Re: [dpdk-dev] [PATCH 0/2] dpdk: Allow for dynamic enablement of some isolated features

[Thomas Monjalon <thomas.monjalon@6wind.com>]

2014-07-31 09:13, Neil Horman:
> On Wed, Jul 30, 2014 at 02:09:20PM -0700, Bruce Richardson wrote:
> > On Wed, Jul 30, 2014 at 03:28:44PM -0400, Neil Horman wrote:
> > > On Wed, Jul 30, 2014 at 11:59:03AM -0700, Bruce Richardson wrote:
> > > > On Tue, Jul 29, 2014 at 04:24:24PM -0400, Neil Horman wrote:
> > > > > Hey all-
> > > > >         I've been trying to update the fedora dpdk package to support VFIO 
> > > > > enabled drivers and ran into a problem in which ixgbe didn't compile because the 
> > > > > rxtx_vec code uses sse4.2 instruction intrinsics, which aren't supported in the 
> > > > > default config I have.  I tried to remedy this by replacing the intrinsics with 
> > > > > the __builtin macros, but it was pointed out (correctly), that this doesn't work 
> > > > > properly.  So this is my second attempt, which I actually like a bit better.  I 
> > > > > noted that code that uses intrinsics (ixgbe and the acl library), don't need to 
> > > > > have those instructions turned on build-wide.  Rather, we can just enable the 
> > > > > instructions in the specific code we want to build with support for that, and 
> > > > > test for instruction support dynamically at run time.  This allows me to build 
> > > > > the dpdk for a generic platform, but in such a way that some optimizations can 
> > > > > be used if the executing cpu supports them at run time.
> > > > > 
> > > > > Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
> > > > > CC: Thomas Monjalon <thomas.monjalon@6wind.com>
> > > > >
> > > > I'd prefer if a solution could be found based off your original patch
> > > > set, as it gives us more chance to deprecate the older code paths in
> > > > future. Looking at the Intel Intrinsics Guide site online, it shows that
> > > > the _mm_shuffle_epi8 intrinsic came in with SSSE3, rather than SSE4.x,
> > > > and so should be available on all 64-bit systems, I believe. The
> > > > popcount intrinsic is newer, but it's a much more basic instruction so
> > > > hopefully the __builtin should work for that.
> > > > 
> > > Yes, but as I look at it, thats somewhat counter to my goal, which is to offer
> > > accelerated code paths on systems that can make use of it at run time.  If We
> > > use the __builtin compiler functions, we will either:
> > > 
> > > 1) Build those code paths with advanced instructions that won't work on older
> > > systems (i.e. crash)
> > > 
> > > 2) Build those code paths with less advanced instructions, meaning that we won't
> > > speedup execution on systems that are capable of using the more advanced
> > > instructions.
> > > 
> > > Using this run time check, we can, at least in these situations, make use of the
> > > accelerated paths when the instructions are available, and ignore them when
> > > they're not, at run time.
> > > 
> > > What would be ideal, would be an alternative type macro, like the linux kernel
> > > employs, but implementing that would require some pretty significant work and
> > > testing.  This seems like a much simpler approach.

[...]

> Now, a macro that selected an instruction optimized or generic path is fine, as
> long as it can happen at run time.  The Linux kernel has such a feature, called
> alternatives.  But its a complex subsystem that does run time replacement of
> instructions based on cpu feature flags.  It would be great to have in the DPDK,
> but its a significant code base and difficult to maintain, which goes against
> your desire to reduce code.

[...]

> > Even though the code is written using intrinsics which correspond to SSE
> > operations, the compiler is free to use AVX instructions where necessary
> Not if you use the default machine target.
> 
> > to improve performance. Therefore, if we go down this road, we need to
> > look to compile up the code for all microarchitectures, rather than just
> > assuming that we will get equivalent performance to "native" by turning
> > on the instruction set indicated by the primitives in the code. This is
> No, you compile for the least common demonitor system, and enable more
> performant paths opportunistically as run time checks allow.
> 
> > where having one codepath recompiled multiple times will work far better
> > than having multiple code paths.
> Only if you're only concern is performance.  As noted above, my goal is more
> than just performance, its compatibility accross systems.  Multiple builds for
> multiple cpu flag availability is simply a non-starter for a generic
> distribution.

Neil, we are mixing 2 different problems here.
1) we have to fix default build (without SSE-4.2)
2) we could try to have performance with default build

Please, let's focus on the first item and we could discuss about performance
later. Having some different code path choosed at runtime is a big rework and
imply changing the compilation model (RFC welcome).

-- 
Thomas