pmTool, the process run by Activity Monitor to actually collect stats, appears to leak memory. If I leave Activity Monitor running for a good period of time, when I check up on it pmTool is often using over 100MiB of Real Memory.

I just checked my laptop, pmTool was using over 100MiB of Real Memory. Right now on my desktop it’s using 41MiB of Real Memory, but I don’t remember how long it’s been running for. I also believe a good deal of memory is currently paged out.

After checking up on it, pmTool on my desktop has a Private Memory size of 91MiB.

If I run leaks on pmTool on my desktop, I see 27931 leaks for 85799936 total leaked bytes, which works out to about 81.8MiB of leaked memory. All but 2 of these leaks appear to be blocks of memory of size 3072 that start with '?\0'.

In any case, this seems to be a pretty significant leak in pmTool.

A highly non-scientific test shows it to be adding one new leak every time the UI in Activity Monitor updates (default is every 2 seconds). So if you haven’t changed the update frequency then pmTool will leak 3KiB of memory every 2 seconds.

That’s a lot.

I haven’t noticed pmTool taking more than about 100MiB of Real Memory so far, but I never thought to check Private Memory until now so it may have simply been paging out a lot of leaks. If my calculations are correct, every 24 hours pmTool will have leaked 126.6MiB.

If you keep Activity Monitor running constantly like I do, you may want to check what your current memory usage is. If you see anything close to what I am, you may want to re-evaluate the importance of Activity Monitor (or set up a cron job to periodically restart Activity Monitor). On the other hand if you’re not seeing a leak problem I’d be interested to hear the details of your system (I’m currently running OS X 10.4.8). I’m not entirely certain whether this leak is Apple’s fault or whether one of the myriad customizations to my system is somehow causing this. As far as I know none of the hacks I use that inject into other code processes affect pmTool (seeing as how it’s not an AppKit application, so it doesn’t load Input Managers, and it’s run as root so it won’t be affected by anything that tries code injection), so this seems likely to be Apple’s fault, but if so I’m really surprised that they haven’t fixed it.

Update: I just checked the current memory usage of pmTool. I haven’t restarted it in at least a day, and yet it was listing a reasonable 1.2MiB of Real memory, and roughly the same Private Memory. This is the memory usage I’d want it to have, and yet completely different than the observed behaviour I documented in this post. I double-checked and the memory usage on my laptop was up to 55MiB. I wish I had thought of running leaks on pmTool when I saw this, but instead I quit and relaunched Activity Monitor, and immediately the leaky behaviour came back.

Update 2: I feel silly now. I completely forgot I’d set up a root crontab entry with @hourly killall pmTool to minimize the leak. That’s why it was showing such a small memory usage.

Update 3: I just got a brand new 2GHz Core 2 Duo MacBook, running 10.4.8 with the latest security update (i.e. the same OS as my iMac), and quite mysteriously, this leak isn’t showing itself. pmTool has a respectable 960KiB of Real memory usage, and no leaks. I can’t explain it.