[dpdk-dev] [PATCH 10/17] librte_acl: add AVX2 as new rte_acl_classify() method

[Neil Horman]

On Tue, Dec 16, 2014 at 04:16:48PM +0000, Ananyev, Konstantin wrote:
> 
> 
> > -----Original Message-----
> > From: Neil Horman [mailto:nhorman at tuxdriver.com]
> > Sent: Monday, December 15, 2014 8:21 PM
> > To: Ananyev, Konstantin
> > Cc: dev at dpdk.org
> > Subject: Re: [dpdk-dev] [PATCH 10/17] librte_acl: add AVX2 as new rte_acl_classify() method
> > 
> > On Mon, Dec 15, 2014 at 04:33:47PM +0000, Ananyev, Konstantin wrote:
> > > Hi Neil,
> > >
> > > > -----Original Message-----
> > > > From: Neil Horman [mailto:nhorman at tuxdriver.com]
> > > > Sent: Monday, December 15, 2014 4:00 PM
> > > > To: Ananyev, Konstantin
> > > > Cc: dev at dpdk.org
> > > > Subject: Re: [dpdk-dev] [PATCH 10/17] librte_acl: add AVX2 as new rte_acl_classify() method
> > > >
> > > > On Sun, Dec 14, 2014 at 06:10:52PM +0000, Konstantin Ananyev wrote:
> > > > > Introduce new classify() method that uses AVX2 instructions.
> > > > > From my measurements:
> > > > > On HSW boards when processing >= 16 packets per call,
> > > > > AVX2 method outperforms it's SSE counterpart by 10-25%,
> > > > > (depending on the ruleset).
> > > > > At runtime, this method is selected as default one on HW that supports AVX2.
> > > > >
> > > > > Signed-off-by: Konstantin Ananyev <konstantin.ananyev at intel.com>
> > > > > ---
> > > > >  lib/librte_acl/Makefile       |   9 +
> > > > >  lib/librte_acl/acl.h          |   4 +
> > > > >  lib/librte_acl/acl_run.h      |   2 +-
> > > > >  lib/librte_acl/acl_run_avx2.c |  58 +++++
> > > > >  lib/librte_acl/acl_run_avx2.h | 305 ++++++++++++++++++++++++
> > > > >  lib/librte_acl/acl_run_sse.c  | 537 +-----------------------------------------
> > > > >  lib/librte_acl/acl_run_sse.h  | 533 +++++++++++++++++++++++++++++++++++++++++
> > > > >  lib/librte_acl/rte_acl.c      |   5 +-
> > > > >  lib/librte_acl/rte_acl.h      |   2 +
> > > > >  9 files changed, 917 insertions(+), 538 deletions(-)
> > > > >  create mode 100644 lib/librte_acl/acl_run_avx2.c
> > > > >  create mode 100644 lib/librte_acl/acl_run_avx2.h
> > > > >  create mode 100644 lib/librte_acl/acl_run_sse.h
> > > > >
> > > > > diff --git a/lib/librte_acl/Makefile b/lib/librte_acl/Makefile
> > > > > index 65e566d..223ec31 100644
> > > > > --- a/lib/librte_acl/Makefile
> > > > > +++ b/lib/librte_acl/Makefile
> > > > > @@ -45,8 +45,17 @@ SRCS-$(CONFIG_RTE_LIBRTE_ACL) += acl_bld.c
> > > > >  SRCS-$(CONFIG_RTE_LIBRTE_ACL) += acl_gen.c
> > > > >  SRCS-$(CONFIG_RTE_LIBRTE_ACL) += acl_run_scalar.c
> > > > >  SRCS-$(CONFIG_RTE_LIBRTE_ACL) += acl_run_sse.c
> > > > > +SRCS-$(CONFIG_RTE_LIBRTE_ACL) += acl_run_avx2.c
> > > > >
> > > > >  CFLAGS_acl_run_sse.o += -msse4.1
> > > > > +ifeq ($(CC), icc)
> > > > > +CFLAGS_acl_run_avx2.o += -march=core-avx2
> > > > > +else ifneq ($(shell \
> > > > > +test $(GCC_MAJOR_VERSION) -le 4 -a $(GCC_MINOR_VERSION) -le 6 && echo 1), 1)
> > > > > +CFLAGS_acl_run_avx2.o += -mavx2
> > > > > +else
> > > > > +CFLAGS_acl_run_avx2.o += -msse4.1
> > > > > +endif
> > > > >
> > > > This seems broken.  You've unilaterally included acl_run_avx2.c in the build
> > > > list above, but only enable -mavx2 if the compiler is at least gcc 4.6.
> > >
> > > Actually 4.7 (before that version, as I know,  gcc doesn't support avx2)
> > >
> > > >  Unless
> > > > you want to make gcc 4.6 a requirement for building,
> > >
> > > I believe DPDK is required to be buildable by gcc 4.6
> > > As I remember, we have to support it all way down to gcc 4.3.
> > >
> > > > you need to also exclude
> > > > the file above from the build list.
> > >
> > > That means that for  gcc 4.6 and below rte_acl_classify_avx2() would not be defined.
> > > And then at runtime, I have to check for that somehow and (re)populate classify_fns[].
> > > Doesn't seems like a good way to me.
> > There are plenty of ways around that.
> > 
> > At a minimum you could make the classify_fns array the one place that you need
> > to add an ifdef __AVX__ call.
> > 
> > You could also create a secondary definition of rte_acl_classify_avx2, and mark
> > it as a weak symbol, which only returns -EOPNOTSUPP.  That would be good, since
> > the right thing will just automatically happen then if you don't build the
> > actual avx2 classification code
> > 
> > > Instead, I prefer to always build acl_run_avx2.c,
> 
> 
> > But you can't do that.  You just said above that you need to support down to gcc
> > 4.3.  I see you've worked around that with some additional ifdef __AVX__
> > instructions, but in so doing you ignore the possibiity that sse isn't
> > supported, so you need to add __SSE__ checks now as well.  ifdeffing that much
> > just isn't scalable.
> 
> We don't need to worry about compiler without SSE4.1 support.
> I believe that all compilers that DDPDK has to build with, do support SSE4.1.
> So for SSE4.1 we only has to worry about situation when target CPU doesn't support it
> We manage it by runtime selection.
> For AVX2 - situation is a bit different: it could be both compiler and target CPU that don't support it.
> 
> >  And for your effort, you get an AVX2 classification path
> > that potentially doesn't actually do vectorized classification.
> > 
> > It really seems better to me to not build the code if the compiler doesn't
> > support the instruction set it was meant to enable, and change the
> > classification function pointer to something that informs the user of the lack
> > of support at run time.
> > 
> > > but for old compilers that don't support AVX2 -
> > > rte_acl_classify_avx2() would simply be identical to rte_acl_classify_sse().
> > >
> > That doesn't make sense to me, for two reasons:
> > 
> > 1) What if the machine being targeted doesn't support sse either?
> > 
> 
> Exactly the same what is happening now on the machine with now SSE4.1 support.
> There is absolutely no difference here.
> 
> > 2) If an application selects an AVX2 classifier, I as a developer expect to
> > either get AVX2 based classification, or an error indicating that I can't do
> > AVX2 classification, not a silent performance degradation down to scalar
> > classification.
> 
> In fact I was considering both variants for compilers not supporting AVX2:
> 1. silently degrade to SSE method.
> 2. create  a dummy function rte_acl_classify_error() and put it  into classify_fns[RTE_ACL_CLASSIFY_AVX2].
> 
> I choose #1 because it seems like a less distraction for the user -
> all would keep working as before, user just wouldn't see any improvement comparing to SSE method. 
> Again didn't want to spread "ifdef __AVX2__" into rte_acl.c
> Though I don't have any strong opinion here.
> So if you can provide some good reason why #2 is preferable, I am ok to switch to #2. 
> 
Because 2 doesn't require any ifdeffing.  As you note above the problem here is
that AVX2 support is both compiler and machine dependent.  If you make a weak
symbol version of rte_acl_classify_avx2 that always gets built, then you've
reduced the problem to just being compiler support, which you can check in the
makefile.

> > 
> > > >  That in turn I think allows you to remove a
> > > > bunch of the ifdeffing that you've done in some of the avx2 specific files.
> > >
> > > Actually there are not many of them.
> > > One in acl_run_avx2.h and another in acl_run_avx2.c.
> > >
> > 2 in acl_run_avx2.h and 1 in rte_acl_osdep_alone.h, which is really 3 more than
> > you need if you just do an intellegent weak classifier function defintion.
> 
> grep -n __AVX2__ lib/librte_acl/*.[c,h] | grep -v endif
> lib/librte_acl/acl_run_avx2.c:45:#ifdef __AVX2__
> lib/librte_acl/acl_run_avx2.h:36:#ifdef __AVX2__
> 
> rte_acl_osdep_alone.h - is a different story.
> It needs to be there anyway, as in rte_common_vect.h.
> In fact  rte_acl_osdep_alone.h is only needed for cases when RTE_LIBRTE_ACL_STANDALONE=y.
> That comes from the old days, when we had to to support building librte_acl library without the rest of DPDK.  
> I think we don't need it anymore and plan to remove it.
> Just thought it should  be in a separate patch. 
> Konstantin
> 
> > 
> > Neil
>