Re: [Gems-users] Time spent in synchronization

[Hemayet Hossain <hossain@xxxxxxxxxxxxxxxx>]

Hi Dan,
Thanks a lot. Yes, my real system is a 16 processors Sun-FireSystem. I agree with your explanation; but I was wondering whether the difference can be that  much. Like, I am getting  12% -->42%, 28%->75%, 54%->82%, 13%->60% for four different applications (from RUBY_CYCLES). Do you think this is reasonable?
Thanks once again,
Hemayet


Dan Gibson wrote:

I think that is the right approach for measuring simulation time. But is 
the increased synchronization time all that surprising? I see from your 
response to Mike that your real machine is a 16-processor v9-based 
(sun4u -- is it a SunFire of some sort?), and your target is a 
16-processor CMP with a big shared L2.

Depending on the performance disparity between real and target machines, 
it might be correct to show increased synchronization time under 
simulation. (Recall that Simics+Ruby uses a very simple processor model 
(IPC=1 for all non-memory operations). This can artificially inflate the 
apparent performance of a processor by abstracting away pipeline details.)

Regards,
Dan

Hemayet Hossain wrote:
  

Hi Dan,
I have collected the time through RUBY_CYCLES also. For that I have 
passed the binded proc id (kept in an array in program) with Magic 
calls for start and finish of lock/barriers calls and used that proc 
id for keeping track of which proc is in synchronization and which one 
is not. Do you think my approach is wrong?

Thanks,
Hemayet

Dan Gibson wrote:
    

gethrtime() is bogus under simulation. Solaris's view of time is 
horribly skewed under Simics alone or Simics+Ruby. Try measuring using 
RUBY_CYCLES instead.

Hemayet Hossain wrote:
  
      

Hi All,
I am simulating some splash2 benchmarks by using ruby with simics 2.2.19 
(Solaris 10) and to characterize the time spent in synchronization, I 
have instrumented the synchronization calls like locks and barrier. I 
have binded each thread to a specific processor (one-to-one) and 
collecting the time by calling high resolution timer gethrtime().  In 
real machine run (having 16 processors) for 16 threads I get around 19% 
time spent on synchronization for a program. If I run the same program 
in simics without ruby, I also get similar percentage of time spent in 
synchronization.

But If I run the same program in simics with ruby, the time spent in 
synchronization is much higher (goes around 75% of total).  I have 
collected the time from both programs and from ruby. Both are getting 
almost same percentage number. I am using MESI_SCMP_directory like 
protocol having 2 cycles for L1 and 14 cycles for L2 access.

Does anyone have any idea what's going on? What wrong with my setup? I 
would really appreciate your reply.
Thanks,
Hemayet

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