Horton

Re: LogwrtResult contended spinlock

Alvaro Herrera <alvherre@alvh.no-ip.org>

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From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Alexander Lakhin <exclusion@gmail.com>
Cc: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>, Andres Freund <andres@anarazel.de>, Jeff Davis <pgsql@j-davis.com>, pgsql-hackers@lists.postgresql.org, Jaime Casanova <jcasanov@systemguards.com.ec>
Date: 2024-06-29T10:23:02Z
Lists: pgsql-hackers 
Hi

On 2024-Jun-29, Alexander Lakhin wrote:

> ----------------------------------- stderr -----------------------------------
> TRAP: failed Assert("TYPEALIGN(8, (uintptr_t)(&currval)) ==
> (uintptr_t)(&currval)"), File: "...\src\include\port/atomics.h", Line: 597,
> PID: 7556
> child process was terminated by exception 0xC0000409

Oh.  This is the new assertion in pg_atomic_monotonic_advance_u64() and
therefore the only possible culprit could be logInsertResult in
XLogCtlData.  Does it work if you do this?

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 8dcdf5a764..e581488d57 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -468,7 +468,7 @@ typedef struct XLogCtlData
 	XLogRecPtr	lastSegSwitchLSN;
 
 	/* These are accessed using atomics -- info_lck not needed */
-	pg_atomic_uint64 logInsertResult;	/* last byte + 1 inserted to buffers */
+	pg_atomic_uint64 logInsertResult pg_attribute_aligned(8);	/* last byte + 1 inserted to buffers */
 	pg_atomic_uint64 logWriteResult;	/* last byte + 1 written out */
 	pg_atomic_uint64 logFlushResult;	/* last byte + 1 flushed */
 

Thanks

-- 
Álvaro Herrera         PostgreSQL Developer  —  https://www.EnterpriseDB.com/
"Use it up, wear it out, make it do, or do without"

Commits

  1. Remove bogus assertion in pg_atomic_monotonic_advance_u64

  2. Add XLogCtl->logInsertResult

  3. Operate XLogCtl->log{Write,Flush}Result with atomics

  4. Split XLogCtl->LogwrtResult into separate struct members

  5. Introduce atomic read/write functions with full barrier semantics.

  6. Reduce the number of GetFlushRecPtr() calls done by walsenders.

  7. Remove most volatile qualifiers from xlog.c