EXCP count and block size



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EXCP count and block size

Postby ndashore20 » Wed Apr 26, 2017 6:55 pm

Hi


< Please correct me wherever i am wrong >

As far as i know the EXCP count indicates the number of I-O operation executed for your program. So if anyhow we could decrease the EXCP count then it is possible to save the time for execution. The EXCP count increases by one when the system fetch one block of data from hard disk(DASD) to system memory. If we increases the block size then then it will cover more number of bytes in one block and hence require less EXCP count. Could you please clarify my concept about EXCP count and its relation with block size and BUFNO and is it possible that i was thinking???


Thanks in Advance
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Re: EXCP count and block size

 

Re: EXCP count and block size

Postby Robert Sample » Wed Apr 26, 2017 8:20 pm

You are focusing on one aspect of performance, possibly to the detriment of the overall job. Yes, a larger block size can reduce EXCP count but these days SMS-managed data sets are already at an optimal block size, along with data sets created with BLKSIZE=0 to let the system choose it. Furthermore, the amount of time to transfer the data from disk to memory goes up as the block size goes up, and the amount of memory required to store the block of data goes up so you may run into other issues; some sites, for example, limit the memory in certain job classes so you may have to change your job class when you increase the block size. BUFNO indicates how many buffers should be used -- in general, the more the better (up to a point); at least 30 buffers will almost always help over the QSAM default of 5 and I've seen cases where up to 250 buffers continued to improve performance.

One key note about performance to remember: jobs are either CPU-bound or I/O-bound. If your job is CPU-bound, that means it needs more CPU to reduce the job elapsed time, so no changes to block size or BUFNO will impact the elapsed time (even though EXCP count may go down).

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Re: EXCP count and block size

Postby Robert Sample » Wed Apr 26, 2017 8:53 pm

I found this in the 2015 Hot Topics Share presentation :
z/OS 2.2: New zFS Support for EXCP Counts
 z/OS 2.1 and Prior: zFS updates EXCP counts for every cache page touched by
the current operation on the user task
̶ Number of cached pages really touched not consistent even for single system cases
̶ In directory operations, pages touched is determined by current layout of the directory and
what was cached in memory
• Also if forwarding directory operations to owners, the client did not touch pages so if some directory
operations were run on a client instead of an owner the EXCP counts may be way off
 In z/OS 2.2 changes are made:
© Copyright IBM Corporation 2015
̶ File reads and writes operate the same - number of cached pages touched because
clients do direct file IO so this is an exact count of user cache pages touched
̶ For directory updates and reads, zFS will determine the minimum number of meta cache
pages required for the operation and use that number
• Ensures client and owner are consistent and the accounting is simple and reasonably accurate
So if your COBOL code is accessing a zFS (Unix) file, and you're not on z/OS 2.2, the EXCP counts may have no relationship to actual I/O counts.
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Re: EXCP count and block size

Postby lawnless » Tue May 23, 2017 10:00 pm

Programs fetching data from DB2 seem to behave in a similar fashion. I haven't done any research into it, but they will run a long time without any change to EXCP count.
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