Difference between use of while() and sleep() to put program into sleep mode

Question

I have created a shared object and access it from two different program and measuring the time.

DATA array is the shared object between two processes.

Case 1: Use of while inside program1

program1 :

access shared DATA array ;// to load into memory and avoid page fault during access time calculation

start=timer;
access shared DATA array
end=timer;

Time_needed= end-start
printf("Inside Program1, Time1=%d\n",Time_needed);    


start=timer;
access shared DATA array   
end=timer;

Time_needed= end-start
printf("Inside Program1, Time2=%d\n",Time_needed);    

while(1){}; // I replace this by sleep(1000) in CASE-2

Program2 :

access shared DATA array ;// to load into memory and avoid page fault during access time calculation

start=timer;
access shared DATA array
end=timer;

Time_needed= end-start
printf("Inside Program2, Time1=%d\n",Time_needed);    
start=timer;
access shared DATA array      
end=timer;

Time_needed= end-start
printf("Inside Program2, Time1=%d\n",Time_needed);    

OUTPUT : First I run program1, then Program2

Inside Program1, Time1 = 17620
Inside Program1, Time1 = 17680

Inside Program2, Time1 = 7620
Inside Program2, Time1 = 7600

Case 2: Use of sleep() inside program1

program1 :

access shared DATA array ;// to load into memory and avoid page fault during access time calculation

start=timer;
access shared DATA array
end=timer;

Time_needed= end-start
printf("Inside Program1, Time1=%d\n",Time_needed);    


start=timer;
access shared DATA array
end=timer;

Time_needed= end-start
printf("Inside Program1, Time2=%d\n",Time_needed);    

sleep(1000);

Program2 :

access shared DATA array ;// to load into memory and avoid page fault during access time calculation

start=timer;
access shared DATA array
end=timer;

Time_needed= end-start
printf("Inside Program2, Time1=%d\n",Time_needed);    
start=timer;
access shared DATA array   
end=timer;

Time_needed= end-start
printf("Inside Program2, Time1=%d\n",Time_needed);    

OUTPUT : First I run program1, then Program2

Inside Program1, Time1 = 17620
Inside Program1, Time1 = 17680

Inside Program2, Time1 = 17620
Inside Program2, Time1 = 17600

From the output in case -1, I can say shared data DATA array is loaded into memory/cache by 1st program and second program access it from cache. Whereas this also true for CASE-2, but the result looks like it is flushed out from cache while Program1 goes into sleep.

I am using GCC under linux.

Any clue ? Thanks in advance .

Answer 1

You didn't describe exactly how you run the different versions (different processes?), but assuming they're sequential - It is possible that you're seeing the affect of sleep()

It depends of course on the exact implementation and HW, but it's very likely to send your CPU into some power-saving/sleep state (that's what it's designed for). If that's the case, then the core caches will have to be flushed as part of the process, and you'll wake-up with cold caches. The whie loop on the other hand is intended to do a busy wait loop while grinding your CPU and keeping it alive (along with the caches), unless you happen to get a context switch along the way.

The exact details would again depend on implementation, on x86 you can use inline assembly to invoke monitor+mwait instructions that allow you to specify the exact C-state depth you want to achieve. The deeper it is, the more caches will get closed (mostly relevant for the L3).

Answer 2

I my opinion we can't predict the cache behavior as per your case.

It also depends upon your H/W which, you have not mentioned (like how many CPU cores(physical or logical) are present).

We can't say that program_2 will be scheduled on the same core and just after program_1 because it totally depends upon OS scheduler. So program_2 may use cache filled by program_1 or not.

It's possible that the cache is flushed out due to some other program which was scheduled just after program_1.