PostgreSQL 17 preview - WAL锁竞争优化 - reading WAL buffer contents without a lock, Additional write barrier in AdvanceXLInsertBuffer()
digoal
2023-12-20
PostgreSQL , PolarDB , DuckDB , WAL , lock , barrier , pg_atomic_write_u64
Additional write barrier in AdvanceXLInsertBuffer(). master github/master
author Jeff Davis <jdavis@postgresql.org>
Wed, 20 Dec 2023 01:35:54 +0000 (17:35 -0800)
committer Jeff Davis <jdavis@postgresql.org>
Wed, 20 Dec 2023 01:35:54 +0000 (17:35 -0800)
commit 766571be16598b401b5527208847145edc6be1f4
tree 8b996fa24f62b557b285ff07123a1d7e79a56092 tree
parent c3a8e2a7cb16d55e3b757934b538cb8b8a0eab02 commit | diff
Additional write barrier in AdvanceXLInsertBuffer().
First, mark the xlblocks member with InvalidXLogRecPtr, then issue a
write barrier, then initialize it. That ensures that the xlblocks
member doesn't appear valid while the contents are being initialized.
In preparation for reading WAL buffer contents without a lock.
Author: Bharath Rupireddy
Discussion: https://postgr.es/m/CALj2ACVfFMfqD5oLzZSQQZWfXiJqd-NdX0_317veP6FuB31QWA@mail.gmail.com
Reviewed-by: Andres Freund
+ /*
+ * Mark the xlblock with InvalidXLogRecPtr and issue a write barrier
+ * before initializing. Otherwise, the old page may be partially
+ * zeroed but look valid.
+ */
+ pg_atomic_write_u64(&XLogCtl->xlblocks[nextidx], InvalidXLogRecPtr);
+ pg_write_barrier();
+
Use 64-bit atomics for xlblocks array elements.
author Jeff Davis <jdavis@postgresql.org>
Wed, 20 Dec 2023 01:35:42 +0000 (17:35 -0800)
committer Jeff Davis <jdavis@postgresql.org>
Wed, 20 Dec 2023 01:35:42 +0000 (17:35 -0800)
commit c3a8e2a7cb16d55e3b757934b538cb8b8a0eab02
tree d22d792b0b99be09bb267399ede72c5f16419de8 tree
parent 1301c80b2167feb658a738fa4ceb1c23d0991e23 commit | diff
Use 64-bit atomics for xlblocks array elements.
In preparation for reading the contents of WAL buffers without a
lock. Also, avoids the previously-needed comment in GetXLogBuffer()
explaining why it's safe from torn reads.
Author: Bharath Rupireddy
Discussion: https://postgr.es/m/CALj2ACVfFMfqD5oLzZSQQZWfXiJqd-NdX0_317veP6FuB31QWA@mail.gmail.com
Reviewed-by: Andres Freund
- XLogRecPtr *xlblocks; /* 1st byte ptr-s + XLOG_BLCKSZ */
+ pg_atomic_uint64 *xlblocks; /* 1st byte ptr-s + XLOG_BLCKSZ */
+ * We don't hold a lock while we read the value. If someone is just about
+ * to initialize or has just initialized the page, it's possible that we
+ * get InvalidXLogRecPtr. That's ok, we'll grab the mapping lock (in
+ * AdvanceXLInsertBuffer) and retry if we see anything other than the page
+ * we're looking for.
