THIN CLIENTS

THIN CLIENTS

For many years, the main computing model has oscillated between centralized and decentralized computing. The first computers, such as the ENIAC, were, in fact, personal computers, albeit large ones, because only one person could use one at once. Then came timesharing systems, in which many remote users at simple terminals shared a big central  computer. Next came the PC era, in which the users had their own personal computers again.

While the decentralized PC model has advantages, it also has some severe disadvantages that are only beginning to be taken seriously. Probably the biggest problem is that each PC  has a large hard disk and complex software that must be maintained. For instance, when a new release of the operating system comes out, a great deal of work has to be done to  perform the upgrade on each machine separately. At most corporations, the labor costs of doing this kind of software maintenance dwarf the actual hardware and software costs. For home users, the labor is technically free, but few people are capable of doing it correctly and fewer still enjoy doing it. With a centralized system, only one or a few machines have to be updated and those machines have a staff of experts to do the work.

A related issue is that users should make regular backups of their gigabyte file systems, but few of them do. When disaster strikes, a great deal of moaning and wringing of hands  tends to follow. With a centralized system, backups can be made every night by automated tape robots. Another advantage is that resource sharing is easier with centralized systems. A system with 256 remote users, each with 256 MB of RAM will have most of that RAM idle most of the  time. With a centralized system with 64 GB of RAM, it never happens that some user temporarily needs a lot of RAM but cannot get it because it is on someone else's PC. The same  argument holds for disk space and other resources.

Finally, we are starting to see a shift from PC-centric computing to Web-centric computing. One area where this shift is very far along is e-mail. People used to get their e-mail delivered  to their home machine and read it there. These days, many people log into Gmail, Hotmail, or Yahoo and read their mail there. The next step is for people to log into other Websites to  do word processing, build spreadsheets, and other things that used to require PC software. It is even possible that eventually the only software people run on their PC is a Web  browser, and maybe not even that.

It is probably a fair conclusion to say that most users want high-performance interactive computing, but do not really want to administer a computer. This has led researchers to  reexamine timesharing using dumb terminals (now politely called thin clients) that meet modern terminal expectations. X was a step in this direction and dedicated X terminals were  popular for a little while but they fell out of favor because they cost as much as PCs, could do less, and still needed some software maintenance. The holy grail would be a high-performance interactive computing system in which the user machines had no software at all. Interestingly enough, this goal is achievable. Below we will explain one such thin client system, called THINC, developed by researchers at Columbia University (Baratto et al., 2005; Kim et al., 2006; and Lai and Nieh, 2006).

The basic idea here is to strip the client machine of all it smarts and software and just use it as a display, with all the computing (including building the bitmap to be displayed) done on  the server side. The protocol between the client and the server just tells the display how to update the video RAM, nothing more. Five commands are used in the protocol between the  two sides. They are listed in Figure 1.

The THINC protocol display commands

Let us examine the commands now. Raw is used to transmit pixel data and have them display verbatim on the screen. In principle this is the only command needed. The others are just optimizations. Copy instructs the display to move data from one part of its video RAM to another part. It is useful for scrolling the screen without having to retransmit all the data.

Sfill fills a region of the screen with a single pixel value. Many screens have a uniform background in some color and this command is used to first generate the background, after  which, text, icons, and other items can be painted.

Pfill replicates a pattern over some region. It is also used for backgrounds, but some backgrounds are slightly more complex than a  single color, in which case this command does the job.

Finally, Bitmap also paints a region, but with a foreground color and a background color. All in all, these are very simple commands, requiring very little software on the client side. All the complexity of building the bitmaps that fill the screen are done on the server. To improve efficiency, multiple  commands can be aggregated into a single packet for transmission over the network from server to client.

On the server side, graphical programs use high-level commands to paint the screen. These are intercepted by the THINC software and translated into commands that can be sent to the client. The commands may be reordered to improve efficiency. The paper gives extensive performance measurements running many common applications on servers at distances ranging from 10 km to 10,000 km from the client. In general  performance exceeded other wide-area network systems, even for real-time video.


Tags

thinc, eniac, albeit, timesharing systems, video ram