Victor Yodaiken on "What is real-time?"

Click here to learn
about this Sponsor:

Home | News | Articles | Polls | Forum

Keywords: Match:

Victor Yodaiken, of FSMLabs (Sept. 14, 2000)

Here are Victor Yodaiken's answers:

What is real-time? . . . and . . . What is hard real-time? -- Real-time software can meet timing deadlines. Then there is "soft", "firm", "usually", or "most of the time" or "kind of" real-time. These all seem to be ways of saying "not" real-time. The term "hard" is sometimes used to try to distinguish software that really is real-time from these various not really real time types of things.

RTLinux is a hard real-time operating system. We can characterize the system in terms of the worst-case delays between interrupts and start of interupt handling software and the worst case jitter in realtime threads that run periodically. Operating systems that offer "typical" interrupt response times are not hard realtime. Consider what happens if a chip is "typically" placed in the right place on a circuit board or an E-commerce transaction is "usually" restarted on a failure before the network can time out, or a rocket motor "almost always" shuts down fast enough when an abort sequence is started.
Who needs real-time? -- I'll stick to real real-time, and not worry about the other kinds. You need real-time if you want to control machinery (including communications equipment) that behaves poorly when software misses deadlines. Many types of systems can tolerate missed deadlines and in most cases average case (typical) performance is more imporant than worst case. For example, TCP is an inherently non-realtime protocol. But a robot arm goes out of position if signals come late, and it does not correct if you make the next couple of signals come early. And think about the failure detection in a cluster of computers connected by a network. If the network message system is realtime, you can detect a failure if a "heartbeat" message does not show up after a fixed interval: perhaps detecting failure in milliseconds instead of seconds. While computers are getting faster all the time, demands on computers are getting more exacting. A modern PC does not need real-time software to act as an ordinary 10BaseT router, but routing in a fault tolerant switch serving voice-over-IP is something else. Gigabit Ethernet can produce a packet every 1.5 microseconds and if you start being serious about exploiting this for industrial control or video serving, you better have software that can guarantee to not drop packets -- ever.
What is "soft real-time"? -- Another kind of "not" real-time. Some real-time systems can tolerate a certain level of failures and these are sometimes called "soft" realtime systems. For example, consumer video display can tolerate dropped frames now and then. Note, however, that there is a big difference between real-time QoS systems that can tolerate some level of failure but have absolute realtime limits and soft real-time that does not have to meeting deadlines. If there is a skip in a consumer video now and then, nobody will notice, but if you drop 5 packets in a row in a professional audio system, you may have ruined a recording.
How should real-time performance be characterized? -- Real-time performance should be characterized by worst case interrupt response times and worst case periodic jitter. Measures of how many events/time can be processed are also useful.

Panelist's bio: Dr. Victor Yodaiken is President of FSMLabs, a software development company headquartered in New Mexico and with developers in the US and Europe. Dr. Yodaiken is the creator of RTLinux. He has been working on operating systems in both industry and academia since the early 1980s when he was one of the developers of one of the first commercial distributed fault tolerant UNIX systems.

(Click here for further information)

7 Advantages of D2D Backup
For decades, tape has been the backup medium of choice. But, now, disk-to-disk (D2D) backup is gaining in favor. Learn why you should make the move in this whitepaper.

4 Legal Reasons to Control Internet Access
The Internet is obviously a valuable resource for many organizations. However, many are exposed to legal liability concerns because they fail to control Internet access. Learn if you're safe in this white paper.

Rapidly Resolve J2EE Application Problems
Whether you are in the process of building J2EE applications or have J2EE applications already running in production, you must ensure that they deliver the expected ROI. Learn how in this white paper.

Load Testing 2.0 for Web 2.0
There are many unknowns in stress testing Web 2.0 applications. Find out how to test the performance of Web 2.0 in this white paper.

Build Better Games Online
For the game infrastructure providers, life is complex. Making money from games has become more complicated. Why? Find out in this white paper.

Building a Virtual Infrastructure from Servers to Storage
This white paper discusses the virtual storage solutions that reduce cost, increase storage utilization, and address the challenges of backing up and restoring Server environments.

Gaining Faster Wireless Connections with WiMAX
Welcome to what is quickly becoming the hyperconnected world where anything that would benefit from being connected to the network will be connected. Learn more in this white paper.

Is Your Desktop a Security Threat?
The new wave of sophisticated crimeware not only targets specific companies, but also targets desktops and laptops as backdoor entryways into those business’ operations and resources. Learn how to stay safe in this white paper.

Increasing SAN Reliability by 100 Percent
Storage area networks (SAN) are a strong part of storage plans. Learn how to increase your reliability and uptime by 100 percent in this case study.

Got a HOT tip? please tell us!

Click here for a profile of each sponsor:

PLATINUM SPONSORS
<a href="http://ad.doubleclick.net/jump/entzd.linuxdevices/desktop;kw=timesys;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?"" style="border-width:0px;height:150px;width:30px;"><img src="http://ad.doubleclick.net/ad/entzd.linuxdevices/desktop;kw=timesys;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?" style="border:0; height: 150; width:30;" /></a>		<a href="http://ad.doubleclick.net/jump/entzd.linuxdevices/desktop;kw=windriver;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?"" style="border-width:0px;height:150px;width:30px;"><img src="http://ad.doubleclick.net/ad/entzd.linuxdevices/desktop;kw=windriver;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?" style="border:0; height: 150; width:30;" /></a>

<a href="http://ad.doubleclick.net/jump/entzd.linuxdevices/desktop;kw=versalogic;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?"" style="border-width:0px;height:150px;width:30px;"><img src="http://ad.doubleclick.net/ad/entzd.linuxdevices/desktop;kw=versalogic;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?" style="border:0; height: 150; width:30;" /></a>		<a href="http://ad.doubleclick.net/jump/entzd.linuxdevices/desktop;kw=winsystems;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?"" style="border-width:0px;height:150px;width:30px;"><img src="http://ad.doubleclick.net/ad/entzd.linuxdevices/desktop;kw=winsystems;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?" style="border:0; height: 150; width:30;" /></a>

<a href="http://ad.doubleclick.net/jump/entzd.linuxdevices/desktop;kw=virtuallogix;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?"" style="border-width:0px;height:150px;width:30px;"><img src="http://ad.doubleclick.net/ad/entzd.linuxdevices/desktop;kw=virtuallogix;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?" style="border:0; height: 150; width:30;" /></a>		<a href="http://ad.doubleclick.net/jump/entzd.linuxdevices/desktop;kw=trolltech;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?"" style="border-width:0px;height:150px;width:30px;"><img src="http://ad.doubleclick.net/ad/entzd.linuxdevices/desktop;kw=trolltech;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?" style="border:0; height: 150; width:30;" /></a>

<a href="http://ad.doubleclick.net/jump/entzd.linuxdevices/desktop;kw=montavista;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?"" style="border-width:0px;height:150px;width:30px;"><img src="http://ad.doubleclick.net/ad/entzd.linuxdevices/desktop;kw=montavista;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?" style="border:0; height: 150; width:30;" /></a>		<a href="http://ad.doubleclick.net/jump/entzd.linuxdevices/desktop;kw=technicalsolutions;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?"" style="border-width:0px;height:150px;width:30px;"><img src="http://ad.doubleclick.net/ad/entzd.linuxdevices/desktop;kw=technicalsolutions;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?" style="border:0; height: 150; width:30;" /></a>

GOLD SPONSORS
<a href="http://ad.doubleclick.net/jump/entzd.linuxdevices/desktop;kw=ultimate;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?"" style="border-width:0px;height:150px;width:30px;"><img src="http://ad.doubleclick.net/ad/entzd.linuxdevices/desktop;kw=ultimate;pos=top;dcopt=ist;zdid=a221772;tile=1;sz=150x30;ord=<script=JavaScript>document.write(Math.round(Math.random()10000000000));</script>?" style="border:0; height: 150; width:30;" /></a>		(Become a sponsor)

ADVERTISEMENT
(Advertise here)

Check out the latest Linux powered...

mobile phones!

other cool
gadgets

BREAKING NEWS

• Linux video camera geo-tags, writes to SATA drives
• Garmin Nav devices run Gnome Linux
• Ten LiMo phones this month?
• It's a Yankee Doodle Linux phone
• Wind River to host "Developer Day"
• Dev boards gain Linux support
• 802.11n zooms ahead
• Low-power mini-ITX board runs Linux
• Pico-ITX board bears twins
• Mass-market WiFi router invites Linux hackers
• LiMo phone specialist buys app stack
• "PDA phone" runs Linux
• ST, NXP spin phone chip JV
• Military-grade USB key supports Linux
• USB Linux systems expand

Linux-Watch headlines:
• Microsoft tactics push India toward Linux
• Bell, SuperMicro sued over GPL
• "Business intelligence" software goes GPL
• Will Atom bomb?
• LF Summit videos posted
• Linux gains "embedded" maintainers
• Virtualization on tap in SLES and RHEL upgrades
• Linux gets security black eye
• Verizon chooses Linux "platform of choice"
• Hats off to Fedora 9

Also visit our sister site:

Sign up for LinuxDevices.com's...

news feed

Home \| News \| Articles \| Polls \| Forum \| About \| Contact

	Ziff Davis Enterprise Home \| Contact Us \| Advertise \| Link to Us \| Reprints \| Magazine Subscriptions \| Newsletters Tech RSS Feeds \| White Papers \| ROI Calculators \| Tech Podcasts \| Tech Video \| VARs \| Channel News
	Baseline \| Careers \| Channel Insider \| CIO Insight \| DesktopLinux \| DeviceForge \| DevSource \| eSeminars \| eWEEK \| Enterprise Network Security \| LinuxDevices \| Linux Watch \| Microsoft Watch \| Mid-market \| Networking \| PDF Zone \| Publish \| Security IT Hub \| Strategic Partner \| Web Buyer's Guide \| Windows for Devices
	Developer Shed \| Dev Shed \| ASP Free \| Dev Articles \| Dev Hardware \| SEO Chat \| Tutorialized \| Scripts \| Code Walkers \| Web Hosters \| Dev Mechanic \| Dev Archives \| igrep
Use of this site is governed by our Terms of Service and Privacy Policy. Except where otherwise specified, the contents of this site are copyright © 1999-2008 Ziff Davis Enterprise Holdings Inc. All Rights Reserved. Reproduction in whole or in part in any form or medium without express written permission of Ziff Davis Enterprise is prohibited. Linux is a registered trademark of Linus Torvalds. All other marks are the property of their respective owners.

Home \| News \| Articles \| Polls \| Forum \| About \| Contact

	Ziff Davis Enterprise Home \| Contact Us \| Advertise \| Link to Us \| Reprints \| Magazine Subscriptions \| Newsletters Tech RSS Feeds \| White Papers \| ROI Calculators \| Tech Podcasts \| Tech Video \| VARs \| Channel News
	Baseline \| Careers \| Channel Insider \| CIO Insight \| DesktopLinux \| DeviceForge \| DevSource \| eSeminars \| eWEEK \| Enterprise Network Security \| LinuxDevices \| Linux Watch \| Microsoft Watch \| Mid-market \| Networking \| PDF Zone \| Publish \| Security IT Hub \| Strategic Partner \| Web Buyer's Guide \| Windows for Devices
	Developer Shed \| Dev Shed \| ASP Free \| Dev Articles \| Dev Hardware \| SEO Chat \| Tutorialized \| Scripts \| Code Walkers \| Web Hosters \| Dev Mechanic \| Dev Archives \| igrep
Use of this site is governed by our Terms of Service and Privacy Policy. Except where otherwise specified, the contents of this site are copyright © 1999-2008 Ziff Davis Enterprise Holdings Inc. All Rights Reserved. Reproduction in whole or in part in any form or medium without express written permission of Ziff Davis Enterprise is prohibited. Linux is a registered trademark of Linus Torvalds. All other marks are the property of their respective owners.