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

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CHAPTER 4 ■ ■ ■ Memory Structures In this chapter, we’ll look at Oracle’s three major memory structures: • System Global Area (SGA): This is a large, shared memory segment that virtually all Oracle processes will access at one point or another. • Process Global Area (PGA): This is memory that is private to a single process or thread, and is not accessible from other processes/threads. • User Global Area (UGA): This is memory associated with your session. It will be found either in the SGA or the PGA depending on whether you are connected to the database using shared server (then it will be in the SGA), or dedicated server (it will be in the PGA, in the process memory). ■Note In earlier releases of Oracle, shared server was referred to as Multi-Threaded Server or MTS. In this book, we will always use the term “shared server.” We’ll first discuss the PGA and UGA, and then we’ll move on to examine the really big structure: the SGA. The Process Global Area and User Global Area The PGA is a process-specific piece of memory. In other words, it is memory specific to a single operating system process or thread. This memory is not accessible by any other process/thread in the system. It is typically allocated via either of the C runtime calls malloc() or memmap(), and it may grow (and shrink even) at runtime. The PGA is never allocated in Oracle’s SGA—it is always allocated locally by the process or thread. The UGA is, in effect, your session’s state. It is memory that your session must always be able to get to. The location of the UGA is wholly dependent on how you connected to Oracle. If you connected via a shared server, then the UGA must be stored in a memory structure that every shared server process has access to—and that would be the SGA. In this way, your session can use any one of the shared servers, since any one of them can read and write your session’s data. On the other hand, if you are using a dedicated server connection, this need for universal access to your session state goes away, and the UGA becomes virtually synonymous with the PGA; it will, in fact, be contained in the PGA of your dedicated server. When you look 115
116 CHAPTER 4 ■ MEMORY STRUCTURES at the system statistics, you’ll find the UGA reported in the PGA in dedicated server mode (the PGA will be greater than or equal to the UGA memory used; the PGA memory size will include the UGA size as well). So, the PGA contains process memory and may include the UGA. The other areas of PGA memory are generally used for in-memory sorting, bitmap merging, and hashing. It would be safe to say that, besides the UGA memory, these are the largest contributors by far to the PGA. Starting with Oracle9i Release 1 and above, there are two ways to manage this other non- UGA memory in the PGA: • Manual PGA memory management, where you tell Oracle how much memory is it allowed to use to sort and hash any time it needs to sort or hash in a specific process • Automatic PGA memory management, where you tell Oracle how much memory it should attempt to use systemwide The manner in which memory is allocated and used differs greatly in each case and, as such, we’ll discuss each in turn. It should be noted that in Oracle9i, when using a shared server connection, you can only use manual PGA memory management. This restriction was lifted with Oracle 10g Release 1 (and above). In that release, you can use either automatic or manual PGA memory management with shared server connections. PGA memory management is controlled by the database initialization parameter WORKAREA_SIZE_POLICY and may be altered at the session level. This initialization parameter defaults to AUTO, for automatic PGA memory management when possible in Oracle9i Release 2 and above. In Oracle9i Release 1, the default setting was MANUAL. In the sections that follow, we’ll take a look at each approach. Manual PGA Memory Management In manual PGA memory management, the parameters that will have the largest impact on the size of your PGA, outside of the memory allocated by your session for PL/SQL tables and other variables, will be as follows: • SORT_AREA_SIZE: The total amount of RAM that will be used to sort information before swapping out to disk. • SORT_AREA_RETAINED_SIZE: The amount of memory that will be used to hold sorted data after the sort is complete. That is, if SORT_AREA_SIZE was 512KB and SORT_AREA_ RETAINED_SIZE was 256KB, then your server process would use up to 512KB of memory to sort data during the initial processing of the query. When the sort was complete, the sorting area would be “shrunk” down to 256KB, and any sorted data that did not fit in that 256KB would be written out to the temporary tablespace. • HASH_AREA_SIZE: The amount of memory your server process would use to store hash tables in memory. These structures are used during a hash join, typically when joining a large set with another set. The smaller of the two sets would be hashed into memory and anything that didn’t fit in the hash area region of memory would be stored in the temporary tablespace by the join key. These parameters control the amount of space Oracle will use to sort or hash data before writing (swapping) it to disk, and how much of that memory segment will be retained after 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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338 CHAPTER 10 ■ DATABASE TABLES
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CHAPTER 11 ■ ■ ■ Indexes Inde
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CHAPTER 13 ■ ■ ■ Partitioning
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Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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