精确计时内核模型的摘要、执行片段和介绍

Abstract 摘要

This memorandum is a substantial revision and update of RFC-1589, "A Kernel Model for Precision Timekeeping," [MIL94a]. It contains information from a technical report of the same name in PostScript format [MIL94b], together with some new information. This revision includes several changes to the daemon and user interfaces to provide more detail in performance monitoring and to support symmetric multiprocessor systems. It provides a new feature which disciplines the CPU clock oscillator in both time and frequency to a source of precision time signals, as well as provisions to operate with good accuracy at much higher poll intervals up to several hours.

这个备忘录是对rfc-1589实质性的修订和更新,“一个精确计时内核模型,”[MIL94a]。它包含以PostScript格式[MIL94b]命名的相同名称的技术报告的信息，加上一些新的信息。该版本包括一些更改守护程序和用户界面，以提供更详细的性能监测，并支持对称多处理器系统。它提供了一个新特性，训练CPU时钟振荡器在时域和频域达到精确时间信号的来源,同时提供适当良好的操作，来支持几个小时的poll间隔。

The version of this memorandum dated 14 October 1994 describes a revised clock discipline model useful for update intervals greater than 1024 s, which was formerly the maximum consistent with reasonable accuracy using standard onboard clock oscillators. The new model is a hybrid in which the old design based on a phase-lock loop is used at and below 1024 s, while a new design based on a frequency-lock loop is used above this.

这个备忘录的版本日期为1994年10月14日，它描述了修订的时钟约束模型描述，此模型用于更新大于1024秒的间隔,这个间隔原来是匹配标准板载时钟振荡器的最大一致合理精度。新模型是混合了旧模型的设计，基于锁相环路，用于和低于1024秒,而基于新设计，频率锁定循环会大于1024秒。

The version of this memorandum dated 13 December 1994 describes a new model which supports symmetric multiprocessor systems in which each processor contains an integral counter which runs at some multiple of 1 MHz. In this model, a master processor maintains the time of day using the standard Unix logical clock, while the counters are used to interpolate the microseconds between ticks of the logical clock. The new model explicitly compensates for the small differences between the operating frequencies of the counters in order to synthesize a consistent, reliable system clock. The version of this memorandum dated 31 January 1996 describes an alternative model peculiar to the Sun Microsystems Solaris kernel for symmetric multiprocessor systems. This model uses a single oscillator, together with two counters to implement a tick interrupt and a time-of-day clock.

这份备忘录是1994年12月13日版本，它描述了支持对称多处理器系统中，每个处理器包含在1 MHz的某个倍数运行的积分计数器的新模式。在该模型中，一个主处理器保持一天的使用标准的Unix逻辑时钟的时间，而计数器用于逻辑时钟的节拍之间内插微秒。新模型明确补偿小额差异的操作频率计数器，为了合成一个一致的、可靠的系统时钟。这个备忘录日期为1996年1月31日的版本，描述特殊的替代模型对称多处理器系统的Sun Solaris内核。这个模型使用一个单一的振荡器,连同两个计数器来实现节拍中断和天周期时间时钟。

Information on measurment technique is contained in a companion memorandum [MIL96a]; a candidate operating systems interface for precision timing signals is contained in [MIL96b]. This memorandum is included in the documentation for the NTP Version 3 distribution for Unix, as well as distributions for SunOS, Ultrix, OSF/1 and HP-UX kernel modifications which support precision time functions. Detailed technical information, including source code segments implementing these functions, is also available. Availability of the kernel distributions, which involve licensed code, will be announced separately.

计量技术之上的信息被包含在一个同伴备忘录(MIL96a);候选操作系统接口来实现人为精确计时信号的方法被包含在(MIL96b)。这个备忘录是包含在为Unix版本服务的NTP版本3文档内,以及为SunOS Ultrix OSF/1和hp-ux服务，这些系统进行了内核修改,支持精确时间函数。详细的技术信息,包括源代码部分实现这些函数,也可用。包括内核发行版的可用性授权代码,将另行公布。

1. Executive Summary 执行概要

This memorandum describes an engineering model which implements a precision time-of-day function for a generic operating system. The model is based on the principles of disciplined oscillators using phase-lock loops (PLL) and frequency-lock loops (FLL) often found in the engineering literature. It has been implemented in the Unix kernels for several workstations, including those made by Sun Microsystems, Digital and Hewlett Packard. The model changes the way the system clock is adjusted in time and frequency, as well as provides mechanisms to discipline its time and frequency to an external precision timing source, such as a pulse-per-second (PPS) signal. The model incorporates a generic system-call interface for use with the Network Time Protocol (NTP) or similar time synchronization protocol. The NTP Version 3 daemon xntpd operates with this model to provide synchronization limited in principle only by the accuracy and stability of the external timing source.

本备忘录描述一个工程模型，它为一个通用的操作系统实现精确时段函数。模型是基于自律的原则振荡器，此振荡器使用经常出现在工程技术文档里的锁相环(PLL)和频率锁定环(FLL)。它已经在Unix内核中实现几个工作站,包括那些由Sun Microsystems、Digital和惠普。该模型改变了系统时钟的时间和频率调整方式，以及提供相关机制的时间和频率，以归整外部精确定时源，如pulse-per-second(PPS)信号。模型包含一个通用的系统调用接口，接口使用网络时间协议(NTP)或类似的时间同步协议。NTP 3守护进程xntpd运作与这个模型提供同步。原则上，同步受限于外部时间源的准确性和稳定性。

This memorandum does not propose a standard protocol, specification or algorithm. It is intended to provoke comment, refinement and implementations for kernels not considered herein. While a working knowledge of NTP is not required for an understanding of the design principles or implementation of the model, it may be helpful in understanding how the model behaves in a fully functional timekeeping system. The architecture and design of NTP is described in [MIL91], while the current NTP Version 3 protocol specification is given in RFC-1305 [MIL92a] and a subset of the protocol, the Simple Network Time Protocol (SNTP), is given in RFC-1361 [MIL92c].

本备忘录没有提出一个标准协议、规范或算法。它的目的是引起评论、细化和实现此处还没捍到的内核。虽然NTP工作知识的设计原理和执行模式的理解不是必需的，它可能对完全理解模型的行为功能计时系统有帮助。NTP的体系结构和设计在[MIL91]描述，而当前的NTP版本3协议规范在RFC-1305[MIL92a]给出的协议的一个子集，在RFB-1361[MIL92c]中给出。

The model has been implemented in the Unix kernels for Sun Microsystems, Digital and Hewlett Packard workstations. In addition, for the Digital machines the model provides improved precision to one microsecond (us). Since these specific implementations involve modifications to licensed code, they cannot be provided directly. Inquiries should be directed to the manufacturer’s representatives. However, the engineering model for these implementations, including a simulator with code segments almost identical to the implementations, but not involving licensed code, is available via anonymous FTP.

该模型已在Sun微系统、Digital和惠普工作站UNIX内核的得到落实。此外,对于Digital机器模型提供了改进的精度，为一微秒(美国)。因为这些具体的实现涉及修改授权代码,它们不能直接提供。调查应该针对制造商的代表。然而,这些实现的工程模型,包括一个模拟器实现代码段几乎相同,但不涉及许可的代码,通过匿名FTP是可用的。

The NTP Version 3 distribution and technical information distributions can be obtained via anonymous ftp from louie.udel.edu in the directory pub/ntp. The compressed tar archive xntp3.v.tar.Z contains the NTP Version 3 distribution, where v is the version identifier and may be incremented in future versions. In order to utilize all features described in this memorandum, the NTP version identifier should be 4f or later. The compressed tar archive kernel.tar.Z contains additional technical information, as well as this file.

NTP版本3配送和技术信息的分布可以通过从louie.udel.edu在目录pub/ NTP匿名ftp来获得。压缩的tar存档xntp3.v.tar.Z包含NTP版本3的分布。其中v是标识符并且可以在未来的版本中增加。为了利用本备忘录中描述的所有功能，NTP版本标识符应该是4f或更高版本。压缩的tar存档kernel.tar.Z包含额外的技术信息,以及该文件。

2. Introduction 介绍

This memorandum describes a model and programming interface for generic operating system software that manages the system clock and timer functions. The model provides improved accuracy and stability for most computers using the Network Time Protocol (NTP) or similar time synchronization protocol. This memorandum describes the design principles and implementations of the model, while related technical reports discuss the design approach, engineering analysis and performance evaluation of the model as implemented in Unix kernels for modern workstations. The NTP Version 3 daemon xntpd operates with these implementations to provide improved accuracy and stability, together with diminished overhead in the operating system and network. In addition, the model supports the use of external timing sources, such as precision pulse-per-second (PPS) signals and the industry standard IRIG timing signals. The NTP daemon automatically detects the presence of the new features and utilizes them when available.

本备忘录描述了通用的操作系统软件、管理系统时钟和定时器功能模型和编程接口。模型提供了对于大多数电脑使用网络时间协议(NTP)或类似的时间同步协议改进的精度和稳定性。本备忘录描述模型的设计原则和实现,而相关的技术报告，讨论了该模型的设计方法，工程分析和绩效评估作为UNIX内核现代工作站实施。NTP版本3后台FTP这些操作的实现提供更高的精度和稳定性，减少在操作系统和网络的开销。此外,模型支持使用外部定时源,如精确pulse-per-second(PPS)信号和行业标准IRIG定时信号。NTP守护进程会自动检测新特征的存在，并在使用时可以用。

There are four prototype implementations of the model presented in this memorandum, one each for the Sun Microsystems SPARCstation with the SunOS 4.x kernel and Solaris 2.x kernel, Digital DECstation 5000 with the Ultrix 4.x kernel, Digital 3000 AXP Alpha with the OSF/1 V3.x kernel, and Hewlett Packard 9000 with the HP-UX 9.x kernel. In addition, for the DECstation 5000/240 and 3000 AXP Alpha machines, a special feature provides improved precision to 1 us (stock Sun and HP kernels already do provide this precision). Other than improving the system clock accuracy, stability and precision, these implementations do not change the operation of existing Unix system calls which manage the system clock, such as gettimeofday(), settimeofday() and adjtime(); however, if the new features are in use, the operations of gettimeofday() and adjtime() can be controlled instead by new system calls ntp_gettime() and ntp_adjtime() as described below.

还有在这个备忘录中提出的模型有四个原型实现，对应每一个为Sun Microsystems SPARCstation SunOS 4.x内核和Solaris 2.x内核、Digital5000年DECstation Ultrix 4.x内核、Digital 3000 AXPαOSF / V3.x内核和惠普9000 hp-ux9.x内核。此外,对于DECstation 5000/240和3000年AXP阿尔法机,一个特殊的特性提供了改进的精度1微妙(stock 太阳和惠普内核已经提供这个精度)。除了提高系统时钟的准确性,这些实现的稳定性和精度,不改变现有的Unix的操作系统时钟管理函数，如gettimeofday(),settimeofday()和adjtime();但是,如果使用了新特性,gettimeofday()和adjtime()的操作可由新系统调用控制,新系统调用是指ntp_gettime()和ntp_adjtime()，如下所述。

A detailed description of the variables and algorithms that operate upon them is given in the hope that similar functionality can be incorporated in Unix kernels for other machines. The algorithms involve only minor changes to the system clock and interval timer routines and include interfaces for application programs to learn the system clock status and certain statistics of the time synchronization process. Detailed installation instructions are given in a specific README files included in the kernel distributions.

关于这2个函数变量和算法的详细描述,希望类似的函数可以并入内核来支持其他机器。该算法只涉及轻微改变系统时钟和间隔计时器例程,包括感知系统时钟状态接口状态和某些时间同步过程的统计数据。给出详细的安装说明，说明包含在一个特定的README文件在内核中发布。

In this memorandum, NTP Version 3 and the Unix implementation xntp3 are used as an example application of the new system calls for use by a synchronization daemon. In principle, these system calls can be used by other protocols and implementations as well. Even in cases where the local time is maintained by periodic exchanges of messages at relatively long intervals, such as with modem services operated by NIST [LEV89] and USNO in the U.S. and PTB in Germany. In these cases the ability to precisely adjust the system clock frequency simplifies the synchronization procedures and allows the toll telephone call frequency to be considerably reduced.

在本备忘录,NTP 版本3和Unix实现xntp3作为一个例子新系统调用的应用程序使用，这个程序是一个同步守护进程。原则上,这些系统调用可以使用其他协议来实现。即使在某些情况下，当地的时间是在比较长的时间间隔，由信息的定期交流保持,比如在美国和德国PTB由NIST[LEV89]和USNO操作调制解调器服务。在这些情况下,能够精确地调整系统时钟频率简化了同步程序，并允许长途电话的频率大大降低。