Predictable Scheduling for a Soft Modem Michael B. Jones and Stefan Saroiu December 2000 Technical Report MSR-TR-2000-88 Microsoft Research Microsoft Corporation One Microsoft Way Redmond, WA 98052 Predictable Scheduling for a Soft Modem Michael B. Jones Stefan Saroiu Microsoft Research, Microsoft Corporation Department of Computer Science & Engineering One Microsoft Way University of Washington Redmond, WA 98052 Seattle, WA 98195-2350 USA USA [email protected] [email protected] http://research.microsoft.com/~mbj/ http://www.cs.washington.edu/homes/tzoompy/ Abstract • INT – the signal processing routines are executed in Soft Modems use the main processor to execute mo- an interrupt handler (a.k.a. in interrupt context). This dem functions traditionally performed by hardware on the is the original version. modem card. To function correctly, soft modems require • DPC – the signal processing routines are executed in that ongoing signal processing computations be performed the context of a Deferred Procedure Call (DPC) on the host CPU in a timely manner. Thus, signal proc- [Solomon & Russinovich 00, pages 107-111]. essing is a commonly occurring background real-time • THR – the signal processing routines are executed in application—one running on systems that were not de- the context of a kernel thread scheduled using the signed to support predictable real-time execution. This standard priority-based Windows 2000 scheduler. paper presents a detailed study of the performance char- • RES – the signal processing routines are executed in acteristics and resource requirements of a popular soft the context of a kernel thread scheduled using a Ri- modem. Understanding these requirements should inform alto/NT CPU Reservation [Jones & Regehr 99b]. the efforts of those designing and building operating sys- tems needing to support soft modems. Furthermore, we We captured performance profiles of each of the four believe that the conclusions of this study also apply to versions of the driver and compared and contrasted the other existing and upcoming soft devices, such as soft modem performance. We report on the benefits and the Digital Subscriber Line (DSL) cards. We conclude that (1) problems encountered with each of the driver versions signal processing in an interrupt handler is not only un- analyzed. One of the goals of this study is to make the necessary but also detrimental to the predictability of other detailed performance characteristics of a popular soft mo- computations in the system and (2) a real-time scheduler dem available to the industry, allowing this data to inform can provide predictability for the soft modem while mini- their work on providing predictable execution on con- mizing its impact on other computations in the system. sumer and general-purpose operating systems. While the soft modem’s 14.7% CPU load is not high 1. Introduction per se, a problem with the vendor version is that all of this Soft modems use the main processor to execute mo- time is spent in interrupt context. Once connected, the dem functions traditionally performed by the digital signal execution of the interrupt handler typically lasts 1.8ms processor (DSP) on the modem card. Soft modems have with a repeatable worst case of 3.3ms during connection. enjoyed large success in the home computer market. Two This study shows that signal processing in interrupt reasons for their success are low cost and the flexibility of context is not only unnecessary but also detrimental to the migrating to newer technologies by simple software up- predictability of other computations in the system. While grade. Given recent advances in CPU processing power, DPCs and priority-based scheduling cause milder side the impact of a soft modem on the throughput of the sys- effects upon the rest of the system, they nevertheless suf- tem is reasonable—we measured a 14.7% sustained CPU fer from some of the same drawbacks as the original ver- load on a 450 MHz Pentium II. Because soft modems need sion. This study supports the conclusion that certain kinds periodic real-time computations on the host CPU in order of real-time scheduling abstractions provide a good an- to maintain line connection and transmit data, a mecha- swer to the observed predictability problems that can be nism ensuring predictable scheduling is essential. caused by the soft modem. This paper presents a detailed study of the perform- Indeed, their real-time requirements and omnipres- ance characteristics and resource requirements of a popu- 1 ence make the soft modems an excellent environment for lar soft modem . We analyzed the vendor-supplied version testing the feasibility and the practicality of soft real-time of the driver and three additional versions that we created. scheduling for commodity operating systems. One section The four versions of the soft modem driver are: of our results is dedicated specifically to studying the ef- fectiveness of using a particular set of real-time schedul- ing abstractions for supporting the predictability require- ments of the soft modem. This paper demonstrates a con- crete set of benefits when real-time scheduling is applied 1 Our agreement with the manufacturer prevents us from to the computations employed by a soft modem. identifying this soft modem. Finally, we believe many of the lessons learned from Modems can be classified into hardware-based mo- studying soft modems are applicable to a wider class of dems (traditional modems) or software-based modems, problems. Other soft devices are already in use, with more depending on where each of these functions are executed. coming soon. For instance, software-based Digital Sub- 2.1.1 Hardware-based Modems scriber Line (Soft DSL) [Tramontano 00] devices have Traditional modems implement all the modem func- been announced. As will be detailed, Soft DSL has similar tionality in hardware on the modem card. Dedicated chips execution period requirements, but significantly larger provide signal modulation and interpret the AT command overall CPU requirements than soft modems. set. The card also provides A/D and D/A converters. On The remainder of this paper is structured as follows: older modems, the Universal Asynchronous Re- Section 2 provides background on soft modems and oper- ceiver/Transmitter (UART) chip implements the asyn- ating systems support for predictable execution. Section 3 chronous interface between the modem and the computer. describes the hardware and software tools used for our Today, the PCI bus interface often provides this function- study. Section 4 details how the soft modem used in these ality, replacing the UART chip. experiments operates. Section 5 discusses the four soft 2.1.2 Software-based Modems modem driver versions used in this study. Section 6 con- tains our results. A roadmap of the results can be found at Software-based analog modems use the host proces- the beginning of Section 6. Section 7 tells one of the “war sor to perform some of the modem functions traditionally stories” from our investigation. Section 8 discusses possi- performed on the modem card. Two types of software- bilities for further related research. Section 9 relates some based modems have emerged [Dell 99]: industry perspectives on the findings of the study. And 1. Controllerless modems (alsoknownaswinmodems or Section 10 presents our conclusions. Then, following the linmodems), which perform and interpret the standard Acknowledgements and References, Appendix A presents attention (AT) commands on the main processor. Sig- some supplemental results. nal modulation, A/D, and D/A are implemented by hardware on the modem card. 2. Background 2. Soft modems, which perform signal processing (as 2.1 Modem Taxonomy well as AT commands) on the host CPU, unlike both A modem is a peripheral device that enables comput- regular and controllerless modems. Modem data buff- ers to communicate with each other over conventional ers may reside in host memory. Soft modems still have phone lines. The term modem stands for Modula- hardware-based A/D and D/A converters. tor/Demodulator. The purpose of a modem is to convert Today, the software-based analog modems are very (modulate) the digital signal that a computer understands common on the new computer systems, both for worksta- into an analog signal that can be carried over a phone line, tions and especially for laptops. Some of the reasons for and to re-convert (demodulate) the analog into a digital their success are low cost, low power consumption, and signal at the other end [Anderson et al. 96]. Demodulation maximum upgrade flexibility. Drawbacks are high CPU consists of digitizing analog waveforms using an A-to-D and memory usage. The scant availability of drivers for converter followed by the application of signal processing operating systems other than Windows has also contrib- algorithms. Modulation consists of a different set of signal uted to their limited use on non-Windows platforms. processing algorithms to produce a digitized waveform, 2.2 Commodity Operating Systems and Real- whichissentthroughaD-to-A converter. Time Applications Traditional modem communication standards assume General-purpose operating systems such as Windows that both ends of a data connection are linked to the public 2000, Linux, and Solaris are increasingly being used to switched telephony network (PSTN) by analog lines. This run time-dependent tasks such as audio and video proc- limits the communication bandwidth to 33.6Kbps in each essing despite good arguments against doing so [Nieh et direction [3Com 98]. By assuming that one of the end- al. 93]. This is the case even though many such systems, points is connected digitally to the PSTN (like most Inter- and Windows 2000 in particular, were designed primarily net Service Providers are), modern modems are able to to maximize aggregate throughput and to achieve ap- achieve speeds of up to 56Kbps downstream and proximately fair sharing of resources, rather than to pro- 33.6Kbps upstream using the V.90 protocol [ITU 98].