White Papers Was Windows 95 Made for Multitasking SCSI Host Adapters? 1.Introduction Windows 95 is making a lot of users reconsider the way they configure their PCs. Until now, 90% of the 200 million IBM compatible PC owners used single tasking IDE/EIDE disks or CD-ROM drives. Their Windows operating system could only send the disk or CD-ROM drive one request at a time. What did it matter if the CD-ROM ran ten times slower than the hard disk? Only one device could be accessed at one time. The job of the multitasking SCSI adapter, on the other hand, is to allow CD-ROMs and other peripherals to run concurrently with fast hard disks. The slower CD-ROM does not impede the fast hard disk from doing useful work. Since Windows 3.1 could not put this multitasking capability to use anyway, the choice for most PC owners was obvious. What's more, IDE or EIDE adapters came basically free; multitasking SCSI adapters cost extra. The most compelling reason for PC owners to trade up to SCSI was for multitasking operating systems such as NetWare, UNIX, or Windows NT, etc. The introduction of Windows 95 on August 24, 1995 changed the equation. We wondered how much more valuable this new multitasking operating system would make a multitasking SCSI adapter to the average PC owner. How much will performance improve, if at all? Isn't EIDE's speed right there with SCSI? Which is faster: CD-ROM, SCSI or IDE? And what about those low cost single-tasking SCSI adapters that come with scanners and printers? Aren't they fast enough? Lots of new questions to consider. We hope this article provides some answers. 2.Performance Measurements In order to investigate these questions, AdvanSys conducted performance tests. We compared four different I/O adapters. The AdvanSys ABP-5150, a multitasking SCSI host adapter that uses bus master technology to handle all instructions on-board. The Future Domain TMC-1860, a low-cost SCSI adapter with programmed I/O (PIO) technology that uses the system CPU for instructions. The Adaptec AHA-1530P, a single tasking bus master SCSI adapter, and an EIDE adapter within the Intel Neptune chipset. The EIDE connection of the Intel Neptune chipset also ran in PIO mode -- the bus master driver was not available. Six measurements were made using Windows 95 as the operating system: 1.Loading a 30 MB file to WordPAD, a word processor within Windows 95. 2.Reading 15 slides from a CD-ROM drive. Drives of 1x and 4x speeds were used. 3.Copying a 30 MB file from one hard disk to another. 4.Scanning a 850 MB hard disk. 5.Reading 15 slides while loading a 30 MB file at the same time. 6.Reading 15 slides while copying a 30 MB file at the same time The results appear in the following table. The units of measurement are seconds and MB/sec of data transfer. Naturally, the smaller number in seconds and the larger MB/sec number indicate faster results. 3.Interpreting the Results Some background on the CD-ROM and hard disk performance numbers will be helpful. A PC owner usually buys an I/O adapter, disk drive or CD-ROM device based upon the maximum instantaneous data rate specifications furnished by the manufacturer. Can you realize these performance rates in the typical PC? Generally, no. For example, if you go by the specs alone, a typical EIDE adapter has the maximum possible data rate of 13 to 16 MB/sec. The SCSI adapter specifies 10 MB/sec. Therefore, EIDE is faster than SCSI, right? Then why does the SCSI adapter provide so much more throughput than the EIDE adapter? Hmmm. In another case, a 4x CD-ROM specifies a maximum data rate of 600 KB/sec compared to only 150 KB/sec for a 1x CD-ROM. Does this mean the 4x CD-ROM is four times faster than the 1x CD-ROM? Not according to the tests. The reason is mechanical. Any CD-ROM needs several hundred milliseconds to move its actuator before accessing the data. That movement represents about 90% of the total time needed to access a CD-ROM device. Since most actuators move at similar speed, 1x and 4x CD-ROMs perform nearly alike most of the time. For much the same reason, hard disks specified for an instantaneous data rate of 7-8 MB/sec usually deliver sustained rates below 1 MB/sec. The performance measured in these tests are sustained data rates from the hard disks and CD-ROM devices. One more thing too often ignored is that device performance depends very much on every other component of your PC. Not the least of these is the operating system, be it single tasking or multitasking. So before spending money on an expensive wide SCSI adapter for your very fast hard disks, you might want to check the sustained data rate of your PC. There are special applications to maximize the data transfer from hard disks and CD-ROMs as well, but this is the topic of another article. First, lets examine the time required to load a 30 MB file into WordPAD. The four adapters completed the task in 40.3, 53.1, and 41.8 and 45.7 seconds respectively. Both PIO adapters were slower, although not bad. Note that the average data rate from the hard disk varies from 565 KB/sec to 744 KB/sec, far below the maximum possible disk media data rate. It seems we're still confined in the age of wheels and levers. Reading 15 slides provided no surprises either. The times were (49.9, 40.7), (88, 51.8), (64, 55.6), and 48.8 seconds respectively. Again, both PIO adapters were slower. But the difference was larger. The time varied from 40.7 to 88 seconds depending on the adapter and the CD-ROM used. What is more interesting is that the data rates of the 1x CD-ROM drive varies from 67 to 118 KB/sec and the 4x CD-ROM varies from 106 to 145 KB/sec. Differences that small could rehabilitate the good name of 1x drives! Copying a 30 MB/sec file from one disk to another told us a lot. First, the EIDE drives took exactly twice the time of the SCSI drives. This is because the EIDE adapter was single tasking; the two EIDE drives ran in serial rather than in parallel like the SCSI drives. Second, the PIO SCSI adapter performed about three times slower than the bus master multitasking adapter. This is because of the double jeopardy on the system CPU created by PIO and single tasking. We are beginning to see why a multitasking SCSI adapter pays off in Windows 95. Scanning a 850 MB hard disk gives us another piece of the picture. This is a very mundane task. The system will run as fast as the disk device allows, or the system allows, whichever is the weakest link. Except for the PIO SCSI product, all the adapters performed very much alike -- with data rates between 711 and 731 KB/sec. The EIDE adapter won by a hair, because it simply needs less overhead. One could even conclude that the scan rates of all the adapters were limited by the scan program itself and the Windows 95 I/O overhead. Without this overhead, scanning a disk drive sequentially should produce several MB/sec of throughput. Makes you wonder if spending more money on fast disks is worth it. The next measurement introduced two concurrent tasks under Windows 95. We loaded the same slide show while loading a 30 MB file in the background. The tasks were completed in between 46.9 and 98 seconds, depending upon the adapter and the CD-ROM used. The remarkable thing was that loading the 30 MB file only delayed the slide show by 10% to 15%. Multitasking operating systems do make a difference. In the final test, we substituted the 30 MB file copy background routine and again loaded the slide show. The two PIO adapters, both SCSI and EIDE, took extra time to complete the slide show, because the computer processor was needed to help the 30 MB file copy. The two bus master SCSI adapters completed the slide show with little or no delay. In other words, the I/O activities of the 30 MB file copy totally overlapped the slide show activities, true testimony to the value of bus master SCSI technology. The careful reader cannot help but notice that the single tasking SCSI adapter loaded the slide show faster while copying the 30 MB than it did when the slide show was being loaded alone. How could this be possible? We could not explain it either. There is so much yet to learn about Windows 95! 4.Conclusion No one argues a bus master multitasking SCSI can help a PC's performance. The question becomes, can you afford not to have a multitasking I/O adapter when you have a multitasking operating system? It boils down to total system speed and how important that is to you. Our measurements show that you can cut file copy time in half with multiple disk drives and multitasking I/O adapters. Not so apparent is that this 100% improvement in performance also holds when moving large files, either a graphic presentation or a video image. By the way, if you've read any of the published articles on Windows 95, you know it needs a large amount of memory to run efficiently. The operating system itself is so large, it constantly swaps itself in and out to a disk file, called a swapfile. Large amounts of RAM memory reduce these time-consuming swaps and improve performance. But many graphics files are still too big. By placing the swapfile on a separate disk and using a multitasking I/O adapter, you can always cut large file access time in half. Check this out yourself. The AdvanSys mission is to put high-performance multitasking I/O adapters into every PC. The introduction of the Windows 95 multitasking operating system brings us closer to the goal. Because, when we can offer an extremely fast multitasking SCSI adapter below $100 the decision is much easier.