Re: [dpdk-users] [dpdk-dev] time taken for allocation of mempool.

[Venumadhav Josyula <vjosyula@gmail.com>]

Hi Anatoly,

After using iova-mode=va, i see my ports are not getting detected ? I
thought it's working but I see following problem

what could be the problem?
i) I see allocation is faster
ii) But my ports are not getting detected
I take my word back that it entirely working..

Thanks,
Regards,
Venu

On Thu, 14 Nov 2019 at 15:27, Burakov, Anatoly <anatoly.burakov@intel.com>
wrote:

> On 14-Nov-19 9:50 AM, Venumadhav Josyula wrote:
> > Hi Anatoly,
> >
> > Thanks for quick response. We want to understand, if there will be
> > performance implications because of iova-mode being va. We want to
> > understand,  specifically in terms following
> >
> >   * cache misses
> >   * Branch misses etc
> >   * translation of va addr -> phy addr when packet is receieved
> >
>
> There will be no impact whatsoever. You mentioned that you were already
> using VFIO, so you were already making use of IOMMU*. Cache/branch
> misses are independent of IOVA layout, and translations are done by the
> hardware (in either IOVA as PA or IOVA as VA case - IOMMU doesn't care
> what you program it with, it still does the translation, even if it's a
> 1:1 IOVA-to-PA mapping), so there is nothing that can cause degradation.
>
> In fact, under some circumstances, using IOVA as VA mode can be used to
> get performance /gains/, because the code can take advantage of the fact
> that there are large IOVA-contiguous segments and no page-by-page
> allocations. Some drivers (IIRC octeontx mempool?) even refuse to work
> in IOVA as PA mode due to huge overheads of page-by-page buffer offset
> tracking.
>
> TL;DR you'll be fine :)
>
> * Using an IOMMU can /theoretically/ affect performance due to hardware
> IOVA->PA translation and IOTLB cache misses. In practice, i have never
> been able to observe /any/ effect whatsoever on performance when using
> IOMMU vs. without using IOMMU, so this appears to not be a concern /in
> practice/.
>
> >   Thanks and regards
> > Venu
> >
> > On Thu, 14 Nov 2019 at 15:14, Burakov, Anatoly
> > <anatoly.burakov@intel.com <mailto:anatoly.burakov@intel.com>> wrote:
> >
> >     On 13-Nov-19 9:01 PM, Venumadhav Josyula wrote:
> >      > Hi Anatoly,
> >      >
> >      > By default w/o specifying --iova-mode option is iova-mode=pa by
> >     default ?
> >      >
> >      > Thanks
> >      > Venu
> >      >
> >
> >     In 18.11, there is a very specific set of circumstances that will
> >     default to IOVA as VA mode. Future releases have become more
> >     aggressive,
> >     to the point of IOVA as VA mode being the default unless asked
> >     otherwise. So yes, it is highly likely that in your case, IOVA as PA
> is
> >     picked as the default.
> >
> >     --
> >     Thanks,
> >     Anatoly
> >
>
>
> --
> Thanks,
> Anatoly
>