CLARIFY TERMS IN PERFORMANCE TOOLBOX/6000: RBLKS, WBLKS

ITEM: RTA000036974

                                                                                
                                                                                
                                                                                
QUESTION:                                                                       
I am using 3D-Monitor in the Performance ToolBox/6000.                          
Please further clarify the following terms:                                     
                                                                                
1.disk xfer: The List of statistics defines it as "transfers from/to            
             disk".  What is the interval of this measurement?                  
             Is it the same as the sampling interval?                           
                                                                                
             I noticed that sometimes there were read and write                 
             activities (rblks and wblks bar), but there was no                 
             activity on xfer?  Why?                                           
                                                                                
A:  PTX reports rblks and wblks in 512 byte blocks per second.                  
    "Disk xfers" counts the number of read and write disk transfer              
    requests, but not necessarily the actual physical transfers                 
    during the period, since the vmm can do read-aheads and                     
    write-behinds.  In general this should be very unusual (the majority        
    of disk transfers seem to be closely correlated to rblk                     
    and wblk activity.                                                          
                                                                                
2. minflt/majflt: defined as "fault with or without I/O".                       
                  I thought all page faults occur by paging memory              
                  into disks, which always involved I/O.                        
                  Am I missing something?                                       
                                                                               
A: Answer to question 2:  There can be a resolution to a page fault             
    without doing any I/O under at least two conditions we can                  
    think of (possibly more):                                                   
      1.  Application requests a page.  Page is read in, used, and              
          then discarded.  Page becomes available.  Pages goes                  
          on the free list for reuse by another process.                        
          AIX then realizes another process wants the same page.                
          Page is taken off the free list and given to the                      
          application (reclaimed) without re-reading the page                   
          from disk.                                                            
      2.  Application requests a page from disk.  The inode for that            
          page marks the page as a "zero filled page", which means it           
         is a page full of zeros.  AIX does not actually store the              
          page with the zeros, but instead just marks the fact in the          
         inode.  When this page is accessed the operating system                
          realizes it can create a page of memory full of zeros without         
          doing the physical I/O.                                               
                                                                                
                                                                                
This item was created from library item Q649170      CMZSH                      
                                                                                
Additional search words:                                                        
CLARIFY CMZSH IX LPPS MEASURE NOV93 OZNEW PERFORMANCE RBLKS RISCO               
RISCSYSTEM SOFTWARE SUPT TERM TERMS TOOLBOX TUNE WBLKS 6000                     
                                                                                
                                                                                
                                                                                
                                                                               



WWQA: ITEM: RTA000036974 ITEM: RTA000036974

Dated: 01/1995 Category: RISCO

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