On Jan 4, 2005, at 5:56 AM, Jack Jansen wrote:
On 3 Jan 2005, at 23:40, Bob Ippolito wrote:Most people on Mac OS X have a lot of memory, and Mac OS X generally does a good job about swapping in and out without causing much of a problem, so I'm personally not very surprised that it could go unnoticed this long.
*Except* when you're low on free disk space. 10.2 and before were really bad with this, usually hanging the machine, 10.3 is better but it's still pretty bad when compared to other unixen. It probably has something to do with the way OSX overcommits memory and swapspace, for which it apparently uses a different algorithm than FreeBSD or Linux.
I wouldn't be surprised if the bittorrent problem report in this thread was due to being low on diskspace. And that could also be true for the original error report that sparked this discussion.
I was able to trigger this bug with a considerable amount of free disk space using a laptop that has 1GB of RAM, although I did have to increase the buffer size from the given example quite a bit to get it to fail. After all, a 32-bit process can't have more than 4 GB of addressable memory. I am pretty sure that OS X is never supposed to overcommit memory. The disk thrashing probably has a lot to do with the fact that Mac OS X will grow and shrink its swap based on demand, rather than having a fixed size swap partition as is common on other unixen. I've never seen the problem myself, though.
From what I remember about Linux, its malloc implementation merely increases the address space of a process. The actual allocation will happen when you try and access the memory, and if it's overcommitted things will fail in a bad way.
-bob
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