How an application runs, and what gains in speed may be seen on a multi-processor (MP) Mac OS computer, can be confusing. Understanding a little of how multi-processing works with software applications will help you properly set your expectations when selecting applications designed for MP and begin using them.
“Twice the processors, not twice the speed” is the bottom line for the S900 DP systems. It’s important to understand why this is often true and yet how a dual-processor configuration can still significantly increase your productivity. Customers often expect a 2x performance increase due to the second processor. This is not the case. Adding a second processor card to the bus in no way increases the transfer rate across the bus, or the bus speed. Further, some processor performance overhead is created in splitting up the functions between the two processors (the primary processor has to manage the secondary processor in addition to everything else it has to do, so there’s a minor performance hit there).
For these reasons, customers using a DP system can expect an “up to 80% performance increase.” This is according to our literature. However, this does not mean that customers will see a guaranteed increase – this is strictly dependent on a particular application’s use of multiple processors, and performance increase(s) may be limited to only particular filters or functions within that application. In fact, some customers may see a performance decrease using applications which do not support the use of multiple processors. As stated further on, benchmarks conducted typically showed up to a 50-55% increase with Photoshop filters.
Mixing and Matching Processors
Secondary Processor cards are currently available in 180 MHz, 200 MHz, 233/225 MHz (the P233), and 250 MHz speeds. Customers who purchase an S900/225 MHz, for example, and want secondary processor capabilities may:
- Use a slower secondary, and less expensive processor, i.e. the 200MHz card (which will run “MP aware” applications like a dual 200 MHz machine, but could be removed for faster single processor performance).
- Use a faster secondary processor, i.e. the 233MHz card; which was designed with them in mind: it automatically runs at 225 MHz in a S900/225 system. (This is considered a P225/233 card.)
- Stay with a uniprocessor system, knowing that their applications’ performance may be better off with a fast single processor, if they do not often use an “MP aware” applications, such as Photoshop.
Here’s why. “There are a wide range of processor speeds which can be used together. The processor clock to bus frequency ratio is determined by the design of each individual processor card. However, the processor bus speed is dependent upon the design of the primary processor card. For instance, if the primary processor is a 225 MHz processor card and the second processor is a 200 MHz processor card, then the primary processor will run at 225 MHz clock frequencey at a bus speed of 45 MHz. The second processor will use the 45 MHz system bus speed determined by the primary processor at a ratio of 4:1 which will yield an actual operating speed of 180 MHz.” For a table of actual processor speed yields with multiple processors, see the conclusion of this KnowledgeBase record.
Summary
Multi-processing is a relatively new, although promising technology for the desktop market, and isn’t for everyone. There are customers who will be better off with a single processor. As long as we understand the benefits, and more importantly, the limitations of multiple processing, our customers will be better for it.
Usually, confusion and concern over MP performance is related to Photoshop performance. The MP plug-in must be correctly loaded, and even with this as the case, the customer will not see a 2x performance increase. A recent MacWeek test of an Unsharp Mask filter of a Photoshop 50 MB file (192 MB RAM application memory) showed a 55% speed increase with a dual-processor system (200 MHz) over a single-processor system (225 MHz). The actual render times were 31.43 seconds as opposed to 48.75 seconds. To get the most from Photoshop you typically need to add RAM and make other configuration changes. Adobe recommends at least 3 times more RAM partitioned to Photoshop than the size of the image used, AND, with MP, Adobe says you should use over 5.5 times more RAM i.e. for a 20 MB image an S900/200 needs 110 MB of RAM allocated to Photoshop and probably at least 132 MB in the machine. What Adobe has done in Photoshop is concentrate its MP programming resources on certain filters where using both processors will demonstrate the most noticeable gains to the user. UnSharp Masks, Small Increment Rotation and Gaussian Blurring are examples of filters that may be noticeably faster when operating with multi-processors.
A recent MacUser test of an Unsharp Mask filter applied to a 50 MB Photoshop file (with 192 MB RAM allocated to Photoshop) showed a 55% speed increase with a 200 MHz dual-processor system over a 225 MHz single-processor system. A test of ElectricImage demonstrated an 80% increase in a dual-processor configuration.
We recommend that prospective customers contact their local Umax Computer Corporation dealer in order to perform tests with single and dual processors using customer-supplied sample files. This is the only reliable way for a customer to make a fully informed decision.
Secondary and Primary Processor Speed Interrelationships (S900 and S910 Only)
In a dual-processor system, the speed of the secondary processor card is determined by the processor/bus speed ratio of the system – which is, itself, determined by the speed of the primary processor. Therefore, while it is (usually) possible to mix and match primary and secondary processors of different speeds, only identical primary and secondary processor speeds will yield optimal results.
For example, if the primary processor runs at 225 MHz and the secondary processor runs at 200 MHz, then the primary processor will establish a 5:1 processor/bus speed ratio for an actual operating bus speed of 45 MHz for the system, and the secondary processor will yield an actual operating speed of 180 MHz (from the 4:1 bus ratio, which is the optimal ratio at which the two processors can interact).
The following table lists the combinations of processor speeds which can be used together and the actual operating speeds of the secondary processor resulting from each individual combination. Note that secondary processors usually only acheive their optimal speed when paired with a primary processor of the same speed.
S900/S910 Primary and Secondary Processor Speed Combinations
| Primary Processor Speed | Primary Processor Clock/Bus Ratio | Primary Processor Bus Speed | ASPD Secondary Processor Speed | ASPD Secondary Processor Bus Ratio | Actual Speed of ASPD Secondary Processor |
| 150 MHz | 3:1 | 50 MHz | N/A | N/A | N/A |
| 180 MHz | 7:2 | 51.428 MHz | 180 MHz | 7:2 | 180 MHz |
| 200 MHz | 4:1 | 50 MHz | 180 MHz | 7:2 | 175 MHz |
| 200 MHz | 4:1 | 50 MHz | 200 MHz | 4:1 | 200 MHz |
| 200 MHz | 4:1 | 50 MHz | 233 MHz | 5:1 | 225 MHz |
| 225 MHz | 5:1 | 45 MHz | 180 MHz | 7:2 | 156 MHz |
| 225 MHz | 5:1 | 45 MHz | 200 MHz | 4:1 | 180 MHz |
| 225 MHz | 5:1 | 45 MHz | 233 MHz | 5:1 | 225 MHz |
| 233 MHz | 5:1 | 46.6 MHz | 180 MHz | 7:2 | 163 MHz |
| 233 MHz | 5:1 | 46.6 MHz | 200 MHz | 4:1 | 186 MHz |
| 233 MHz | 5:1 | 46.6 MHz | 233 MHz | 5:1 | 233 MHz |
| 250 MHz | 5:1 | 50 MHz | 200 MHz | 4:1 | 200 MHz |
| 250 MHz | 5:1 | 50 MHz | 250 MHz | 5:1 | 250 MHz |
This article was originally posted in the SuperMac Knowledge Base at http://lowendmac.com/1999/multi-processing-benefits-of-a-secondary-processor/
copyright©1999 UMAX Technologies, Inc. All Rights Reserved.
