i’m so full of ideas

cocoa: app memory usage

May 3, 2009 6:30 pm

My least favorite part of Cocoa programming is its reference-counted memory management scheme. If you can exclusively target Mac OS X 10.5 or later, then you can use garbage collection instead, which is better. But it doesn’t work on Mac OS X 10.4 or earlier or the iPhone, so garbage collection might as well not exist, as far as I’m concerned. Yes, I know Cocoa’s reference-counting scheme has “only a few simple rules” you have to follow … yet Apple’s own apps tend to leak pretty badly. Back when I was using Mac OS X 10.4, I could only run Safari for a few hours before I had to restart it. Seems to be less of a problem in Mac OS X 10.5, but they’re likely using garbage collection these days.

So, early versions of your Cocoa programs are probably going to leak. There are ways to combat this. The primary ones are the leaks command-line tool and the Instruments app that comes bundled with Xcode, neither of which I like very much. I’d prefer that the app itself report its bad behavior. To that end, I’d like my apps to be able to tell how much memory they are using.

Surprisingly, an app’s memory usage is subject to interpretation. Suppose your app and another are both using one in-memory copy of a shared framework. Should your app’s memory total include the size of the framework or not? What if your app has a lot of memory allocated that currently lives on disk in a swap file — should you count that?

I’ve spent some time in the past studying this issue, and I’ve decided to go with a figure called the “resident set size.” This is more-or-less how much memory your app is using. Not perfect, but plenty close enough for my needs. Here’s how you can get it.

#include <mach/mach_init.h> #include <mach/task.h> #include <sys/time.h> #include <sys/resource.h> #include <stdint.h> #include <string.h> #include <unistd.h> int64_t MemoryUsage() {     task_basic_info         info;     kern_return_t           rval = 0;     mach_port_t             task = mach_task_self();     mach_msg_type_number_t  tcnt = TASK_BASIC_INFO_COUNT;     task_info_t             tptr = (task_info_t) &info;         memset(&info, 0, sizeof(info));         rval = task_info(task, TASK_BASIC_INFO, tptr, &tcnt);     if (!(rval == KERN_SUCCESS)) return 0;         return info.resident_size; }

This was difficult to write. It makes use of Darwin kernel APIs, which Google knows almost nothing about.

This works on any version of Mac OS X back to about 10.2, I think. It also works on the iPhone simulator, as well as on actual iPhone hardware. I’ve tried it myself on all these, including my own phone. The fact that this function works unmodified on both Macs and live iPhone hardware is proof positive that the two platforms use very similar kernels.

If you want to use this to detect leaks, you have to track your apps’ memory usage over time. Take a snapshot of your app’s size near the beginning of a run, then put your app through its paces for half an hour or so. Is the app’s memory usage trending up?

In addition to being useful for tracking leaks, I simply appreciate knowing how much memory my apps are using. There’s a definite upper limit on how much RAM you can allocate on an iPhone, and there’s no virtual memory at all. If your iPhone app exhausts all physical memory, it can’t start swapping to disk, it’ll just get killed.

I’ve noticed that the app I’m working on now uses 14MB in the simulator, but only 8MB on real iPhone hardware. Probably because simulator apps are really just modified Mac apps. Windows, views, and other user interface elements on the Mac are no doubt heavier than their iPhone counterparts.

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