On Tue, 14 Jan 2025 17:22:08 +0800
huangdengdui <huangdeng...@huawei.com> wrote:
> On 2025/1/11 1:20, Stephen Hemminger wrote:
> > This will make it slower for many platforms.
> > GCC will unroll a loop of fixed small size, which is what we want.
>
> Do you mean to replace option with a macro?
> But most of prefetch_offset are used with the nb_rx, So using macros is the
> same as using options.
>
> const int32_t k = RTE_ALIGN_FLOOR(nb_rx, FWDSTEP);
> for (j = 0; j != k; j += FWDSTEP) {
> for (i = 0, pos = j + prefetch_offset;
> i < FWDSTEP && pos < k; i++, pos++)
> rte_prefetch0(rte_pktmbuf_mtod(pkts_burst[pos], void *));
> processx4_step1(&pkts_burst[j], &dip, &ipv4_flag);
> processx4_step2(qconf, dip, ipv4_flag, portid,
> &pkts_burst[j], &dst_port[j]);
> if (do_step3)
> processx4_step3(&pkts_burst[j], &dst_port[j]);
> }
>
> The option can dynamically adjust the prefetch window, which makes it easier
> to find the prefetch window for a HW platform.
> So I think it's better to use option.
The tradeoff is that loop unrolling most often is only done on small fix sized
loops.
And the cost of a loop with variable small values (branch prediction) is high
enough that it
could make things slower.
Prefetching is a balancing act, and more is not better especially on real
workloads.
