TweetFollow Us on Twitter

January 93 - OOPC Memory Management

OOPC Memory Management

Gary Odom

The November 1992 issue of Frameworks had an article by Jeff Alger about memory management in C++ with MacApp. Presented as a contrast to that article, this article describes memory management with OOPC.

Programming with objects involves allocating thousands of blocks. By the time the base classes in OOPC have been initialized, 7000 blocks have been allocated. Within a few minutes of working with a document, 20,000 blocks or more have been allocated. This is typical of object-oriented applications.

Macintosh Memory Management

The top-priority design goal for Macintosh memory management was to fully utilize available memory (space efficiency), even at the expense of other considerations, such as speed efficiency. This was sensible for the original Macintosh, a machine limited to 128 KB, but with relatively large memory demands. The chosen solution was to have relocatable blocks of memory, accessed using pointers to pointers, called handles.

The danger with handles is using them. To access a block, the handle must be dereferenced to its pointer. If the data is accessed a lot in a function, it is more efficient to dereference a handle to its pointer, and repeatedly use the pointer. The pointer is valid as long as the Memory Manager doesn't have a chance to move memory. But if you call any function, you have to know whether that function can move memory. If the function can move memory, you need to dereference the handle to get the pointer again. If memory is moved, the master pointer may change, making the pointer you were using invalid. The problem pointer is called a 'dangling pointer'.

Dangling pointers can be very hard to find, because memory may not move consistently (it depends on how tight memory is at the time). You could be writing into an invalid memory location and not discover a problem until much later. Dangling pointers are a debugging nightmare.

The Macintosh Memory Manager keeps track of every allocated block. This means memory management is optimized for a small number of blocks. With the relocatable block memory scheme, keeping track of a large number of moving blocks imposes significant overhead. This penalty is paid most in trying to allocate non-relocatable (pointer) blocks. To maintain efficient memory use, the Memory Manager always tries to allocate a non-relocatable block toward the bottom or top of the heap, so it won't get in the way during a memory shuffle. Finding space at the edge of the heap while a program is running typically involves a major memory move.

The result is that Memory Manager allocation time per block is distinctly non-linear. While handle allocation drags its tail, the time needed to allocate non-relocatable blocks is atrocious. Chart 1 below illustrates block allocation times in ticks (a tick is 1/60th of a second).

So, the Macintosh Memory Manager optimizes memory usage, but at a significant cost in speed, and with danger lurking in the form of dangling pointers.

OOPC Low-Level Memory Management

OOPC has its own memory manager, using pointers. This avoids the dangling pointer problem caused by using handles. The OOPC memory manager grabs a large chunk of memory from the heap, then slices it up for use. The first chunk is 128 KB. Other chunks are as big as needed, but at least 32 KB. (Chunk sizes are adjustable by an OOPC programmer). Chunks are locked at the top of the heap so they don't get in the way of operating system memory management. Chunks are allocated and freed as needed.

The result is pointer block allocation that does not fragment the heap. Because empty space is managed, not allocated blocks, allocation time is linear. Chart 2 below compares OOPC memory management times to Macintosh Memory Manager times. The times shown are for allocation and deallocation.At 20,000 blocks, OOPC is 20 times faster than Mac OS handles, and 100 times faster than Mac OS pointers.

Low-level memory management is implemented in OOPC using platform-independent functions, such as get_block and free_block. Handle functions (such as get_handle and free_handle) are also supported (using the Mac OS Memory Manager for the Mac version of OOPC). The OS requires handles for resources and some other tasks (such as color pixel maps).

Low-level memory management is only occasionally called directly (by an application programmer). More often, objects are created and released. (Of course, internal methods rely upon low-level memory management.) Let's look at how objects are allocated and deallocated in OOPC.

OOPC Object Memory Management

OOPC object memory management is quite simple. There are six functions/methods involved: new_object (a function), and new, bind, trash, release and empty (all methods).

The new_object function call creates an object, allocating a small amount memory required for all objects (36 bytes), plus any application data the object keeps. new_object calls the new function, which dispatches to an initial data assignment method (the method depends on the class of the object being created). After the new method finishes, an object is fully allocated and initialized.

Some objects need to keep track of other objects. A document object, for example, owns the window object that displays the document. A document object also keeps track of the pages in the document. These links between objects must be maintained.

OOPC's object system has built-in support for maintaining object links. Links between objects are established by calling bind(thisobject, thatobject). By this call, thatobject has created a link (or bond) to thisobject. thisobject cannot be released until thatobject trashes the bond between the objects by calling trash(thisobject, thatobject). Once all links to an object are trashed, the object is released by a trash (or release) call. Using bind and trash functions provides simple garbage collection, preventing objects from being released prematurely (and risking access to 'dead' objects).

release releases an object, but only if there are no links to other objects. When an object is released, empty is called (by release) to deallocate any data allocated in the new method. The release method disposes of the object itself.

Conclusion

Having its own low-level memory management gives a significant speed boost to OOPC-built applications and keeps programming chores simple. At the high level, built-in support for object linking provides garbage collection and safety with object access.
 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Check out the new Pirate Attack update i...
Love pirates and board games? Well, you'll love the new Pirate Attack themed update that just launched in Game of Dice. It adds a bunch of new content themed around pirates, like an all new event map based on a pirate ship which revamps the toll... | Read more »
Splash Cars guide - How to paint the tow...
Splash Cars is an arcade driving game that feels like a hybrid between Dawn of the Plow and Splatoon. In it, you'll need to drive a car around to repaint areas of a town that have lost all of their color. Check out these tips to help you perform... | Read more »
The best video player on mobile
We all know the stock video player on iOS is not particularly convenient, primarily because it asks us to hook a device up to iTunes to sync video in a world that has things like Netflix. [Read more] | Read more »
Four apps to help improve your Super Bow...
Super Bowl Sunday is upon us, and whether you’re a Panthers or a Broncos fan you’re no doubt gearing up for it. [Read more] | Read more »
LooperSonic (Music)
LooperSonic 1.0 Device: iOS Universal Category: Music Price: $4.99, Version: 1.0 (iTunes) Description: LooperSonic is a multi-track audio looper and recorder that will take your loops to the next level. Use it like a loop pedal to... | Read more »
Space Grunts guide - How to survive
Space Grunts is a fast-paced roguelike from popular iOS developer, Orange Pixel. While it taps into many of the typical roguelike sensibilities, you might still find yourself caught out by a few things. We delved further to find you some helpful... | Read more »
Dreii guide - How to play well with othe...
Dreii is a rather stylish and wonderful puzzle game that’s reminiscent of cooperative games like Journey. If that sounds immensely appealing, then you should immediately get cracking and give it a whirl. We can offer you some tips and tricks on... | Read more »
Kill the Plumber World guide - How to ou...
You already know how to hop around like Mario, but do you know how to defeat him? Those are your marching orders in Kill the Plumber, and it's not always as easy as it looks. Here are some tips to get you started. This is not a seasoned platform... | Read more »
Planar Conquest (Games)
Planar Conquest 1.0 Device: iOS Universal Category: Games Price: $12.99, Version: 1.0 (iTunes) Description: IMPORTANT: Planar Conquest is compatible only with iPad 3 & newer devices, iPhone 5 & newer. It’s NOT compatible with... | Read more »
We talk to Cheetah Mobile about its plan...
Piano Tiles 2 is a fast-paced rhythm action high score chaser out now on iOS and Android. You have to tap a series of black tiles that appear on the screen in time to the music, being careful not to accidentally hit anywhere else. Do that and it's... | Read more »

Price Scanner via MacPrices.net

12-inch 1.2GHz Silver Retina MacBook on sale...
B&H Photo has the 12″ 1.2GHz Silver Retina MacBook on sale for $1399 including free shipping plus NY sales tax only. Their price is $200 off MSRP, and it’s the lowest price for this model from... Read more
iPads on sale at Target: $100 off iPad Air 2,...
Target has WiFi iPad Air 2s and iPad mini 4s on sale for up to $100 off MSRP on their online store for a limited time. Choose free shipping or free local store pickup (if available). Sale prices for... Read more
Target offers Apple Watch for $100 off MSRP
Target has Apple Watches on sale for $100 for a limited time. Choose free shipping or free local store pickup (if available). Sale prices for online orders only, in-store prices may vary: - Apple... Read more
Apple refurbished 2014 13-inch Retina MacBook...
Apple has Certified Refurbished 2014 13″ Retina MacBook Pros available for up to $400 off original MSRP, starting at $979. An Apple one-year warranty is included with each model, and shipping is free... Read more
Macs available for up to $300 off MSRP, $20 o...
Purchase a new Mac or iPad using Apple’s Education Store and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free, and... Read more
Watch Super Bowl 50 Live On Your iPad For Fre...
Watch Super Bowl 50 LIVE on the CBS Sports app for iPad and Apple TV. Get the app and then tune in Sunday, February 7, 2016 at 6:30 PM ET to catch every moment of the big game. The CBS Sports app is... Read more
Two-thirds Of All Smart Watches Shipped In 20...
Apple dominated the smart watch market in 2015, accounting for over 12 million units and two-thirds of all shipments according to Canalys market research analysts’ estimates. Samsung returned to... Read more
12-inch 1.2GHz Retina MacBooks on sale for up...
B&H Photo has 12″ 1.2GHz Retina MacBooks on sale for $180 off MSRP. Shipping is free, and B&H charges NY tax only: - 12″ 1.2GHz Gray Retina MacBook: $1499 $100 off MSRP - 12″ 1.2GHz Silver... Read more
12-inch 1.1GHz Gray Retina MacBook on sale fo...
B&H Photo has the 12″ 1.1GHz Gray Retina MacBook on sale for $1199 including free shipping plus NY sales tax only. Their price is $100 off MSRP, and it’s the lowest price available for this model... Read more
Apple now offering full line of Certified Ref...
Apple now has a full line of Certified Refurbished 2015 21″ & 27″ iMacs available for up to $350 off MSRP. Apple’s one-year warranty is standard, and shipping is free. The following models are... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
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* Subject Matter Expert - Experis (Uni...
This position is for an Apple Subject Matter Expert to assist in developing the architecture, support and services for integration of Apple devices into the domain. Read more
*Apple* Macintosh OSX - Net2Source Inc. (Uni...
…: * Work Authorization : * Contact Number(Best time to reach you) : Skills : Apple Macintosh OSX Location : New York, New York. Duartion : 6+ Months The associate would Read more
Computer Operations Technician ll - *Apple*...
# Web Announcement** Apple Technical Liaison**The George Mason University, Information Technology Services (ITS), Technology Support Services, Desktop Support Read more
Restaurant Manager - Apple Gilroy Inc./Apple...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.