Re: LogwrtResult contended spinlock

Hi Alvaro,

Thank you for looking at this!

29.06.2024 13:23, Alvaro Herrera wrote:
>> 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 */

It doesn't, but the following works for me:
  static inline uint64
  pg_atomic_monotonic_advance_u64(volatile pg_atomic_uint64 *ptr, uint64 target_)
  {
-       uint64          currval;
+       pg_attribute_aligned(8) uint64          currval;

because the failed assertion is:
#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
     AssertPointerAlignment(&currval, 8);
#endif

Best regards,
Alexander