Re: [PATCH V2 0/6] pipeline: make the hash function configurable per table

[Stephen Hemminger <stephen@networkplumber.org>]

On Fri, 19 Aug 2022 19:52:19 +0000
Cristian Dumitrescu <cristian.dumitrescu@intel.com> wrote:

> Also, since this flexibility has some performance cost, this patch set
> also introduces key comparison functions specialized for each key size
> value. Since the key size is fixed for each table, the key comparison
> function can be selected at initialization as opposed to using a
> generic function that can handle any key size. This strategy result in
> a performance improvement for the table lookup operation of around 5%.

I wonder if DPDK should start to adopt the Linux kernel optimizations
around indirect calls. For most all cases, the function pointer will be
a certain value and the cpu can do direct rather than indirect call.

As in:

     if (likely(hash_func == crc32_hash))
            crc32_hash(x, y)
     else
            (*hash_func)(x, y)

This was done in Linux kernel because of the overhead of the Spectre/Meltdown
mitigation's, but could apply more generally in DPDK.