Re: Queues when accessing disks

[M. Edward Borasky]

On Fri, 2004-12-31 at 04:32 +0100, Felipe Erias wrote:
> Hi,
> 
> I'm trying to apply queuing theory to the study of the GNU/Linux kernel.
> Right now, I'm focusing in the queue of processes that appears when they
> try to access an I/O device (specifically, an IDE HD). When they want to
> read data, it behaves as a usual queue: several clients (processes) that
> require attention from a server (disk / driver / ...). The case when they want
> to write data is a bit more tricky, because of the cache buffers used by the OS,
> and maybe could be modelized by a network of queues. Both cases are
> interesting for my work, but I'll take the reading one first, just
> because it seems
> a bit more simple 'a priori'.
> 
> To modelize the queue, I need to get some information:
>  - what processes claim attention from the disk
>  - when they do it
>  - when they begin to be served
>  - when they finish being served
> 
> To get all this information, maybe I could hack my kernel a bit to write
> a line to a log on every access to the HD, or account the IRQs from
> the IDE channels... I also have the feeling that this queuing problem could
> dissappear o became more hidden if DMA were enabled.
> 
> To be true, I'm a bit lost and that's why I ask for your help.
> 
> Yours sincerely,
> 
>   Felipe Erias

I have a similar goal -- building an operational queueing model of a
running Linux system. At this point, though, I'm not really trying to do
per-process work, just model the system overall, including processors,
I/O and virtual memory. As I noted in a recent posting, I'm currently
blocked by the incorrect statistics gathered by a 2.4 kernel in
"/proc/partitions". You can compute the throughputs, but not the average
wait and service times, or the utilizations, which depend on the service
times.

I'm rather strongly considering establishing a mailing list just devoted
to this topic -- queuing models of Linux kernels based on statistics the
kernel collects. If such a list already exists, please let me know -- I
don't want to re-invent any wheels, but there *is* a wheel I do want to
invent! Please e-mail me off-list if you know of such a list or would
join in if I create it.

Thanks,

Ed Borasky
http://www.borasky-research.net/



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