Re: [dpdk-dev] [PATCH] mempool: fix mempool obj alignment for non x86

[Olivier Matz <olivier.matz@6wind.com>]

Hi,

On Sat, Dec 21, 2019 at 10:36:15AM +0530, Jerin Jacob wrote:
> On Sat, Dec 21, 2019 at 2:37 AM Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com> wrote:
> >
> > <snip>
> > > > > From: Jerin Jacob <jerinj@marvell.com>
> > > > >
> > > > > The exiting optimize_object_size() function address the memory
> > > > > object alignment constraint on x86 for better performance.
> > > > >
> > > > > Different (Mirco) architecture may have different memory alignment
> > > > > constraint for better performance and it not same as the existing
> > > > > optimize_object_size() function. Some use, XOR(kind of CRC) scheme
> > > > > to enable DRAM channel distribution based on the address and some
> > > > > may have a different formula.

typo: Mirco -> Micro
Maybe the whole sentence can be reworded a bit (I think a word is missing).

> > > > If I understand correctly, address interleaving is the characteristic of the
> > > memory controller and not the CPU.
> > > > For ex: different SoCs using the same Arm architecture might have different
> > > memory controllers. So, the solution should not be architecture specific, but
> > > SoC specific.
> > >
> > > Yes.  See below.
> > >
> > > > > -static unsigned optimize_object_size(unsigned obj_size)
> > > > > +static unsigned
> > > > > +arch_mem_object_align(unsigned obj_size)
> > > > >  {
> > > > >       unsigned nrank, nchan;
> > > > >       unsigned new_obj_size;
> > > > > @@ -99,6 +101,13 @@ static unsigned optimize_object_size(unsigned
> > > > > obj_size)
> > > > >               new_obj_size++;
> > > > >       return new_obj_size * RTE_MEMPOOL_ALIGN;  }
> > > > > +#else
> > > > This applies to add Arm (PPC as well) SoCs which might have different
> > > schemes depending on the memory controller. IMO, this should not be
> > > architecture specific.
> > >
> > > I agree in principle.
> > > I will summarize the
> > > https://www.mail-archive.com/dev@dpdk.org/msg149157.html feedback:
> > >
> > > 1) For x86 arch, it is architecture-specific
> > > 2) For power PC arch, It is architecture-specific
> > > 3) For the ARM case, it will be the memory controller specific.
> > > 4) For the ARM case, The memory controller is not using the existing
> > > x86 arch formula.
> > > 5) If it is memory/arch-specific, Can userspace code find the optimal
> > > alignment? In the case of octeontx2/arm64, the memory controller does  XOR
> > > on PA address which userspace code doesn't have much control.
> > >
> > > This patch address the known case of (1), (2),  (4) and (5). (2) can be added to
> > > this framework when POWER9 folks want it.
> > >
> > > We can extend this patch to address (3) if there is a case. Without the actual
> > > requirement(If some can share the formula of alignment which is the
> > > memory controller specific and it does not come under (4))) then we can
> > > create extra layer for the memory controller and abstraction to probe it.
> > > Again there is no standard way of probing the memory controller in
> > > userspace and we need platform #define, which won't work for distribution
> > > build.
> > > So solution needs to be arch-specific and then fine-tune to memory controller
> > > if possible.
> > >
> > > I can work on creating an extra layer of code if some can provide the details
> > > of the memory controller and probing mechanism or this patch be extended
> > Inputs for BlueField, DPAAx, ThunderX2 would be helpful.
> 
> Yes. Probably memory controller used in n1sdp SoC also.
> 
> >
> > > to support such case if it arises in future.
> > >
> > > Thoughts?
> > How much memory will this save for your platform? Is it affecting performance?

Currently, I think Arm-based architectures use the default (nchan=4,
nrank=1). The worst case is for object whose size (including mempool
header) is 2 cache lines, where it is optimized to 3 cache lines (+50%).

Examples for cache lines size = 64:
  orig     optimized
  64    -> 64           +0%
  128   -> 192          +50%
  192   -> 192          +0%
  256   -> 320          +25%
  320   -> 320          +0%
  384   -> 448          +16%
  ...
  2304  -> 2368         +2.7%  (~mbuf size)

> No performance difference.
> 
> The existing code adding the tailer for each objs.
> Additional space/Trailer space will be function of number of objects
> in mempool  and its obj_size, its alignment and selected
> rte_memory_get_nchannel() and rte_memory_get_nrank()
> 
> I will wait for inputs from Bluefield, DPAAx, ThunderX2 and n1sdp(if
> any) for any rework on the patch.

If there is no performance impact on other supporter Arm-based
architectures, I think it is a step in a right direction.

> > > > > +static unsigned
> > > > > +arch_mem_object_align(unsigned obj_size) {
> > > > > +     return obj_size;
> > > > > +}
> > > > > +#endif

I'd prefer "unsigned int" for new code.
Also, the opening brace should be on a separate line.

The documentation of the MEMPOOL_F_NO_SPREAD flag in the .h could be
slightly modified, as you did for the comment above
arch_mem_object_align().

> > > > >
> > > > >  struct pagesz_walk_arg {
> > > > >       int socket_id;
> > > > > @@ -234,8 +243,8 @@ rte_mempool_calc_obj_size(uint32_t elt_size,
> > > > > uint32_t flags,
> > > > >        */
> > > > >       if ((flags & MEMPOOL_F_NO_SPREAD) == 0) {
> > > > >               unsigned new_size;
> > > > > -             new_size = optimize_object_size(sz->header_size + sz-
> > > > > >elt_size +
> > > > > -                     sz->trailer_size);
> > > > > +             new_size = arch_mem_object_align
> > > > > +                         (sz->header_size + sz->elt_size +
> > > > > + sz->trailer_size);
> > > > >               sz->trailer_size = new_size - sz->header_size - sz->elt_size;
> > > > >       }
> > > > >
> > > > > --
> > > > > 2.24.1
> > > >