[Gems-users] Relation between SIMICS cycle & GEMS cycle

["Weihang Jiang" <weihang.jiang@xxxxxxxxx>]

Hi,
  Can anybody help me understand the relation between SIMICS
cycle and GEMS cycle.

  SIMICS cycle means the number of cycles SIMICS thinks the
simulation has lasted, which can be calculated through
calling SIM_cycle_count() at the beginning and the end of
one simulation.
  GEMS cycle is referring to the number of cycles opal/ruby
has advanced.

  Understanding the relation between SIMICS cycle and GEMS
cycle is important for utilizing system level profiling
tools. For example, vmstat reports both number of events
(e.g. bi, bo) and relative execution time (e.g. us,sy,id).
Other profiling tools report absolute execution time.

  I did a small experiment to testify the relation. I ran
SIMICS+GEMS with two different cache configurations for
10,000,000 GEMS cycles.
  In experiment 1, using fast cache, 407,808 instructions
have been executed and the SIMICS cycle number is 1,711,569.
In experiment 2, using slow cache, 401,245 have been
executed. However, the SIMICS cycle number is 1,664,671
(different from exp1).

   Based on this experiment, my understanding is that,
SIMICS cycle is not an accurate measurement and those system
level profiling tools can not be used without care.

   So, I have 2 questions:

1. Is my understanding correct?
2. How much effort does it require to let SIMICS cycle have
more sense(i.e. let SIMICS cycle == GEMS cycle)?


--
Weihang Jiang,  UIUC