Create 20240330_04.md

Author: digoal

202403/20240330_04.md

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+## PostgreSQL 17 preview - 支持在申请时指定动态共享内存区域初始、最大段size
+                                                                                                
+### 作者                                                                                                
+digoal                                                                                                
+                                                                                                
+### 日期                                                                                                
+2024-03-30                                                                                      
+                                                                                                
+### 标签                                                                                                
+PostgreSQL , PolarDB , DuckDB , dsa , DSA_DEFAULT_INIT_SEGMENT_SIZE , DSA_MIN_SEGMENT_SIZE , DSA_MAX_SEGMENT_SIZE                      
+                                                                                                
+----                                                                                                
+                                                                                                
+## 背景       
+https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=bb952c8c8b13279eca039499249cb5dc60991603  
+```  
+Allow specifying initial and maximum segment sizes for DSA.  
+  
+author	Masahiko Sawada <[email protected]>	  
+Wed, 27 Mar 2024 02:43:29 +0000 (11:43 +0900)  
+committer	Masahiko Sawada <[email protected]>	  
+Wed, 27 Mar 2024 02:43:29 +0000 (11:43 +0900)  
+commit	bb952c8c8b13279eca039499249cb5dc60991603  
+tree	ba7cc3ced86501cb0da23a66b9170ffae19dcf6c	tree  
+parent	1f42337be535243e665f85916ce21b2d85d9f2b3	commit | diff  
+Allow specifying initial and maximum segment sizes for DSA.  
+  
+Previously, the DSA segment size always started with 1MB and grew up  
+to DSA_MAX_SEGMENT_SIZE. It was inconvenient in certain scenarios,  
+such as when the caller desired a soft constraint on the total DSA  
+segment size, limiting it to less than 1MB.  
+  
+This commit introduces the capability to specify the initial and  
+maximum DSA segment sizes when creating a DSA area, providing more  
+flexibility and control over memory usage.  
+  
+Reviewed-by: John Naylor, Tomas Vondra  
+Discussion: https://postgr.es/m/CAD21AoAYGGC1ePjVX0H%2Bpp9rH%3D9vuPK19nNOiu12NprdV5TVJA%40mail.gmail.com  
+```  
+    
+```  
++/*  
++ * The number of bits used to represent the offset part of a dsa_pointer.  
++ * This controls the maximum size of a segment, the maximum possible  
++ * allocation size and also the maximum number of segments per area.  
++ */  
++#if SIZEOF_DSA_POINTER == 4  
++#define DSA_OFFSET_WIDTH 27        /* 32 segments of size up to 128MB */  
++#else  
++#define DSA_OFFSET_WIDTH 40        /* 1024 segments of size up to 1TB */  
++#endif  
++  
++/*  
++ * The default size of the initial DSM segment that backs a dsa_area created  
++ * by dsa_create.  After creating some number of segments of the initial size  
++ * we'll double this size, and so on.  Larger segments may be created if  
++ * necessary to satisfy large requests.  
++ */  
++#define DSA_DEFAULT_INIT_SEGMENT_SIZE ((size_t) (1 * 1024 * 1024))  
++  
++/* The minimum size of a DSM segment. */  
++#define DSA_MIN_SEGMENT_SIZE   ((size_t) (256 * 1024L))  
++  
++/* The maximum size of a DSM segment. */  
++#define DSA_MAX_SEGMENT_SIZE ((size_t) 1 << DSA_OFFSET_WIDTH)  
+  
++/* Create dsa_area with default segment sizes */  
++#define dsa_create(tranch_id) \  
++   dsa_create_ext(tranch_id, DSA_DEFAULT_INIT_SEGMENT_SIZE, \  
++                  DSA_MAX_SEGMENT_SIZE)  
++  
++/* Create dsa_area with default segment sizes in an existing share memory space */  
++#define dsa_create_in_place(place, size, tranch_id, segment) \  
++   dsa_create_in_place_ext(place, size, tranch_id, segment, \  
++                           DSA_DEFAULT_INIT_SEGMENT_SIZE, \  
++                           DSA_MAX_SEGMENT_SIZE)  
++  
+```    
+