One of the unintended side effects of my previous post on the horrible memory situation on the iPhone, was that some people pointed out it was possible to hook up a disk storage back end to the iPhone’s virtual memory system. That’s quite ironic because I think of it as compounding the already dismal situation rather than a solution or a even a stopgap measure.

As far as I’m concerned, virtual memory paging is the lazy man’s caching scheme. There, I’ve said it. Now let me qualify it a bit and justify why I feel that way.

A lot of applications, and especially games, can’t keep in memory every bit of data they need for their execution. There’s just too much data and not enough RAM to keep all the levels, all the textures, all the characters, and all the movies. Even the thought of keeping everything in memory is ridiculous. Instead, games are architected to strike a balance between memory usage and responsive interfaces. Usually that means loading levels, or parts of levels on demand, and keeping them in memory while they can be needed. Some other times it means loading levels of detail for textures, meshes, and animations depending on the player position. The point is, nobody knows as well as the game itself what needs to be in memory, what can be unloaded, and when the best time to do it is.

A different approach would be to ignore thinking about managing memory as a scarce resource and use as much of it as we need. If we ever go over the amount of available physical memory, the virtual memory system will kick in and page out memory to disk to make room for the memory we’ve requested. But there lies the problem: The virtual memory system is a lower-level system that knows absolutely nothing about our game or application. It can only make guesses about what memory is OK to evict and when it’s a good time to do it. If you’re unlucky it will choose to page out memory when you need performance the most, and it might evict a page that you’re about to use in a few milliseconds. If that happens, you can kiss your performance bye-bye.

Because of that, virtual memory paging will always do a much worse job than we could have done ourselves. But hey, it takes no effort or thinking on the part of the programmers, hence the “lazy man’s caching scheme” I was referring to earlier.

However, virtual memory paging is sometimes a necessary evil. The assumption running through the previous three paragraphs is that we, as programmers of the game, can always manage the physical memory so our game runs efficiently on it. That is true for certain platforms, usually with fixed hardware specs, and ones with minimal or no background processes running. Game consoles are perfect examples of this: We know how much RAM we have to start, we know how much the system needs, how much video memory there is, and we can deal with the rest ourselves. We’ll decide what to load and when to load it.

On something like a modern PC (whether it’s Windows, Mac, Linux, or any other flavor), we have none of those guarantees. We don’t know how much memory we’re going to encounter when we run our game, and, what’s even worse, we have no idea how that available memory is going to change during the execution of the program [1]. In a situation like that, we can plan for some minimum memory requirements, but we’re going to need that virtual memory paging to bail us out of tricky low-memory situations. It might sound like a good idea, but that’s one of the main reasons why PC games are often choppy and with inconsistent frame rates (buggy drivers being the other main reason).

Finally coming back to the iPhone, how does virtual memory paging fit there? There’s no doubt that it would help because it would reduce the number of crashes due to programs unexpectedly running out of memory. But at the same time, it would cause most games and apps to be choppy and unresponsive, especially when it’s just launched. But the iPhone is mostly a fixed spec platform, with minimal background processes [2], so we can always do better than the dumb virtual memory system. Maybe that’s not a big deal if you’re writing an app that interfaces with a web site and writes data to a database, but it’s crucial to be able to write responsible games, which are considered near real-time apps.

On the iPhone there should be no need for virtual memory paging (not to mention that it would probably drain the battery a lot faster). Instead of solving the memory problem by throwing more and more complex systems at it, what we really need is a guaranteed amount of memory for our app to run and we can take care of the rest. But Apple needs to take the first step and set that memory aside.

We’re waiting, Apple.

[1] We could go totally hard core and allocate the memory we need and then mark it as not swappable by the virtual memory system. That wouldn’t do any good because if other processes start requesting memory and they run out, the virtual memory system will swap out other pages (and possibly cause other processes to thrash) and kill our performance as well. So things are pretty much out of our hands as soon as virtual memory paging comes into play.

[2] There are a lot of background processes, but they should be relatively well-behaved. None of them should be pulling in massive amounts of data while the game is running.

My recent rant about the dismal situation of memory in the iPhone quickly became quite popular. Most people were either unaware of the situation, or completely agreed that it was a major stumbling block for any app that tries to make good use of the hardware.

As a response to that post, some people suggested some intriguing ways to increase the available memory on the iPhone. I’ve been experimenting with that a bit, but I don’t have any conclusive solutions yet. Right now it’s all totally unreliable hacks. Hopefully in a followup post I can present some solutions.

In the meanwhile, I wanted to talk about a piece of the puzzle, which is keeping track of the memory status on the iPhone. How much memory is available? How much memory does the program use? We need to be able to answer those questions accurately in order to do anything about memory. Unfortunately, there doesn’t appear to be a good answer even for this!

Finding Total Memory

Let’s start with the easy part: Finding the overall memory on the device. Right now all iPhones and iPod Touch models have 128MB RAM, so it’s kind of pointless, but it’s interesting anyway. To accomplish that we need to dig into some of the low-level functions that query hardware capabilities. In particular, the function sysctl() reports both physical memory and user memory.

    int mem;
    int mib[2];
    mib[0] = CTL_HW;
    mib[1] = HW_PHYSMEM;
    size_t length = sizeof(mem);
    sysctl(mib, 2, &mem, &length, NULL, 0);
    NSLog(@"Physical memory: %.2fMB", mem/1024.0f/1024.0f);

    mib[1] = HW_USERMEM;
    length = sizeof(mem);
    sysctl(mib, 2, &mem, &length, NULL, 0);
    NSLog(@"User memory: %.2fMB", mem/1024.0f/1024.0f);

The output of that code is:

Physical memory: 116.00MB
User memory: 91.30MB

Interesting. Physical memory is not quite reported as 128MB. That’s probably because the video memory takes up 12MB of the total. The kernel apparently uses 24.7MB, which is not unreasonable if that was all the memory used by the OS. Unfortunately, that’s only for the kernel. The different processes and apps are going to use up more than that. A LOT more than that.

Incidentally, sysctl() is a pretty rocking function. It will give you all sorts of cool information, such as CPU and bus frequency (412MHz and 103MHz respectively for my iPhone 3G), cache sizes, and whether a vector unit is present (it claims it isn’t, so it must not be referring to the vfp).

Finding Available Memory

As cool a function as it is, sysctl() only returns static information about the hardware. If we want to find information about the current status of the memory, we need to look elsewhere.

As far as I can tell, the best function to get this information host_statistics() with HOST_VM_INFO_COUNT to get virtual memory statistics. Querying that function fills out the following structure:

struct vm_statistics {
    natural_t	free_count;		/* # of pages free */
    natural_t	active_count;		/* # of pages active */
    natural_t	inactive_count;		/* # of pages inactive */
    natural_t	wire_count;		/* # of pages wired down */
    natural_t	zero_fill_count;	/* # of zero fill pages */
    natural_t	reactivations;		/* # of pages reactivated */
    natural_t	pageins;		/* # of pageins */
    natural_t	pageouts;		/* # of pageouts */
    natural_t	faults;			/* # of faults */
    natural_t	cow_faults;		/* # of copy-on-writes */
    natural_t	lookups;		/* object cache lookups */
    natural_t	hits;			/* object cache hits */

    /* added for rev1 */
    natural_t	purgeable_count;	/* # of pages purgeable */
    natural_t	purges;			/* # of pages purged */

    /* added for rev2 */
    /*
     * NB: speculative pages are already accounted for in "free_count",
     * so "speculative_count" is the number of "free" pages that are
     * used to hold data that was read speculatively from disk but
     * haven't actually been used by anyone so far.
     */
    natural_t	speculative_count;	/* # of pages speculative */
};

Lots of good info there! (at least for a hardware geek like me). The most interesting one is free_count. That number combined with the page size (which is defined in the handy global variable vm_page_size–which is unsurprisingly 4K on the iPhone) should give us the amount of available memory, right? Right?

Kind of. Unfortunately virtual memory is managed at many different levels. The OS will keep some pages on reserve to use on a rainy day. So it’s possible that there is more memory available than reported in this function. It will however give you a good minimal bound on the amount of free memory. You can count on that amount for sure.

This function puts it together to return the amount of available memory in KB:

int getAvailableMemoryInKB()
{
    vm_statistics_data_t vmStats;
    mach_msg_type_number_t infoCount = HOST_VM_INFO_COUNT;
    kern_return_t kernReturn = host_statistics(mach_host_self(),
                             HOST_VM_INFO, (host_info_t)&vmStats, &infoCount);
    if(kernReturn != KERN_SUCCESS)
        return -1;
    return (vm_page_size * vmStats.free_count) / 1024;
}

To check that available memory roughly matches what I do in the app, I allocate a 1MB block, run it again and… surprise! I get the same amount of memory available. Apparently the OS is waiting to actually set those memory pages aside until you really need them (I hate it when machines try to out-think me). So to really get them to count as allocated, we need to write to every page of the memory we just allocated. The easiest way is just go bzero(mem, 1024*1024);. Running the function again correctly shows that the available memory has gone down by 1MB.

(Not) Making Sense Of Used and Total Memory

An old habit I picked up during my engineering days is to always confirm my calculations through a different path. If the total amount of memory reported by the virtual memory system matched up with the total amount of memory reported by sysctl() I would leave this happy and be able to sleep soundly tonight.

The total amount of virtual memory is calculated by adding together the free, used, wired, and inactive pages. It should come up to be roughly around 91MB. The answer: 89MB. OK, close enough, right? Not really.

Here’s the biggest mystery so far: If I malloc a chunk of memory (and write to it to have it marked as not available), host_statistics() correctly shows that those pages are not available anymore, but they don’t show up as used, wired, or anything else!!! So simply adding free, used, wired, and inactive reports a totally bogus number that doesn’t take into account allocations from your program. What’s going on there? Obviously I’m misunderstanding something, so maybe someone with more knowledge of kernel and vm features can help me out. Otherwise I won’t be getting much sleep I’m afraid 🙂

Just to prove that I’m not crazy, here’s a code snippet and its output:

- (void)updateStatus
{
	vm_statistics_data_t vmStats;
	mach_msg_type_number_t infoCount = HOST_VM_INFO_COUNT;
	host_statistics(mach_host_self(), HOST_VM_INFO, (host_info_t)&vmStats, &infoCount);

	const int totalPages = vmStats.wire_count + vmStats.active_count +
                               vmStats.inactive_count + vmStats.free_count;
	const int availablePages = vmStats.free_count;
	const int activePages = vmStats.active_count;
	const int wiredPages = vmStats.wire_count;
	const int purgeablePages = vmStats.purgeable_count;

	NSMutableString* txt = [[NSMutableString alloc] initWithCapacity:512];
	[txt appendFormat:@"Total: %d (%.2fMB)", totalPages, pagesToMB(totalPages)];
	[txt appendFormat:@"nAvailable: %d (%.2fMB)", availablePages, pagesToMB(availablePages)];
	[txt appendFormat:@"nActive: %d (%.2fMB)", activePages, pagesToMB(activePages)];
	[txt appendFormat:@"nWired: %d (%.2fMB)", wiredPages, pagesToMB(wiredPages)];
	[txt appendFormat:@"nPurgeable: %d (%.2fMB)", purgeablePages, pagesToMB(purgeablePages)];

	NSLog(txt);
	[txt release];
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    [self updateStatus];
    const int sizeInBytes = 1024*1024;
    int* mem = (int*)malloc(sizeInBytes);
    bzero(mem, sizeInBytes);
    [self updateStatus];
}

And the output is:

Total: 22128 (86.44MB)
Available: 8124 (31.73MB)
Active: 6361 (24.85MB)
Wired: 6408 (25.03MB)
Purgeable: 359 (1.40MB)

Total: 21880 (85.47MB)
Available: 7888 (30.81MB)
Active: 6344 (24.78MB)
Wired: 6412 (25.05MB)
Purgeable: 359 (1.40MB)

Notice how the available pages go down by about 1MB but everything else stays about the same?

While we’re at it, does someone know of a better, lower-level way to allocate memory than malloc()? I’d like to just allocate memory pages directly. I looked into vm_allocate() but I wasn’t able to get the correct port rights. Anyone?

Update: Thanks to Colin for pointing out that mmap is the low-level memory allocation function I was looking for. You can allocate pages directly without going through malloc. Perfect!

Next Up…

This is a stepping stone towards the memory experiments I’m running. In the next day or so I should be able to report some good ways to clear up as much memory as possible for our apps.