[Disksim-users] Problem: No. of in-flight requests bounded by 20

[Yipkei Kwok]

Hi,

I want to use synthetic workload generators to simulate a scenario
where 50 single-thread independent processes are concurrently
accessing a single-disk storage system. All I/O requests are
synchronous. I expect that the number of pending request in the entire
I/O hierarchy (I/O driver + controller + disk) to be 50. However, I
found that the number of pending requests drops from 50, at the
beginning, to 20, later in the simulation.

Here is the summary of the DiskSim parv file
(http://utminers.utep.edu/ykwok2/tmp/20130710-0.parv.txt)
* I/O driver: FCFS
* single disk: MAXTOR_ATLAS10K5_300 (disk model provided by Anjo at
http://ovahldy.blogspot.com/2011/03/recent-diskmodels-for-disksim.html)
* Think time from call to request = 0.0
* Think time from request to return = 0.0
* number of synthetic workload generators = 50
* System call/return with each request = 1 (i.e., each request occurs
within the context of a system call)
Note: I gave a try to set "System call/return with each request" to 0
but I encountered the same problem.
* For each generator, Probability of time-critical request = 1.0

This is how I measure the number of pending requests in the I/O
hierarchy. I keep a counter in the simulator. When a request arrives
the I/O driver, iodriver_request() increments the counter. When a
request is serviced, iodriver_access_complete() decrements the
counter.

The plot of the number of pending requests vs. time is here
(http://utminers.utep.edu/ykwok2/tmp/Slide1.PNG). I repeated the
experiment with 35, 20, and 15 generators (all results are shown on
the same slide). The number of pending requests with 35 generators,
too, drops to 20, while the numbers of pending requests with 20 and 15
generators remain constant.

I want the number of pending requests to be 50 when 50 generators are
used. Why is the number of pending requests dropping?

Please kindly help. Thank you very much.

Regards,
Yipkei

--
********************************************
Yipkei Kwok
Ph.D. Candidate
HiPerSys Lab
Department of Computer Science
The University of Texas at El Paso
Phone: 915 747 6433 (O)
E-mail: ykwok2 at miners dot utep dot edu
********************************************