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From: "Mattias Rönnblom" <mattias.ronnblom@ericsson.com>
To: "Wiles, Keith" <keith.wiles@intel.com>
Cc: "dev@dpdk.org" <dev@dpdk.org>,
	"stephen@networkplumber.org" <stephen@networkplumber.org>
Subject: Re: [dpdk-dev] [RFC v2 2/2] eal: introduce random generator function with upper bound
Date: Mon, 22 Apr 2019 09:07:37 +0200	[thread overview]
Message-ID: <e55398f8-3fcb-7c12-2a60-6174194e8692@ericsson.com> (raw)
Message-ID: <20190422070737.Mq3fF7w0RSTurvWuybIgYKvy6m5AzZ7ujneJ_9Zgsm4@z> (raw)
In-Reply-To: <4440C41D-23E4-4A46-8C8A-3AA2ECB59C40@intel.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.

  parent reply	other threads:[~2019-04-22  7:07 UTC|newest]

Thread overview: 137+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-05 13:45 [dpdk-dev] [RFC] eal: make rte_rand() MT safe Mattias Rönnblom
2019-04-05 13:45 ` Mattias Rönnblom
2019-04-05 13:51 ` Mattias Rönnblom
2019-04-05 13:51   ` Mattias Rönnblom
2019-04-05 14:28   ` Bruce Richardson
2019-04-05 14:28     ` Bruce Richardson
2019-04-05 14:56     ` Mattias Rönnblom
2019-04-05 14:56       ` Mattias Rönnblom
2019-04-05 16:57 ` Stephen Hemminger
2019-04-05 16:57   ` Stephen Hemminger
2019-04-05 18:04   ` Mattias Rönnblom
2019-04-05 18:04     ` Mattias Rönnblom
2019-04-05 20:50     ` Stephen Hemminger
2019-04-05 20:50       ` Stephen Hemminger
2019-04-06  5:52       ` Mattias Rönnblom
2019-04-06  5:52         ` Mattias Rönnblom
2019-04-08 12:30       ` [dpdk-dev] [RFC 1/3] Replace lrand48-based rte_rand with LFSR generator Mattias Rönnblom
2019-04-08 12:30         ` Mattias Rönnblom
2019-04-08 12:30         ` [dpdk-dev] [RFC 2/3] Add 32-bit version of rte_rand Mattias Rönnblom
2019-04-08 12:30           ` Mattias Rönnblom
2019-04-08 12:30         ` [dpdk-dev] [RFC 3/3] Introduce random generator functions with upper bound Mattias Rönnblom
2019-04-08 12:30           ` Mattias Rönnblom
2019-04-08 12:47         ` [dpdk-dev] [RFC 1/3] Replace lrand48-based rte_rand with LFSR generator Mattias Rönnblom
2019-04-08 12:47           ` Mattias Rönnblom
2019-04-19 21:21         ` [dpdk-dev] [RFC v2 0/2] Pseudo-number generation improvements Mattias Rönnblom
2019-04-19 21:21           ` Mattias Rönnblom
2019-04-19 21:21           ` [dpdk-dev] [RFC v2 1/2] eal: replace libc-based random number generation with LFSR Mattias Rönnblom
2019-04-19 21:21             ` Mattias Rönnblom
2019-04-22 11:34             ` Neil Horman
2019-04-22 11:34               ` Neil Horman
2019-04-22 14:49               ` Stephen Hemminger
2019-04-22 14:49                 ` Stephen Hemminger
2019-04-22 15:52               ` Mattias Rönnblom
2019-04-22 15:52                 ` Mattias Rönnblom
2019-04-22 17:44                 ` Mattias Rönnblom
2019-04-22 17:44                   ` Mattias Rönnblom
2019-04-23 11:33                   ` Neil Horman
2019-04-23 11:33                     ` Neil Horman
2019-04-23 17:13                     ` Mattias Rönnblom
2019-04-23 17:13                       ` Mattias Rönnblom
2019-04-24 11:37                       ` Neil Horman
2019-04-24 11:37                         ` Neil Horman
2019-04-23 15:31                   ` Stephen Hemminger
2019-04-23 15:31                     ` Stephen Hemminger
2019-04-23 17:17                     ` Mattias Rönnblom
2019-04-23 17:17                       ` Mattias Rönnblom
2019-04-24  7:52                       ` Mattias Rönnblom
2019-04-24  7:52                         ` Mattias Rönnblom
2019-04-24 12:33                         ` [dpdk-dev] [RFC v3 0/2] Pseudo-random number generation improvements Mattias Rönnblom
2019-04-24 12:33                           ` Mattias Rönnblom
2019-04-24 12:33                           ` [dpdk-dev] [RFC v3 1/2] eal: replace libc-based random number generation with LFSR Mattias Rönnblom
2019-04-24 12:33                             ` Mattias Rönnblom
2019-05-08 20:12                             ` Stephen Hemminger
2019-05-08 20:12                               ` Stephen Hemminger
2019-05-08 20:30                               ` Mattias Rönnblom
2019-05-08 20:30                                 ` Mattias Rönnblom
2019-05-09  1:10                                 ` Stephen Hemminger
2019-05-09  1:10                                   ` Stephen Hemminger
2019-05-14  9:20                                   ` [dpdk-dev] [PATCH 0/6] Pseudo-random number generation improvements Mattias Rönnblom
2019-05-14  9:20                                     ` Mattias Rönnblom
2019-05-14  9:20                                     ` [dpdk-dev] [PATCH 1/6] eal: replace libc-based random number generation with LFSR Mattias Rönnblom
2019-05-14  9:20                                       ` Mattias Rönnblom
2019-05-14  9:32                                       ` Mattias Rönnblom
2019-05-14  9:32                                         ` Mattias Rönnblom
2019-05-14 14:16                                       ` Neil Horman
2019-05-14 14:16                                         ` Neil Horman
2019-05-14 14:53                                         ` Mattias Rönnblom
2019-05-14 14:53                                           ` Mattias Rönnblom
2019-05-17 19:27                                           ` Neil Horman
2019-05-17 20:57                                             ` Bruce Richardson
2019-05-17 21:10                                             ` Mattias Rönnblom
2019-05-19 18:32                                               ` Neil Horman
2019-05-14 15:27                                       ` Stephen Hemminger
2019-05-14 15:27                                         ` Stephen Hemminger
2019-05-14  9:20                                     ` [dpdk-dev] [PATCH 2/6] eal: add pseudo-random number generation performance test Mattias Rönnblom
2019-05-14  9:20                                       ` Mattias Rönnblom
2019-05-14  9:20                                     ` [dpdk-dev] [PATCH 3/6] eal: improve entropy for initial PRNG seed Mattias Rönnblom
2019-05-14  9:20                                       ` Mattias Rönnblom
2019-05-14  9:36                                       ` Mattias Rönnblom
2019-05-14  9:36                                         ` Mattias Rönnblom
2019-05-14  9:39                                         ` Bruce Richardson
2019-05-14  9:39                                           ` Bruce Richardson
2019-05-14 11:58                                           ` Mattias Rönnblom
2019-05-14 11:58                                             ` Mattias Rönnblom
2019-05-14  9:37                                       ` Bruce Richardson
2019-05-14  9:37                                         ` Bruce Richardson
2019-05-14  9:20                                     ` [dpdk-dev] [PATCH 4/6] eal: introduce random generator function with upper bound Mattias Rönnblom
2019-05-14  9:20                                       ` Mattias Rönnblom
2019-05-14  9:20                                     ` [dpdk-dev] [PATCH 5/6] eal: add bounded PRNG performance tests Mattias Rönnblom
2019-05-14  9:20                                       ` Mattias Rönnblom
2019-05-14  9:20                                     ` [dpdk-dev] [PATCH 6/6] eal: add pseudo-random number generation to MAINTAINERS Mattias Rönnblom
2019-05-14  9:20                                       ` Mattias Rönnblom
2019-05-16 17:55                                     ` [dpdk-dev] [PATCH v2 0/6] Pseudo-random number generation improvements Mattias Rönnblom
2019-05-16 17:55                                       ` [dpdk-dev] [PATCH v2 1/6] eal: replace libc-based random number generation with LFSR Mattias Rönnblom
2019-05-16 17:55                                       ` [dpdk-dev] [PATCH v2 2/6] eal: add pseudo-random number generation performance test Mattias Rönnblom
2019-05-16 17:55                                       ` [dpdk-dev] [PATCH v2 3/6] eal: improve entropy for initial PRNG seed Mattias Rönnblom
2019-05-16 17:55                                       ` [dpdk-dev] [PATCH v2 4/6] eal: introduce random generator function with upper bound Mattias Rönnblom
2019-05-16 17:55                                       ` [dpdk-dev] [PATCH v2 5/6] eal: add bounded PRNG performance tests Mattias Rönnblom
2019-05-16 17:55                                       ` [dpdk-dev] [PATCH v2 6/6] eal: add PRNG to MAINTAINERS and release notes Mattias Rönnblom
2019-05-16 20:35                                       ` [dpdk-dev] [PATCH v2 0/6] Pseudo-random number generation improvements Bruce Richardson
2019-06-05 10:43                                         ` [dpdk-dev] [PATCH v3 " Mattias Rönnblom
2019-06-05 10:43                                           ` [dpdk-dev] [PATCH v3 1/6] eal: replace libc-based random number generation with LFSR Mattias Rönnblom
2019-06-05 10:43                                           ` [dpdk-dev] [PATCH v3 2/6] eal: add pseudo-random number generation performance test Mattias Rönnblom
2019-06-27 21:23                                             ` Thomas Monjalon
2019-06-28  8:20                                               ` Mattias Rönnblom
2019-06-05 10:43                                           ` [dpdk-dev] [PATCH v3 3/6] eal: improve entropy for initial PRNG seed Mattias Rönnblom
2019-06-05 10:43                                           ` [dpdk-dev] [PATCH v3 4/6] eal: introduce random generator function with upper bound Mattias Rönnblom
2019-06-05 10:43                                           ` [dpdk-dev] [PATCH v3 5/6] eal: add bounded PRNG performance tests Mattias Rönnblom
2019-06-05 10:44                                           ` [dpdk-dev] [PATCH v3 6/6] eal: add PRNG to MAINTAINERS and release notes Mattias Rönnblom
2019-06-27 21:27                                             ` Thomas Monjalon
2019-06-28  8:17                                               ` Mattias Rönnblom
2019-06-15 12:23                                           ` [dpdk-dev] [PATCH v3 0/6] Pseudo-random number generation improvements Mattias Rönnblom
2019-06-28  9:01                                           ` [dpdk-dev] [PATCH v4 0/5] " Mattias Rönnblom
2019-06-28  9:01                                             ` [dpdk-dev] [PATCH v4 1/5] eal: replace libc-based random number generation with LFSR Mattias Rönnblom
2019-06-28  9:01                                             ` [dpdk-dev] [PATCH v4 2/5] eal: add pseudo-random number generation performance test Mattias Rönnblom
2019-06-28  9:01                                             ` [dpdk-dev] [PATCH v4 3/5] eal: improve entropy for initial PRNG seed Mattias Rönnblom
2019-06-28 19:01                                               ` Ferruh Yigit
2019-06-28 20:58                                                 ` Mattias Rönnblom
2019-06-28 21:08                                                   ` [dpdk-dev] [PATCH] eal: use 32-bit RDSEED to allow 32-bit x86 usage Mattias Rönnblom
2019-06-29 12:54                                                     ` Thomas Monjalon
2019-06-28  9:01                                             ` [dpdk-dev] [PATCH v4 4/5] eal: introduce random generator function with upper bound Mattias Rönnblom
2019-06-28  9:01                                             ` [dpdk-dev] [PATCH v4 5/5] eal: add bounded PRNG performance tests Mattias Rönnblom
2019-06-28 13:24                                             ` [dpdk-dev] [PATCH v4 0/5] Pseudo-random number generation improvements Thomas Monjalon
2019-04-24 12:33                           ` [dpdk-dev] [RFC v3 2/2] eal: introduce random generator function with upper bound Mattias Rönnblom
2019-04-24 12:33                             ` Mattias Rönnblom
2019-04-19 21:21           ` [dpdk-dev] [RFC v2 " Mattias Rönnblom
2019-04-19 21:21             ` Mattias Rönnblom
2019-04-20 21:08             ` Wiles, Keith
2019-04-20 21:08               ` Wiles, Keith
2019-04-21 19:05               ` Mattias Rönnblom
2019-04-21 19:05                 ` Mattias Rönnblom
2019-04-22  4:33                 ` Wiles, Keith
2019-04-22  4:33                   ` Wiles, Keith
2019-04-22  7:07                   ` Mattias Rönnblom [this message]
2019-04-22  7:07                     ` Mattias Rönnblom
2019-04-22 13:19                     ` Wiles, Keith
2019-04-22 13:19                       ` Wiles, Keith

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