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‎202403/20240302_01.md

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## PostgreSQL 17 preview - 引入读写原子操作函数接口with full barrier semantics
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### 作者
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digoal
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### 日期
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2024-03-02
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### 标签
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PostgreSQL , PolarDB , DuckDB , atomic , 原子操作 , 内存屏障
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----
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## 背景
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https://blog.csdn.net/gp_community/article/details/124636303
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https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=bd5132db558b6c8d11eb838be81e2177a95c7388
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```
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Introduce atomic read/write functions with full barrier semantics.
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author Nathan Bossart <[email protected]>
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Thu, 29 Feb 2024 16:00:44 +0000 (10:00 -0600)
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committer Nathan Bossart <[email protected]>
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Thu, 29 Feb 2024 16:00:44 +0000 (10:00 -0600)
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commit bd5132db558b6c8d11eb838be81e2177a95c7388
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tree c27f73a66003401092c363a65dae5f88e568af2d tree
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parent 5f2e179bd31e5f5803005101eb12a8d7bf8db8f3 commit | diff
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Introduce atomic read/write functions with full barrier semantics.
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Writing correct code using atomic variables is often difficult due
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to the memory barrier semantics (or lack thereof) of the underlying
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operations. This commit introduces atomic read/write functions
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with full barrier semantics to ease this cognitive load. For
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example, some spinlocks protect a single value, and these new
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functions make it easy to convert the value to an atomic variable
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(thus eliminating the need for the spinlock) without modifying the
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barrier semantics previously provided by the spinlock. Since these
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functions may be less performant than the other atomic reads and
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writes, they are not suitable for every use-case. However, using a
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single atomic operation with full barrier semantics may be more
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performant in cases where a separate explicit barrier would
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otherwise be required.
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The base implementations for these new functions are atomic
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exchanges (for writes) and atomic fetch/adds with 0 (for reads).
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These implementations can be overwritten with better architecture-
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specific versions as they are discovered.
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This commit leaves converting existing code to use these new
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functions as a future exercise.
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Reviewed-by: Andres Freund, Yong Li, Jeff Davis
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Discussion: https://postgr.es/m/20231110205128.GB1315705%40nathanxps13
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```
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src/include/port/atomics.h
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```
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+/*
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+ * pg_atomic_read_membarrier_u32 - read with barrier semantics.
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+ *
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+ * This read is guaranteed to return the current value, provided that the value
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+ * is only ever updated via operations with barrier semantics, such as
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+ * pg_atomic_compare_exchange_u32() and pg_atomic_write_membarrier_u32().
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+ * While this may be less performant than pg_atomic_read_u32(), it may be
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+ * easier to reason about correctness with this function in less performance-
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+ * sensitive code.
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+ *
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+ * Full barrier semantics.
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+ */
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+static inline uint32
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+pg_atomic_read_membarrier_u32(volatile pg_atomic_uint32 *ptr)
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+{
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+ AssertPointerAlignment(ptr, 4);
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+
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+ return pg_atomic_read_membarrier_u32_impl(ptr);
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+}
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+
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/*
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* pg_atomic_write_u32 - write to atomic variable.
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*
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@@ -274,6 +294,26 @@ pg_atomic_unlocked_write_u32(volatile pg_atomic_uint32 *ptr, uint32 val)
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pg_atomic_unlocked_write_u32_impl(ptr, val);
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}
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+/*
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+ * pg_atomic_write_membarrier_u32 - write with barrier semantics.
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+ *
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+ * The write is guaranteed to succeed as a whole, i.e., it's not possible to
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+ * observe a partial write for any reader. Note that this correctly interacts
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+ * with both pg_atomic_compare_exchange_u32() and
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+ * pg_atomic_read_membarrier_u32(). While this may be less performant than
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+ * pg_atomic_write_u32(), it may be easier to reason about correctness with
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+ * this function in less performance-sensitive code.
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+ *
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+ * Full barrier semantics.
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+ */
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+static inline void
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+pg_atomic_write_membarrier_u32(volatile pg_atomic_uint32 *ptr, uint32 val)
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+{
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+ AssertPointerAlignment(ptr, 4);
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+
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+ pg_atomic_write_membarrier_u32_impl(ptr, val);
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+}
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+
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/*
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* pg_atomic_exchange_u32 - exchange newval with current value
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*
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@@ -427,6 +467,15 @@ pg_atomic_read_u64(volatile pg_atomic_uint64 *ptr)
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return pg_atomic_read_u64_impl(ptr);
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}
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+static inline uint64
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+pg_atomic_read_membarrier_u64(volatile pg_atomic_uint64 *ptr)
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+{
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+#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
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+ AssertPointerAlignment(ptr, 8);
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+#endif
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+ return pg_atomic_read_membarrier_u64_impl(ptr);
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+}
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+
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static inline void
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pg_atomic_write_u64(volatile pg_atomic_uint64 *ptr, uint64 val)
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{
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@@ -436,6 +485,15 @@ pg_atomic_write_u64(volatile pg_atomic_uint64 *ptr, uint64 val)
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pg_atomic_write_u64_impl(ptr, val);
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}
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+static inline void
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+pg_atomic_write_membarrier_u64(volatile pg_atomic_uint64 *ptr, uint64 val)
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+{
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+#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
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+ AssertPointerAlignment(ptr, 8);
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+#endif
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+ pg_atomic_write_membarrier_u64_impl(ptr, val);
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+}
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+
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static inline uint64
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pg_atomic_exchange_u64(volatile pg_atomic_uin
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```
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#### [期望 PostgreSQL|开源PolarDB 增加什么功能?](https://github.com/digoal/blog/issues/76 "269ac3d1c492e938c0191101c7238216")
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#### [PolarDB 开源数据库](https://openpolardb.com/home "57258f76c37864c6e6d23383d05714ea")
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#### [PolarDB 学习图谱](https://www.aliyun.com/database/openpolardb/activity "8642f60e04ed0c814bf9cb9677976bd4")
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#### [购买PolarDB云服务折扣活动进行中, 55元起](https://www.aliyun.com/activity/new/polardb-yunparter?userCode=bsb3t4al "e0495c413bedacabb75ff1e880be465a")
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#### [PostgreSQL 解决方案集合](../201706/20170601_02.md "40cff096e9ed7122c512b35d8561d9c8")
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#### [德哥 / digoal's Github - 公益是一辈子的事.](https://github.com/digoal/blog/blob/master/README.md "22709685feb7cab07d30f30387f0a9ae")
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#### [About 德哥](https://github.com/digoal/blog/blob/master/me/readme.md "a37735981e7704886ffd590565582dd0")
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![digoal's wechat](../pic/digoal_weixin.jpg "f7ad92eeba24523fd47a6e1a0e691b59")
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‎202403/readme.md

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<a rel="nofollow" href="http://info.flagcounter.com/h9V1" ><img src="http://s03.flagcounter.com/count/h9V1/bg_FFFFFF/txt_000000/border_CCCCCC/columns_2/maxflags_12/viewers_0/labels_0/pageviews_0/flags_0/" alt="Flag Counter" border="0" ></a>
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### 文章列表
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----
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##### 20240302_01.md [《PostgreSQL 17 preview - 引入读写原子操作函数接口with full barrier semantics》](20240302_01.md)

‎README.md

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### 所有文档如下
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----
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##### 202403/20240302_01.md [《PostgreSQL 17 preview - 引入读写原子操作函数接口with full barrier semantics》](202403/20240302_01.md)
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----
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##### 202402/20240229_02.md [《pg_dumpbinary: PostgreSQL copy binary 格式逻辑备份》](202402/20240229_02.md)
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##### 202402/20240229_01.md [《pg_statement_rollback: 服务端auto savepoint和rollback statement支持》](202402/20240229_01.md)
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##### 202402/20240227_02.md [《PostgreSQL 15 Docker镜像学习环境 ARM64版》](202402/20240227_02.md)

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