >
> >We've come up with a compressdev-specific structure using the struct iovec from sys/uio.h, which is
> commonly used by storage
> >applications. This would replace the src and dest mbufs in the  op.
> >I'll not include the code here - Pablo will push that to github shortly and we'd appreciate review
> comments there.
> >https://github.com/pablodelara/dpdk-draft-compressdev
> >Just posting on the mailing list to give a heads-up and ensure this reaches a wider audience than may see
> it on github.
> >
> >Note : We also considered having no data structures in the op, instead the application
> >would supply a callback which the PMD would use to retrieve meta-data (virt address, iova, length)
> >for each next segment as needed. While this is quite flexible and allow the application
> >to keep its data in its native structures, it's likely to cost more cycles.
> >So we're not proposing this at the moment, but hope to benchmark it later while the API is still
> experimental.
> >
> >General feedback on direction is welcome here on the mailing list.
> >For feedback on the details of implementation we would appreciate comments on github.
> >
> >Regards,
> >Fiona.