Re: [dpdk-dev] cuckoo hash in dpdk

[Andriy Berestovskyy <aber@semihalf.com>]

Hey Pragash,
I am not the author of the code, but I guess it is done that way
because modern compilers do recognize power of two constants and do
substitute division and modulo operations with corresponding bit
manipulations.

Just try to compile a small program like the following:

volatile unsigned a = 123, b, c;
int main(int argc, char **argv)
{
    b = a / 4;
    c = a % 4;
    printf("%x %x %x\n", a, b, c);
}


and then disassemble it with gdb:

(gdb) disassemble /s main
[...]
13 b = a / 4;
   0x0000000000400464 <+20>: shr    $0x2,%eax
   0x0000000000400467 <+23>: mov    %eax,0x200bd3(%rip)        # 0x601040 <b>

14 c = a % 4;
   0x000000000040046d <+29>: mov    0x200bc5(%rip),%eax        # 0x601038 <a>
   0x0000000000400473 <+35>: and    $0x3,%eax
   0x0000000000400476 <+38>: mov    %eax,0x200bc8(%rip)        # 0x601044 <c>
[...]

As you can see both division and modulo was substituted with "shr" and "and".

So basically nowadays there is no need to worry about that and
complicate code with explicit low-level optimizations. Hope that
answers your question.

Regards,
Andriy


On Wed, Aug 23, 2017 at 4:15 PM, Pragash Vijayaragavan <pxv3620@rit.edu> wrote:
> Hi,
>
> I got the chance to look at the cuckoo hash used in dpdk and have a query.
>
> would using division and modulo operations be slower than bitwise
> operations on RTE_HASH_BUCKET_ENTRIES, specially since
> RTE_HASH_BUCKET_ENTRIES is a power of 2.
> For example, to do a modulo we can do a "AND" operation on
> (RTE_HASH_BUCKET_ENTRIES - 1), which might be faster. We did a cuckoo
> filter for VPP and doing this gave a slight improvement in speed.
> Is there any particular reason its done this way.
>
> Sorry if i am being wrong in any way, i was just curious.
>
> Thanks,
>
> Pragash Vijayaragavan
> Grad Student at Rochester Institute of Technology
> email : pxv3620@rit.edu
> ph : 585 764 4662



-- 
Andriy Berestovskyy