TweetFollow Us on Twitter

Memman
Volume Number:7
Issue Number:9
Column Tag:Programmer's Forum

Related Info: Memory Manager

A Memory Manager for the Rest of US

By Jordan Zimmerman, Pacific Grove, CA

A Memory Manager for the Rest Of Us: The Evolution of Portable Virtual Memory

[Jordan Zimmerman lives in Burbank, California where he drinks fresh ale, plays Smash T.V. and writes Movie Magic Scheduling for Screenplay Systems, Inc.]

Introduction

Our story begins with a humble Macintosh programmer faced with what is becoming the issue of the 90’s: there are people in the world who insist on having windows on their blue boxes.

I was confronted with the task of reconciling the different memory management schemes of the Macintosh and Windows 3.0. In the process of solving this, a memory management scheme was developed that would be useful regardless of the porting issues. This memory manager automatically handles virtual memory (without the need for System 7 or a PMMU), is portable, and traps a multitude of errors and crashes caused by mistakes in memory usage.

While the full-blown manager is beyond the scope of this article; what follows is an outline that should be all one needs to write such a manager.

At this point, I must give due credit to my co-workers Ken Smith and Mark Guidarelli who helped design our Memory Manager, Memman.

In the Beginning

It has been my experience that the Windows 3 API (Application Programming Interface) is less flexible than the Macintosh’s. A perfect example is the respective memory managers.

While both platforms use the label “Handle” for their basic type of memory, they are really very different animals. On the Mac, a Handle points to a real location in memory. At that location there is another pointer that points to your data. Once the OS returns an allocated Handle, you are free to use it at will - you don’t need to check with the OS before using it (except, of course, to lock it).

Under Windows, the Handle it returns is merely a reference. It doesn’t point to any real memory. In order to get a pointer to real memory, you have to go through an OS call. When you are done using the real memory, you make another call to let the OS know you’re through.

The restrictions of the Windows model didn’t give us a lot of choice. Ultimately, it made a lot more sense to try to fit the Mac model into the Windows model than it did to try it the other way around.

And Then There Was Light

It quickly became apparent how much control the memory manager would have, given the constraints on the user of the manager. I could count on several things:

a) I’d know whenever memory was or wasn’t being used;

b) I’d have knowledge of every allocation made; and

c) I could do whatever I wanted with the data of an allocation when it wasn’t being used so long as I restored its condition before it was needed again.

The Window’s Model

Windows has five basic memory routines. They are:

a) GlobalAlloc() - allocates a block of memory;

b) GlobalReAlloc() - changes the size of an allocation;

c) GlobalLock() - returns a real pointer to memory;

d) GlobalUnLock() - signals that real memory is done being used; and

e) GlobalFree() - disposes of an allocation.

So, it seemed simple enough to fit the Macintosh memory model into Windows’ - just put wrappers around all memory calls.

Memman is born

This is the basic structure of Memman:

/* 1 */

#ifdef windows
typedef cookie_t HANDLE
#elif macintosh
typedef cookie_t Handle
#endif

cookie_t MMAlloc(long size);
void MMRealloc(cookie_t hdl, 
 long new_size);
void *MMUse(cookie_t hdl);
void MMUnuse(cookie_t hdl);
void MMFree(cookie_t hdl);

The cookie_t is what we call around the office a “magic cookie” - a reference to something that is unusable by itself. In Memman, the magic cookie is a Handle on the Macintosh and a HANDLE on Windows. But that doesn’t really matter to the user of the manager.

The Memman model ports perfectly between the two platforms:

MEMMAN Macintosh Windows

-------------------------------------------------

MMAlloc NewHandle GlobalAlloc

MMRealloc SetHandleSize GlobalReAlloc

MMUse HLock GlobalLock

MMUnuse HUnlock GlobalUnLock

MMFree DisposHandle GlobalFree

Memman imposes some constraints on a program that Macintosh programmers won’t be used to.

Before you read to or write from memory, you MUST call MMUse() to get a real pointer to memory. When you are through reading/writing, you MUST call MMUnuse(). This is a very different way of coding. The program becomes a “client” of the Operating System. On the Mac, it’s somewhat the other way around normally.

MMUse() can be unlimitedly nested. However, for every MMUse(), there must be an MMUnuse() eventually.

Here’s an example:

/* 2 */

/* the Mac way of allocating memory and
then writing to it */
. . .
short   **short_array;
short   *short_ptr;

short_array = (short **)NewHandle(10 * 
 sizeof(short);
short_ptr = *short_array;
short_ptr[1] = 1;
short_ptr[2] = 2;
...

/* now, the Memman way */
. . .
cookie_treference;
short   *short_ptr;

reference = MMAlloc(10 * sizeof(short));
short_ptr = (short *)MMUse(reference);
short_ptr[1] = 1;
short_ptr[2] = 2;
/* etc. */
MMUnuse(reference);
. . .

Where’s the VM Beef?

So how does this get us Virtual Memory? Given the control that Memman has over memory allocation and usage, Virtual Memory becomes somewhat simple.

What is Virtual Memory, Anyway?

Virtual Memory is a technique that allows an application to access more memory than is physically present in the system. Data is paged to and from disk as needed, thus giving the appearance of more memory than is really available.

Under System 7, this is done on a hardware level by the Paged Memory Management Unit (PMMU). This is the fastest and most desirable way to implement Virtual Memory. But there is nothing stopping the lowly software programmer from doing it manually.

Today’s operating systems provide sophisticated disk I/O and memory managers. These are all a programmer needs to do Virtual Memory.

Memman knows about every allocation that is made. It also knows whenever an allocation is or isn’t being used. So, the first thing to do is to keep track of every allocation made through MMAlloc().

/* 3 */

typedef longhdl_t;

typedef struct {
 cookie_t platform_hdl;
 long   size;
 void   *ptr;
 short  access_cnt;
} alloc_rec;

Memman keeps an array of alloc_recs. Every time MMAlloc() is called, an entry into this array is stored. platform_hdl is a Handle on the Mac or a HANDLE on Windows. Because there is no equivalent to the Mac’s GetHandleSize() on Windows, size stores the size of the allocation.

Instead of MMAlloc() returning a cookie_t, Memman defines its own “magic cookie”, hdl_t. This is an offset into the array of alloc_recs.

ptr is NULL if the allocation isn’t currently being “used” (i.e. MMUse() hasn’t been called) or a real memory location if it is being used. This is done as an optimization. If MMUse() is called in a nested way, there is no need to go through the OS (HLock() or GlobalLock() ) to get a pointer.

access_cnt is the number of unbalanced times MMUse() has been called for the allocation. This is how Memman determines if an allocation is in use or not. When allocated, the access_cnt is set to zero. Every time MMUse() is called, it is incremented by one. Every time MMUnuse() is called it is decremented by one. When the access_cnt is zero, Memman knows that the allocation is not being used.

It is the knowledge of when an allocation is in use or not that allows us to do VM. When an allocation isn’t in use, its data can be stored on disk (however, you’d probably only want to do this when memory is tight). Let’s change alloc_rec a little.

/* 4 */

typedef longhdl_t;

typedef struct {
 cookie_t platform_hdl;
 long   size;
 void   *ptr;
 short  access_cnt;
 long   location;
} alloc_rec;

Memman uses the location field to determine whether or not an allocation is in memory or on disk. MMUse() is responsible for reading in a paged allocation. If location >= 0, then the allocation’s data is on disk; otherwise, location == -1.

A simple implementation of MMAlloc(), MMUse() and MMUnuse() for the Mac might look like this:

/* 5 */

alloc_rec **alloc_array;

hdl_t MMAlloc(long size)
{

 alloc_rec*alloc_ptr;
 Handle h;
 long   old_size;
 hdl_t  hdl;

 /* get some real memory from the OS */
 h = NewHandle(size);
 if ( MemError() )
 DoError();

 /* add another alloc_rec */
 old_size = GetHandleSize(alloc_array);
 SetHandleSize(alloc_array,old_size + 
 sizeof(alloc_rec));
 if ( MemError() )
 DoError();

 /* get the index into the array */
 hdl = old_size / sizeof(alloc_rec);
 alloc_ptr = (*alloc_array)[hdl];

 /* store away the information */
 alloc_ptr->platform_hdl = h;
 alloc_ptr->size = size;
 alloc_ptr->ptr = NULL;
 alloc_ptr->access_cnt = 0;
 alloc_ptr->location = -1;/* in memory */

 return hdl;

} /* MMAlloc */

void *MMUse(hdl_t hdl)
{

 alloc_rec*alloc_ptr;
 void   *ptr;

 /* hdl is an index into the array of alloc_recs */
 HLock(alloc_array);
 alloc_ptr = (*alloc_array)[hdl];

 /* make sure it’s in memory */
 if ( alloc_ptr->location >= 0 )
 load_from_disk(alloc_ptr);

 /* increment the access_cnt and lock the Handle if necessary */
 if ( ++alloc_ptr->access_cnt > 1 )
 ptr = alloc_ptr->ptr;
 else {
 HLock(alloc_ptr->platform_hdl);
 ptr = *alloc_ptr->platform_hdl;
 }

 HUnlock(alloc_array);

 return ptr;

} /* MMUse */

void MMUnuse(hdl_t hdl)
{

 alloc_rec*alloc_ptr;

 alloc_ptr = (*alloc_array)[hdl];

 if ( --alloc_ptr->access_cnt > 0 )
 return;/* handle is still in use, keep it locked */

 alloc_ptr->ptr = NULL;

 HUnlock(alloc_ptr->platform_hdl);

} /* MMUnuse */

Memman opens a temp file that stores any paged data. Memman defines a function, MMPage(), that is used to page data to disk. This would probably be called from the GrowZone or could be setup to be called automatically by MMAlloc() (if NewHandle() failed).

Here’s a simple implementation of MMPage():

/* 6 */

/* page out “needed” bytes of data */
void MMPage(long needed)
{

 alloc_rec*alloc_ptr;
 long   total = 0;
 long   i;
 long   size;

 size = GetHandleSize(alloc_array);
 HLock(alloc_array);

 alloc_ptr = *alloc_array;

 /* go through all allocations paging them out until total >= needed 
*/
 for ( i = 0; i < size; ++i  ) {
 if ( alloc_ptr->location == -1 ) {
 long   offset;

 offset = get_disk_block(alloc_ptr->size);
 write_data(alloc_ptr->platform_hdl,
 alloc_ptr->size,offset);
 alloc_ptr->location = offset;
 DisposHandle(alloc_ptr->platform_hdl);
 
 if ( (total += alloc_ptr->size) >= needed )
 break;
 }
 }

 HUnlock(alloc_array);

} /* MMPage */

You might consider writing MMPage() so that it pages allocations in a “least recently used” fashion. The way Memman does this is by keeping a field (a short) in the alloc_rec that is incremented every time MMUse() is called on the allocation. Allocations with the smallest “time stamp” are the oldest and are paged first. This reduces the likelihood of a lot of swapping to and from disk because an allocation is paged and then read back in, etc.

Ideally, you’ll keep track of any “free” blocks within your temp file and reuse these (a free block is one to which data was paged and then re-read into memory; thus, the block is no longer being used).

Debugging - The Best Benefit

The final benefit of Memman is the automatic debugging it provides. There are several debugging tools that can be built into this memory model.

The first is inherent in the design: Handles are always locked when they are being used. It is a common plague of the Macintosh that a lot of bugs are caused by unlocked handles. With Memman, this is no longer an issue.

The other tools must be added to the memory manager. The following is a list of things we’ve added to Memman at the office. It is by no means an exhaustive list. It seems we are always finding new debugging code to add. You should surround all your debugging code with

/* 7 */

#ifndef NDEBUG
...
#endif

so that it can be turned off easily for the shipping product.

Overdraft Protection

We’ve changed MMAlloc() so that it always allocates 2 bytes more than requested. These two bytes are then set to some unlikely value like 0x1234. Every time MMUse() or MMUnuse() are called, the last two bytes of the allocation are checked and Memman asserts if the value isn’t 0x1234. This catches those pesky bugs where the program writes past the end of an allocation (at a resolution that even Protected Memory can’t achieve!).

Corruption Police

Our Memman has an extra field (a short) in the alloc_rec. This field is used to store a checksum of the data. A checksum of the allocation’s data is stored at MMUnuse() time when the access_cnt gets set to zero (we use a public domain CRC routine). Whenever MMUse() is called, this checksum is verified and Memman asserts if the checksum doesn’t match. This catches memory corruption errors.

The Enforcers

Whenever MMFree() is called, every byte of the allocation’s data is set to 0xff before DisposHandle() is called. This sets up a condition that will always produce incorrect results if an allocation is accessed after it is disposed.

Whenever MMUnuse() is called and the access_cnt gets set to zero, HandToHand() is called on the allocation to duplicate it. The old Handle has every byte set to 0xff and is then disposed. This is the Memman equivalent of Heap Scrambling.

Conclusion

We are using Memman at our office. It has already proved invaluable in weeding out bugs and cleaning up the way we look at memory allocations. Our Memman has been ported to the Mac, Windows and Unix without a hitch.

Even if porting is not an issue for you, the memory model laid out in this article is valuable for any situation. Indeed, the Memman model, I believe, is ideal for every situation and has become an integral part of all the code that I currently write and plan on writing.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

MarsEdit 3.6.8 - Quick and convenient bl...
MarsEdit is a blog editor for OS X that makes editing your blog like writing email, with spell-checking, drafts, multiple windows, and even AppleScript support. It works with with most blog services... Read more
BBEdit 11.0.3 - Powerful text and HTML e...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more
Microsoft Office Preview 15.8 - Popular...
Welcome to the new and modern Microsoft Office for Mac. You will receive regular updates automatically until the official release in the second half of 2015. With the redesigned Ribbon and your... Read more
Yosemite Cache Cleaner 9.0.5 - Clear cac...
Yosemite Cache Cleaner is an award-winning general purpose tool for OS X. YCC makes system maintenance simple with an easy point-and-click interface to many OS X functions. Novice and expert users... Read more
ExpanDrive 4.3.2 - Access cloud storage...
ExpanDrive builds cloud storage in every application, acts just like a USB drive plugged into your Mac. With ExpanDrive, you can securely access any remote file server directly from the Finder or... Read more
RapidWeaver 6.0.8 - Create template-base...
RapidWeaver is a next-generation Web design application to help you easily create professional-looking Web sites in minutes. No knowledge of complex code is required, RapidWeaver will take care of... Read more
Artlantis Studio 5.1.2.7 - 3D rendering...
Artlantis Studio is a unique and ideal tool for performing very high resolution rendering easily and in real time. The new FastRadiosity engine now lets you compute images in radiosity-even in... Read more
MacUpdate Desktop 6.0.5 - Search and ins...
MacUpdate Desktop 6 brings seamless 1-click installs and version updates to your Mac. With a free MacUpdate account and MacUpdate Desktop 6, Mac users can now install almost any Mac app on macupdate.... Read more
BitTorrent Sync 2.0.82 - Sync files secu...
BitTorrent Sync allows you to sync unlimited files between your own devices, or share a folder with friends and family to automatically sync anything. File transfers are encrypted. Your information... Read more
Google Drive 1.20 - File backup and shar...
Google Drive is a place where you can create, share, collaborate, and keep all of your stuff. Whether you're working with a friend on a joint research project, planning a wedding with your fiancé, or... Read more

Bored? MyLeisure FreeTime Maximizer Will...
Bored? MyLeisure FreeTime Maximizer Will Take Care of That! Posted by Jessica Fisher on March 5th, 2015 [ permalink ] iPhone App - Designed for the iPhone, compatible with the iPad | Read more »
New Publisher Allstar Games Heads West w...
Allstar Games has announced its first mobile title designed for western audiences, Allstar Heroes. The game will be a massive online battle arena (MOBA) that offers dozens of heroes for you to collect and pit against your opponents. As each hero has... | Read more »
RAD Boarding Review
RAD Boarding Review By Jennifer Allen on March 5th, 2015 Our Rating: :: NEARLY RADUniversal App - Designed for iPhone and iPad RAD Boarding isn’t quite one of the greats, but it has potential.   | Read more »
Presenting the International Mobile Gami...
11th Annual International Mobile Gaming Awards ceremony, hosted by actress Allison Haislip, gathered mobile game developers and publishers from around the world. They chose 13 winners out of the 93 nominations. British studio USTWO won the the Grand... | Read more »
AG Drive Review
AG Drive Review By Tre Lawrence on March 5th, 2015 Our Rating: :: FUTURISTIC STREET RACING.Universal App - Designed for iPhone and iPad Futuristic racing… interstellar style.   | Read more »
GDC 2015 – Nightmare Guardians is an Int...
GDC 2015 – Nightmare Guardians is an Interesting Hybrid of MOBA and Lane Defense Posted by Rob Rich on March 5th, 2015 [ permalink ] I have to say that lane defense (i.e. | Read more »
Overkill 3 Review
Overkill 3 Review By Tre Lawrence on March 5th, 2015 Our Rating: :: WHO'S NEXT?Universal App - Designed for iPhone and iPad Cover system gameplay in the third-person.   Developer: Craneballs Price: Free Version Reviewed: 1.1.6... | Read more »
Warner Bros. Interactive Entertainment A...
Warner Bros. has some exciting games coming down the pipe! | Read more »
GDC 2015 – Star Trek Timelines will Prob...
GDC 2015 – Star Trek Timelines will Probably Make Your Inner Trekkie Squeal With Glee Posted by Rob Rich on March 4th, 2015 [ permalink ] Any popular fictional universe has its fair share of fan fiction – where belo | Read more »
Protect Yourself from an Onslaught of Ca...
Surprise Attack Games has announced a Cat-astrophic new physics puzzler called Fort Meow! In the game, a young girl named Nia finds her grandfather’s journal which triggers an all mighty feline attack! Why do the cats want the journal? Who knows,... | Read more »

Price Scanner via MacPrices.net

Apple restocks refurbished 15-inch Retina Mac...
The Apple Store has restocked Apple Certified Refurbished 2014 15″ Retina MacBook Pros, available for up to $400 off the cost of new models. An Apple one-year warranty is included with each model,... Read more
Roundup of MacBook Air sale prices, models up...
B&H Photo has MacBook Airs on sale for up to $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 11″ 128GB MacBook Air: $799 100 off MSRP - 11″ 256GB MacBook Air: $999 $100... Read more
New Firstrade Mobile App Enables On-The-Go Tr...
Firstrade Securities Inc. has announced its new mobile app, which gives investors immediate access to the company’s trading platform on all mobile devices. The app was developed in-house and was... Read more
Sonnet Introduces USB 3.0 + eSATA Thunderbolt...
Sonnet has announced the launch of its new USB 3.0 + eSATA Thunderbolt Adapter for easy connectivity to USB 3.0 devices and eSATA storage, and USB 3.0 + Gigabit Ethernet Thunderbolt Adapter for easy... Read more
Apple restocks refurbished 27-inch 5K iMacs f...
The Apple Store has restocked Apple Certified Refurbished 27″ 3.5GHz 5K iMacs for $2119 including free shipping. Their price is $380 off the cost of new models, and it’s the lowest price available... Read more
Free Clean Reader Mobile App Hides Swear Word...
The new Clean Reader app, now available in the Apple App Store and Google Play, delivers the opportunity of reading any book without being exposed to profanity. By selecting how clean they want their... Read more
Kinsa Launches “Groups” App to Monitor Illnes...
Kinsa, makers of the first FDA approved app-enabled smartphone thermometer thst won the 2013 Cleveland Clinic Medical Innovation Grand Prize and recently appeared in Apple’s “Parenthood” TV... Read more
iPad: A More Positive Outlook – The ‘Book Mys...
It’s good to hear someone saying positive things about the iPad. I’ve been trying to bend my mind around how Apple’s tablet could have gone from zero to bestselling personal computing device on the... Read more
Mac Pros on sale for up to $279 off MSRP
Amazon has Mac Pros in stock and on sale for up to $279 off MSRP. Shipping is free: - 4-Core Mac Pro: $2725.87, $273 off MSRP (9%) - 6-Core Mac Pro: $3719.99, $279 off MSRP (7%) Read more
Sale! 13-inch Retina MacBook Pros for up to $...
B&H Photo has 13″ Retina MacBook Pros on sale for up to $205 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.6GHz/128GB Retina MacBook Pro: $1219.99 save $80 - 13″ 2.... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
Position Opening at *Apple* - Apple (United...
…Summary** As a Specialist, you help create the energy and excitement around Apple products, providing the right solutions and getting products into customers' hands. You Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** The Apple Store is a retail environment like no other - uniquely focused on delivering amazing customer experiences. As an Expert, you introduce people Read more
*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.