TweetFollow Us on Twitter

December 95 - Balance Of Power: Advanced Performance Profiling

Balance Of Power: Advanced Performance Profiling

Dale Evans

There's little that compares to diving headfirst toward the ground at 120 miles per hour. I may have been going even faster when I last went skydiving. Tucking my arms in tightly, with my head back and legs even, I heard a deafening roar from the wind as I sped toward terminal velocity. "Terminal" would have been a good word for the situation if it weren't for the advances that have been made in parachute technology.

Parachutes have come a long way since their debut, when they were billowy round disks of silk sewn with simple cords stretching to a harness. They were greatly improved when the square parachute was invented thirty years ago. The square parachutes look like an airplane's wing, and they create lift in much the same way. Until recently, however, square parachutes weren't improved upon much. Perhaps their superiority over round parachutes left everyone satiated. That lack of progress was unfortunate; if recent improvements -- like many-celled parachutes and automatic activation devices -- had been pursued many years ago, skydiving would be even safer today.

The moral from this is to question satisfaction, and that will be our mantra for this column. In particular, I want you to question the performance gains you've seen by moving to native PowerPC code. In this column we'll look at improved tools for examining PowerPC code performance, and you'll see how such questioning can really enlighten you.

ILLUSIONS

The PowerPC processors can issue multiple instructions at once. You therefore may think they'll tear through your code, executing many instructions per cycle. While this is sometimes true, a number of hurdles keep the PowerPC processors from completing even one instruction per cycle. These hurdles include instruction cache misses, data cache misses, and processor pipeline stalls.

What may surprise you is how often the processor sits idle because of these hurdles. I did some tests and found that while opening new windows in one popular application, a Power Macintosh 8500's processor completed an average of only one instruction for every two cycles. This is not very efficient, considering its PowerPC 604 processor can complete up to four instructions per cycle.

Much of that inefficiency is from instruction and data cache misses. As PowerPC processors reach faster clock rates, these cache misses will have an increasing impact. By minimizing cache misses we could realize a significant performance improvement.

Simply recompiling your 680x0 code to native PowerPC code doesn't typically generate efficient code. Many designs and data structures for the 680x0 architecture work very poorly when ported to PowerPC code. When you port native, you should carefully examine your code. Tuning for a cached RISC architecture is very different than for the 680x0 family. Here are some important things to consider:

  • Redesign your data structures. Use long word-sized elements. Keep commonly used elements together, and keep everything aligned on double long word boundaries.

  • Keep results in local variables, instead of recomputing or calling subroutines to retrieve global variables.

BETTER PROFILING

Until recently you couldn't measure cache misses unless you had a logic analyzer or other expensive hardware. The PowerPC 604 processor, however, includes an extremely useful performance measurement feature: two special registers (plus a register to control them) that can count most events that occur in the processor. Each of these registers can count about 20 events, and there are five basic events that both registers can count.

Here are just a few examples of what you can count with these registers: integer instructions that have completed; mispredicted branch instructions; data cache misses; and floating-point instructions that have been issued.

To use the performance profiling that the PowerPC 604 processor provides, you'll need to have one of the newer Macintosh models that include this processor, such as the Power Macintosh 9500 or 8500. This will cost less than a logic analyzer yet allow you to get detailed performance profiles.

Although these registers will show your software's performance only on a 604-based Power Macintosh, your software's cache usage and efficiency should be similar on other PowerPC processors. Use the 604's special abilities to profile your code and you'll benefit on all Power Macintosh models.

For more accurate performance measurements, you may want to use the DR Emulator control panel, which is provided on this issue's CD. With this control panel you can turn off the dynamic recompilation feature of the new emulator; this feaure, which is described in the Balance of Power column in Issue 23, can affect the performance of your tests over time.

    Also provided on the CD is the POWER Emulator control panel. This control panel lets you turn off the Mac OS support for RS/6000 POWER instructions and thus check for these instructions in your code (they'll cause a crash).*

THE 4PM PERFORMANCE TOOL

To use the new 604 performance registers, you don't need to program in PowerPC assembly language. On this issue's CD we've included a prototype application called 4PM. This tool, which was developed by engineer Tom Adams in Apple's Performance Evaluation Group, uses the PowerPC 604-specific registers to provide various types of performance data.

4PM is very simple to use. It presents three key menus: Control, Config, and Tests, as shown in Figure 1. You use these menus to select the type of performance measurement and an application you'd like to run the tests on. The application you're testing is launched by 4PM, and you can gather data either continuously or, using a "hot key," exactly when you want.


Figure 1. 4PM menus

Once a test completes, 4PM fills a window with the results -- a tabular summary with a different test run on each line. The Save command in the File menu will write the results to a file of type 'TEXT'.

The Control menu. Use the Launch command in this menu to select an application and run it, gathering the test data specified with the Config or Tests menu. The default configuration will measure cycles and instructions completed between when the application launches and when it quits. The Launch Again command simply relaunches the last application you tested.

Check Use Hotkey if you'd like to control exactly when data is gathered. With this option, you start and stop collecting data by holding down the Command key while pressing the Power key. (This key combination is the same way to force entry to MacsBug, which you'll be unable to do during the tests.)

The Repeats command is just a shortcut that's handy if you're repeating a test multiple times. If you specify a repeat value with this command, your test application will be relaunched that many times after you quit it.

The Intervals command allows you to collect data points at regular intervals; a dialog box offers the choices 10 milliseconds, 100 milliseconds, 1 second, or Other. Normally just a total is collected, but by specifying an interval time you'll instead receive a spreadsheet of timings. This will show what your code's performance was as the test progressed.

The Config menu. The commands in the Config menu allow you to tailor the test data by specifying exactly which events each register will count. The Count Select command lets you specify the machine states to collect data in; set this to "User Only" since you'll be tuning application code.

The Tests menu. The commands in the Tests menu are for generating typical reports. Use the calibrate command to count the five basic events that are common to both 604 performance registers, including cycles and instructions completed; with this test selected, the Launch command will run your application five times, successively counting each of these events. You can use one of the remaining tests to collect more specific measurements. The caches, load/store, execution units, and special instructions tests each generate a report for the corresponding aspect of 604 performance. The Describe command displays a window describing which events are counted in the selected test. Use the New command to create your own tests. These new tests are automatically saved; you can use the Delete command to remove any that you've added.

ASSEMBLY USAGE

If you want finer results, you should read and write to the 604 performance registers directly. This requires writing in PowerPC assembly language, but it allows you complete control over what data you'll collect for your time-critical code.

You'll be accessing three new special-purpose registers: MMCR0, PMC1, and PMC2. MMCR0 controls which events will be recorded and when exactly to record. The performance monitor counter registers, PMC1 and PMC2, are the registers in which you'll read the results. I'll give a brief summary of how to use these registers, but you'll need to read Chapter 9 of the PowerPC 604 RISC Microprocessor User's Manual for details.

MMCR0 is a 32-bit register that specifies all the options for performance measurement. Most of these options aren't important to your application profiling, and you should at first leave the high 19 bits of MMCR0 set to 0. The low 13 bits, however, specify which events you want counted in PMC1 and PMC2. Bits 19 through 25 select PMC1, and bits 26 through 31 select PMC2. See Chapter 9 of the 604 user's manual to learn which specific bits to set.

Here's an example of how to measure data cache misses per instruction:

.eq PMC1_InstructionsCompleted   2 << 6
.eq PMC2_DataCacheMisses         6
.eq MMCR0_StopAllRecording      $80000000

   li         r0, MMCR0_StopAllRecording
   mtspr      MMCR0, r0     ; stop all recording
   li         r0, 0
   mtspr      PMC1, r0      ; zero PMC1
   mtspr      PMC2, r0      ; zero PMC2
   li         r0, PMC1_InstructionsCompleted +
                PMC2_DataCacheMisses
   mtspr      MMCR0, r0     ; start recording
Notice that we load MMCR0 with only the most significant bit set to turn off all recording. This holds PMC1 and PMC2 at their current values and allows us to also zero PMC1 and PMC2 before we start recording. When you're done measuring, follow with this code:
   li         r0, MMCR0_StopAllRecording
   mtspr      MMCR0, r0    ; stop all recording
   mfspr      PMC1, r3     ; r3 is number of
                           ; instructions completed
   mfspr      PMC2, r4     ; r4 is data cache misses
Notice again that we turn off recording before reading the results. Otherwise the very act of reading the registers would affect the results; it will slow your code slightly, since the mtspr and mfspr instructions take multiple cycles to complete.

Don't record over very long periods of time, because the PMC1 and PMC2 registers can overflow. To measure over long periods, you should periodically read from the registers, add the result to a 64-bit number in memory, and clear the registers to prevent this overflow.

Don't ship any products that rely on these performance registers. They're supported only in the current 604 processor, and they're not part of the PowerPC architecture specification.

COMPLACENCY

The moral is the same as for my tale of the square parachutes: question satisfaction. Don't become complacent about the performance of your new native PowerPC applications. The profiling tools described here should help you more accurately measure and identify bottlenecks in your PowerPC code. Use that information to tune -- especially paying attention to memory usage -- and you'll be surprised how much faster your product will run. Macintosh users consistently hunger for faster computers and more responsive software; spend some serious time tuning, and they'll thank you for it.

DAVE EVANS likes to go skydiving when he can get away from his job gluing together the Mac OS software at Apple. He has gone a few times now, but he'll always cherish the memory of his first jump. Friends on the ground that day claim to have clearly heard his scream, although he was nearly a mile above them when he left the plane. On his second leap, if he hadn't opened the chute while upside down and then watched it deploy through his legs, he might have noticed more of the surrounding countryside.

Thanks to Tom Adams, Geoff Chatterton, Mike Crawford, and Dave Lyons for reviewing this column.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Adobe Animate CC 2018 18.0.1.115 - Anima...
Animate CC 2018 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Flash Professional customer). Animate CC 2018 (was Flash CC) lets you... Read more
Postbox 5.0.22 - Powerful and flexible e...
Postbox is a new email application that helps you organize your work life and get stuff done. It has all the elegance and simplicity of Apple Mail, but with more power and flexibility to manage even... Read more
Tunnelblick 3.7.4b - GUI for OpenVPN.
Tunnelblick is a free, open source graphic user interface for OpenVPN on OS X. It provides easy control of OpenVPN client and/or server connections. It comes as a ready-to-use application with all... Read more
Carbon Copy Cloner 5.0.5 - Easy-to-use b...
Carbon Copy Cloner backups are better than ordinary backups. Suppose the unthinkable happens while you're under deadline to finish a project: your Mac is unresponsive and all you hear is an ominous,... Read more
Bartender 3.0.32 - Organize your menu-ba...
Bartender lets you organize your menu-bar apps by hiding them, rearranging them, or moving them to Bartender's Bar. You can display the full menu bar, set options to have menu-bar items show in the... Read more
Adobe Lightroom Classic CC 7.1 - Import,...
Adobe Lightroom is available as part of Adobe Creative Cloud for as little as $9.99/month bundled with Photoshop CC as part of the photography package. Lightroom 6 is also available for purchase as a... Read more
Ortelius 2.0.8 - Vector drawing app espe...
Ortelius is a full-featured vector drawing application especially for map design. Draw directly with features such as roads, rivers, coastlines, buildings, symbols and contours. Ortelius is known for... Read more
Tunnelblick 3.7.4b - GUI for OpenVPN.
Tunnelblick is a free, open source graphic user interface for OpenVPN on OS X. It provides easy control of OpenVPN client and/or server connections. It comes as a ready-to-use application with all... Read more
Carbon Copy Cloner 5.0.5 - Easy-to-use b...
Carbon Copy Cloner backups are better than ordinary backups. Suppose the unthinkable happens while you're under deadline to finish a project: your Mac is unresponsive and all you hear is an ominous,... Read more
Postbox 5.0.22 - Powerful and flexible e...
Postbox is a new email application that helps you organize your work life and get stuff done. It has all the elegance and simplicity of Apple Mail, but with more power and flexibility to manage even... Read more

Latest Forum Discussions

See All

Amazing Katamari Damacy guide - beginner...
Amazing Katamari Damacy brings the bizarro world of the original games to mobile and shifts them into an endless format that's just as addictive as the PlayStation entries. Your goal is still to roll as much random stuff as you possibly can, though... | Read more »
Portal Knights guide - crafting tips and...
In Portal Knights, you're only as strong as the items you have at your disposal. This sandbox adventure is all about crafting and building up the next big thing. Whether you're an avid explorer or collector, crafting will likely play a large part... | Read more »
The best deals on the App Store this wee...
A new week means new discounts on the App Store. This week's deals run the gamut of action-adventure titles, puzzle games, and one of the best narrative adventure series out there. If you're looking to fill out your mobile gaming library on a... | Read more »
What you need to know about Animal Cross...
We hope you've been hard at work on collecting all of those holiday items in Animal Crossing: Pocket Camp, because you're about to get a whole new list of fun things to do as the game receives its first big update sometime soon. There are a lot of... | Read more »
Reigns: Her Majesty guide - how to use e...
Ruling a kingdom isn't easy--doubly so for a queen whose every decision is questioned by the other factions seeking a slice of power. Reigns: Her Majesty builds on the original game's swipey tactics, adding items that you can use to move the story... | Read more »
The best new games we played this week -...
Friday has crept up on us once again, so it's time to honor the best new games we've played over the past few days. This past week was a pretty exciting one, with the debut of lots of beautiful new indies and some familiar faces returning to the... | Read more »
Portal Knights guide- beginner tips and...
Portal Knights is finally making the jump to iOS and Android, and it's already climbing the ranks to become the next big MMO experience on mobile. This sprawling sandbox game will let you pursue any adventure you wish, whether you want to sling... | Read more »
Reigns: Her Majesty guide - how to swipe...
Reigns: Her Majesty is storming the App Store this week, bringing more tinder-esque kingdom building to eager players everywhere. If you've played the original Reigns, you'll know that leading a kingdom is never easy. It's a careful balancing act... | Read more »
Getting Over It (Games)
Getting Over It 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: A game I madeFor a certain kind of person To hurt them. • Climb up an enormous mountain with nothing but a hammer and a pot.•... | Read more »
Reigns: Her Majesty (Games)
Reigns: Her Majesty 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »

Price Scanner via MacPrices.net

Apple Watch Series 2, Certified Refurbished,...
Apple has Certified Refurbished Apple Watch Nike+ Series 2s, 42mm Space Gray Aluminum Case with Anthracite/Black Nike Sport Bands, available for $249 (38mm) or $279 (42mm). The 38mm model was out of... Read more
Apple offers Certified Refurbished 2016 12″ R...
Apple has Certified Refurbished 2016 12″ Retina MacBooks available starting at $949. Apple will include a standard one-year warranty with each MacBook, and shipping is free. The following... Read more
B&H drops price on 13″ 256GB MacBook Air...
B&H has the 13″ 1.8GHz/256GB Apple MacBook Air (MQD42LL/A) now on sale for $1079 including free shipping plus NY & NJ sales tax only. Their price is $120 off MSRP, and it’s the lowest price... Read more
Holiday sale: 9″ iPads starting at $299, take...
MacMall has 9″ WiFi iPads on sale for $30 off including free shipping: – 9″ 32GB WiFi iPad: $299 – 9″ 128GB WiFi iPad: $399 Read more
Green Monday deal: 15″ 2.8GHz MacBook Pro on...
B&H Photo has the 15″ 2.8GHz Space Gray MacBook Pro on sale for $250 off MSRP for today only as part of their Green Monday/Holiday sale. Shipping is free, and B&H charges sales tax for NY... Read more
Green Monday sale: B&H offers 12″ Apple i...
B&H Photo has 12″ iPad Pros on sale for up to $150 off MSRP as part of their Green Monday/Holiday sale. Shipping is free, and B&H charges sales tax in NY & NJ only: – 12″ 64GB WiFi iPad... Read more
Holiday deal: 21″ and 27″ Apple iMacs on sale...
MacMall has 2017 21″ and 27″ Apple iMacs on sale for up to $200 off MSRP. Shipping is free: – 21″ 2.3GHz iMac: $999 $100 off MSRP – 21″ 3.0GHz iMac: $1199 $100 off MSRP – 21″ 3.4GHz iMac: $1379 $120... Read more
Holiday deal: Apple Mac minis for up to $150...
MacMall has Mac minis on sale for up to $100 off MSRP, each including free shipping: – 1.4GHz Mac mini: $399 $100 off MSRP – 2.6GHz Mac mini: $599 $100 off MSRP – 2.8GHz Mac mini: $949 $50 off MSRP... Read more
Beats by Dr. Dre – BeatsX Earphones on sale f...
Best Buy has BeatsX Earphones on sale for $109, $40 off, on their online store. Sale price for online orders only. Choose free store pickup, if available, or choose free shipping. Read more
10″ 64GB WiFi Apple iPad Pros on sale for $59...
MacMall has 10.5″ 64GB Apple iPad Pros on sale for $599 including free shipping. That’s $50 off MSRP and among the lowest prices available for these iPads from any Apple reseller. Read more

Jobs Board

QA Automation Engineer, *Apple* Pay - Apple...
# QA Automation Engineer, Apple Pay Job Number: 113202642 Santa Clara Valley, California, United States Posted: 11-Dec-2017 Weekly Hours: 40.00 **Job Summary** At Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Information Security - Security Data...
# Apple Information Security - Security Data Analyst Job Number: 113119545 Austin, Texas, United States Posted: 10-Nov-2017 Weekly Hours: 40.00 **Job Summary** This Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.