Messages in this thread | | | Date | Thu, 01 May 2003 11:52:53 -0400 | From | Timothy Miller <> | Subject | Re: naive questions about thrashing |
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J.A. Magallon wrote: > On 04.30, Timothy Miller wrote: > >>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. >> > > > As I understand it, it is telling you that your programs spends 68% of > its time is kernel space, ie, waiting your pages to come from disk. It > does not mean that the CPU is doing anything, but it is locked by the > kernel.
What would the kernel be locked while waiting on disk I/O? Shouldn't it be running another process? It's not DOING anything. The whole idea behind a multitasking OS is to overlap the I/O of one process with the CPU usage of another whenever possible. Swapping is an I/O operation.
And for that matter, if every runnable process has pages swapped out so that they cannot run, then the CPU should be IDLE.
Am I wrong?
> > If you can't afford to buy more memory, recode the thing. So much thrashing > looks like you access your data very randomly. Try to process the data > in a more sequential way, so you just fault after processing a big bunch > of data. With 700Mb of data and a 512Mb box, at least half of your data > fit in memory, so under an ideal sequential access you just would page > 300Mb one time... >
The process got that large because of a bug in my program. But a side-effect of that was kernel behavior that didn't make sense to me. I decided to ask about it.
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