Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 6 ■ LOCKING AND LATCHING 227 Elapsed: 0.12 (mins) Load Profile ~~~~~~~~~~~~ Per Second Per Transaction --------------- --------------- ... Parses: 8.43 29.50 Hard parses: 0.14 0.50 Top 5 Timed Events ~~~~~~~~~~~~~~~~~~ % Total Event Waits Time (s) Call Time -------------------------------------------- ------------ ----------- --------- CPU time 4 86.86 log file parallel write 49 0 10.51 control file parallel write 4 0 2.26 log file sync 4 0 .23 control file sequential read 542 0 .14 That is quite dramatic: from 26 CPU seconds in the no bind variables example to 4 CPU seconds here. From 807 hard parses per second to 0.14 per second. Even the elapsed time was dramatically reduced from about 45 seconds down to 8 seconds. When not using bind variables, we spent five-sixths of our CPU time parsing SQL. This was not entirely latch related, as much of the CPU time incurred without bind variables was spent parsing and optimizing the SQL. Parsing SQL is very CPU intensive, but to expend five-sixths of our CPU doing something (parsing) that doesn’t really do useful work for us—work we didn’t need to perform—is pretty expensive. When we get to the two-user test, the results continue to look better: Elapsed: 0.20 (mins) Load Profile ~~~~~~~~~~~~ Per Second Per Transaction --------------- --------------- Parses: 6.58 26.33 Hard parses: 0.17 0.67 Top 5 Timed Events ~~~~~~~~~~~~~~~~~~ % Total Event Waits Time (s) Call Time -------------------------------------------- ------------ ----------- --------- CPU time 11 89.11 log file parallel write 48 1 9.70 control file parallel write 4 0 .88 log file sync 5 0 .23 log buffer space 2 0 .05 The amount of CPU time is about 2 to 2.5 times the amount reported by the single-user test case.
228 CHAPTER 6 ■ LOCKING AND LATCHING ■Note Due to rounding, the 4 CPU seconds is really anywhere from 3.5 to 4.49, and the 11 is really anywhere from 10.5 to 11.49 seconds. Further, the amount of CPU used by two users with bind variables is less than half the amount of CPU a single user not using bind variables required! When I went to look at the latch report in this Statspack report, I found it was missing in this report—there was so little contention for the Shared pool and library cache that it was not even reported. In fact, digging deeper turned up the fact that the Shared pool latch was requested 50,367 times versus well over 1,000,000 times in the two-user test just shown. Performance/Scalability Comparison Table 6-1 summarizes the CPU used by each implementation, as well as the latching results as we increase the number of users beyond two. As you can see, the solution using fewer latches will scale much better as the user load goes up. Table 6-1. CPU Usage Comparison With and Without Bind Variables Waits for Latches CPU Seconds/Elapsed Shared Pool Latch (Number of Waits/Time Users Time in Minutes Requests in Wait in Seconds) No Binds Binds No Binds Binds No Binds Binds 1 26/0.52 4/0.10 563,883 25,232 0/0 2 74/0.78 11/0.20 1,126,006 50,367 406/1 3 155/1.13 29/0.37 1,712,280 75,541 2,830/4 4 272/1.50 44/0.45 2,298,179 100,682 9,400/5 5 370/2.03 64/0.62 2,920,219 125,933 13,800/20 6 466/2.58 74/0.72 3,526,704 150,957 30,800/80 17/0 7 564/3.15 95/0.92 4,172,492 176,085 40,800/154 8 664/3.57 106/1.00 4,734,793 201,351 56,300/240 120/1 9 747/4.05 117/1.15 5,360,188 230,516 74,600/374 230/1 10 822/4.42 137/1.30 5,901,981 251,434 60,000/450 354/1 The interesting observation for me is that 10 users using bind variables (and very few latch requests as a result) use the same amount of hardware resources as 2 to 2.5 users that do not use bind variables (i.e., that overuse a latch, or process more than they need to). When you examine the results for 10 users, you see that nonuse of bind variables results in the use of 6 times the CPU and takes 3.4 times the execution time when compared to the bind variable solution. The more users are added over time, the longer each user spends waiting for these latches. We went from an average of 4 seconds/session of wait time for latches with 5 users to an average of 45 seconds/session of wait time with 10 users. However, the implementation that avoided overuse of the latch suffered no ill effects as it scaled up.
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Expert Oracle Database Architecture
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Contents Foreword . . . . . . . . .
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■CONTENTS v Block Buffer Cache .
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■CONTENTS vii ■CHAPTER 9 Redo a
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■CONTENTS ix ■CHAPTER 12 Dataty
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Foreword “THINK.” In 1914, Thom
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■FOREWORD xiii Tom is an aficiona
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About the Technical Reviewers ■JO
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Introduction The inspiration for th
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