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

["Mattias Rönnblom" <mattias.ronnblom@ericsson.com>]

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.