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

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CHAPTER 4 ■ MEMORY STRUCTURES 127 exit when l_status = 0; for x in ( select * from big_table order by 1, 2, 3, 4 ) loop null; end loop; end loop; end; / exit The script to stop these processes from running is as follows: begin dbms_alert.signal( 'WAITING', '' ); commit; end; To observe the differing amounts of RAM allocated to the session I was measuring, I initially ran the SELECT in isolation—as the only session. I captured the same six statistics and saved them into another table, along with the count of active sessions. Then I added 25 sessions to the system (i.e., I ran the preceding benchmark script with the loop in 25 new sessions). I waited a short period of time—one minute for the system to adjust to this new load—and then I created a new session, captured the statistics for it with reset_stat.sql, ran the query that would sort, and then ran watch_stat.sql to capture the differences. I did this repeatedly, for up to 500 concurrent users. It should be noted that I asked the database instance to do an impossible thing here. As noted previously, based on the first time we ran watch_stat.sql, each connection to Oracle, before even doing a single sort, consumed almost .5MB of RAM. At 500 users, we would be very close to the PGA_AGGREGATE_TARGET setting just by having them all logged in, let alone actually doing any work! This drives home the point that the PGA_AGGREGATE_TARGET is just that: a target, not a directive. We can and will exceed this value for various reasons. Table 4-1 summarizes my findings using approximately 25 user increments. Table 4-1. PGA Memory Allocation Behavior with Increasing Numbers of Active Sessions, with PGA_AGGREGATE_TARGET Set to 256MB Active PGA Used by PGA in Use Writes to Temp by Reads from Temp Sessions Single Session by System Single Session by Single Session 1 7.5 2 0 0 27 7.5 189 0 0 51 4.0 330 728 728 76 4.0 341 728 728 101 3.2 266 728 728 126 1.5 214 728 728 151 1.7 226 728 728 177 1.4 213 728 728 Continued
128 CHAPTER 4 ■ MEMORY STRUCTURES Table 4-1. Continued Active PGA Used by PGA in Use Writes to Temp by Reads from Temp Sessions Single Session by System Single Session by Single Session 201 1.3 218 728 728 226 1.3 211 728 728 251 1.3 237 728 728 276 1.3 251 728 728 301 1.3 281 728 728 326 1.3 302 728 728 351 1.3 324 728 728 376 1.3 350 728 728 402 1.3 367 728 728 426 1.3 392 728 728 452 1.3 417 728 728 476 1.3 439 728 728 501 1.3 467 728 728 ■Note You might wonder why only 2MB of RAM is reported in use by the system with one active user. It has to do with the way I measured. The simulation would snapshot the single session’s of interest’s statistics. Next, I would run the big query in the single session of interest and then snapshot that session’s statistics again. Finally, I would measure how much PGA was used by the system. By the time I measured, the single session of interest would have already completed and given back some of the PGA it was using to sort. So, the number for PGA used by the system is an accurate measurement of the system’s PGA memory at the time it was measured. As you can see, when I had few active sessions, my sorts were performed entirely in memory. For an active session count of 1 to somewhere less than 50, I was allowed to sort entirely in memory. However, by the time I got 50 users logged in, actively sorting, the database started reining in the amount of memory I was allowed to use at a time. It took a couple of minutes before the amount of PGA being used fell back within acceptable limits (the 256MB request), but it eventually did. The amount of PGA memory allocated to the session dropped from 7.5MB to 4MB to 3.2MB, and eventually down to the area of 1.7 to 1.3MB (remember, parts of that PGA are not for sorting, but are for other operations—just the act of logging in created a .5MB PGA). The total PGA in use by the system remained within tolerable limits until somewhere around 300 to 351 users. There I started to exceed on a regular basis the PGA_AGGREGATE_ TARGET and continued to do so until the end of the test. I gave the database instance in this case an impossible task—the very act of having 350 users, most executing a PL/SQL, plus the
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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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■INTRODUCTION xix • Exposure to
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■INTRODUCTION xxi Chapter 3: File
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■INTRODUCTION xxiii Next up are t
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Setting Up Your Environment In this
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CHAPTER 6 ■ LOCKING AND LATCHING
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CHAPTER 11 ■ ■ ■ Indexes Inde
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CHAPTER 13 ■ ■ ■ Partitioning
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