[Disksim-users] Request coalescence

[YipKei Kwok]

Hi Ming,

In my case, I use trace workload, instead of synthetic workload. As my
understand, the parameter of "Number of I/O requests to generate" is
only applicable for synthetic workload, as it is a part of the
synthetic generation module configuration. Under trace workload, the
simulation should not terminate until the trace is exhausted. (please
let me know if I am wrong.)

Thanks!

-Yipkei

On 5/13/07, mingju Li <mingjule at gmail.com> wrote:
> We found that the program would stop when it has the number of requests the
> user asks for in the generating list which means some requests are still in
> the queues in different places like in controller or disks waiting for the
> service. So you will find in the result that "total number of requests
> generated" is the number of requests you asked for but "the total number of
> requests handled" is always less than "the total number of requests
> generated".  Although I tested it using synchronous way but I think it
> should work the same way with using trace.
>
> Ming
>
> On 5/13/07, Greg Ganger <ganger at ece.cmu.edu > wrote:
> >
> > Cache hits would cause the number of requests at the disk to be less
> > than the number in the trace.  That's something to look at.
> >
> > Greg
> >
> >
> > On Sun, 13 May 2007, YipKei Kwok wrote:
> >
> > > Hi,
> > >
> > > I am running Disksim w./ a trace workload.
> > >
> > > Problem:
> > > The amount of "Total Requests handled" by the disk is less than the
> > > number of requests specified in the trace.
> > >
> > > I suspect that is because of request
> coalescence/overlapping/combining.
> > >
> > > disksim_ioqueue: Overlapping request scheme
> > >
> -----------------------------------------------------------
> > > - We have already set this parameter 0 so that overlapping requests
> > > are treated as independent.
> > >
> > > disksim_disk: Combine seq writes
> > > -------------------------------------------
> > > - The default value of our HP C3323 hard drive is 1. We experienced
> > > _exactly_ the same problem even when we set it to 0.
> > >
> > > I/O Driver Statistics
> > > -------------------------
> > >> From the statistics, both "IOdriver Overlaps combined" and "IOdriver
> > > Read overlaps combined" are both 0. So, no requests are combined on
> > > the I/O driver level.
> > >
> > >
> > > Is there any other level of the I/O hierarchy could cause request
> > > coalescence? Or, are there some other reasons, other than request
> > > coalescence, which cause the problem that I am facing?
> > >
> > > Thank you very much!
> > >
> > > -Yipkei
> > >
> > > P.S. Here is our experimental setup.
> > > I/O trace-driven
> > > raid 0 - 1 HP C3323A hard drive.
> > > (Originally, there are 4 disks in the RAID. For simplicity, now we
> > > only use 1. So, strictly speaking, it's not a RAID.
> > > Number of processors = 1
> > >
> > > --
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>
>


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