Messages in this thread | | | Date | Wed, 30 Apr 2003 14:02:58 -0400 | From | Timothy Miller <> | Subject | naive questions about thrashing |
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I am running kernel version 2.4.18-26.7.x under Red Hat 7.2.
I wrote a CPU-intensive program which attempts to use over 700 megs of RAM on a 512-meg box, therefore it thrashes.
One thing I noticed was that 'top' reported that the kernel ("system") was using 68% of the CPU. (The offending process was getting about 9%.) How much CPU involvement is there in sending I/O requests to the drive and waiting on an interrupt? Maybe I don't understand what's going on, but I would expect the CPU involvement in disk I/O to be practically NIL, unless it's trying to be really smart about it. Is it? Or maybe the kernel isn't using DMA... this is a Dell Precision 340. I'm not sure what drive is in it, but I would be surprised if it weren't using DMA.
The next thing has to do with interactivity. I don't know how much interactivity code is in the 2.4 kernel, but as I understand it, it's based on a process giving up its timeslice. Well, if a process is causing numerous page faults, is it not, in effect, giving up its timeslice? Is it therefore being scheduled as an interactive process? If so, the process scheduler is very mistaken. In my naive opinion, causing a page fault should NOT be an indication of interactivity. It seemed to me that the process was getting too much attention from the process scheduler. The thrashing process was getting run while other processes (like X, etc.) were being almost completely starved.
Does causing a page fault get interpreted as interactivity?
Going under the assumption that disk I/O should require almost no CPU involvement, it seems to me that memory could be managed so that processes which behave themselves should feel no effect from the thrashing, while the thrashing process continues to thrash and therefore have little opportunity to use the CPU. This would require that the VM not swap out pages which are being used heavily by well-behaved processes. Instead, it seems that the thrashing process is being allowed to completely trash everything else running in the system.
Now, I'm not stupid. Someone's going to tell me that the problem is in user space, and that I should fix the user space program. (I am doing that.) I can certainly understand that perspective, but it seems to me that the fairest result of a process misbehaving is that it should only punish itself, not every other process in the system.
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