Re: [dpdk-dev] [RFC v2 2/2] eal: introduce random generator function with upper bound

["Wiles, Keith" <keith.wiles@intel.com>]

> On Apr 22, 2019, at 2:07 AM, Mattias Rönnblom <mattias.ronnblom@ericsson.com> wrote:
> 
> On 2019-04-22 06:33, Wiles, Keith wrote:
> 
>>> From a performance point of view, the high-loop-count cases are rare enough not to pose a serious threat. For example, being forced to redo rte_rand() more than five times is only a ~3% risk.
>> Even a few loops can have an effect on performance when we are talking about micro-seconds plus it leads to indeterminate results. The numbers you reported here are interesting, but I would be happier if you added a limit to the loop. If you state the likely hood of doing 5 loops is only 3% then adding a loop limit would be reasonable, right?
> 
> Probability is already effectively putting a limit to the loop. The risk of being stuck for >1us is p=~6e-73. The variations in execution times will in most cases be less than a LLC miss.
> 
> A loop variable will not have any effect on performance, pollute the code, and hurt uniformity.
> 
> Here's what rte_rand_max() performance looks like on my Skylake.
> 
> Average rte_rand_max() latency with worst-case upper_bound:
> rte_rand_max() w/o loop limit: 47 cc
> rte_rand_max() w/ max 8 retries: 49 cc
> rte_rand_max() w/ max 4 retries: 47 cc
> rte_rand_max() w/ max 2 retries: 40 cc
> 
> So you need to be very aggressive in limiting the loop count for that loop variable to pay off. Otherwise, you will just be at a loss, doing all that bookkeeping which very rarely turns out to be useful.

The adding of a loop counter is not a performance problem (as you stated only a few cycles are added) and it does not hurt the readability of the code. As for hurting uniformity I do not see that being a problem, as most loops have a visible way to terminate the loop instead of relying on a random number generator to give a valid value in a few loops. It is only reasonable IMO to limit the loop.

But I will let it go and let you add this code to DPDK if someone else wants to ACK it.

Regards,
Keith