Re: [dpdk-dev] [PATCH] ring: empty and count optimizations

["Morten Brørup" <mb@smartsharesystems.com>]

> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Ananyev,
> Konstantin
> Sent: Thursday, May 14, 2020 6:47 PM
> >
> > > > -static inline unsigned
> > > > +static inline unsigned int
> > > >  rte_ring_count(const struct rte_ring *r)
> > > >  {
> > > >  	uint32_t prod_tail = r->prod.tail;
> > > >  	uint32_t cons_tail = r->cons.tail;
> > > >  	uint32_t count = (prod_tail - cons_tail) & r->mask;
> > > > -	return (count > r->capacity) ? r->capacity : count;
> > > > +	return likely(count <= r->capacity) ? count : r->capacity;
> > >
> > > Honestly, I don't see there is any point of that change:
> > > I think it wouldn't change anything in terms of functionality
> > > or performance.
> >
> > Chapter 3.4.1 "Branch Prediction Optimization" in the Intel 64 and
> IA-32 Architectures Optimization Reference Manual recommends this
> > kind of optimization as Assembly/Compiler Coding Rule 3, which is why
> I rearranged the trigraph. Essentially, there is a limit to the number
> > of BTB (Branch Target Buffer) entries, so they should be conserved if
> possible.
> >
> > In addition to that, I have added the likely() because I consider it
> nearly impossible that the count will exceed the capacity.
> >
> > However, it's not the first time I see this kind of response to a
> suggested branch optimization on the DPDK mailing list. Everyone seem
> to
> > think that branch prediction is infinite and always works. It may
> seem as if infinite on trivial applications, but BTB entries may be a
> scarce
> > resource on complex applications. I assume Intel's recommendations
> are not just for the fun of it.
> 
> I think it is better to leave such level of micro-optimizations to the
> compiler.
> BTW, in that particular case, compiler most likely will generate a code
> without any branches at all (at least for IA).
> Let say on my box with gcc 7.3:
> 
> $ cat trc1.c
> #include <stdint.h>
> #include <rte_config.h>
> #include <rte_ring.h>
> 
> uint32_t
> fffx1(const struct rte_ring *r)
> {
>         uint32_t prod_tail = r->prod.tail;
>         uint32_t cons_tail = r->cons.tail;
>         uint32_t count = (prod_tail - cons_tail) & r->mask;
>         return (count > r->capacity) ? r->capacity : count;
> }
> 
> uint32_t
> fffx2(const struct rte_ring *r)
> {
>         uint32_t prod_tail = r->prod.tail;
>         uint32_t cons_tail = r->cons.tail;
>         uint32_t count = (prod_tail - cons_tail) & r->mask;
>         return likely(count <= r->capacity) ? count : r->capacity;
> }
> 
> $ gcc -m64 -O3 -march=native -I${RTE_SDK}/x86_64-native-linuxapp-
> gcc/include -c trc1.c
> 
> $ objdump -d trc1.o
> 
> 0000000000000000 <fffx1>:
>    0:   8b 87 84 00 00 00       mov    0x84(%rdi),%eax
>    6:   8b 97 04 01 00 00       mov    0x104(%rdi),%edx
>    c:   29 d0                   sub    %edx,%eax
>    e:   8b 57 38                mov    0x38(%rdi),%edx
>   11:   23 47 34                and    0x34(%rdi),%eax
>   14:   39 d0                   cmp    %edx,%eax
>   16:   0f 47 c2                cmova  %edx,%eax
>   19:   c3                      retq
>   1a:   66 0f 1f 44 00 00       nopw   0x0(%rax,%rax,1)
> 
> 0000000000000020 <fffx2>:
>   20:   8b 87 84 00 00 00       mov    0x84(%rdi),%eax
>   26:   8b 97 04 01 00 00       mov    0x104(%rdi),%edx
>   2c:   29 d0                   sub    %edx,%eax
>   2e:   8b 57 38                mov    0x38(%rdi),%edx
>   31:   23 47 34                and    0x34(%rdi),%eax
>   34:   39 d0                   cmp    %edx,%eax
>   36:   0f 47 c2                cmova  %edx,%eax
>   39:   c3                      retq
> 
> As you can see, there is no difference.
> 

Thank you for the detailed feedback.

Reality trumps theory, so I will leave the count function as is. :-)


> >
> > Konstantin, please note that I'm letting out my frustration about the
> general misconception about branch prediction here. You are doing a
> > great job, so I feel bad about responding like this to you.
> 
> No worries, in fact I am glad to know that DPDK contributors
> read IA optimization manual that thoughtfully 😊
> 
> Konstantin