Re: Further XLogInsert scaling tweaking

On Mon, Sep 2, 2013 at 10:32 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> On Mon, Sep 2, 2013 at 10:14:03AM +0300, Heikki Linnakangas wrote:
>> diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
>> index 39c58d0..28e62ea 100644
>> --- a/src/backend/access/transam/xlog.c
>> +++ b/src/backend/access/transam/xlog.c
>> @@ -428,8 +428,14 @@ typedef struct XLogCtlInsert
>> uint64 CurrBytePos;
>> uint64 PrevBytePos;
>>
>> - /* insertion slots, see above for details */
>> - XLogInsertSlotPadded *insertSlots;
>> + /*
>> + * Make sure the above heavily-contended spinlock and byte positions are
>> + * on their own cache line. In particular, the RedoRecPtr and full page
>> + * write variables below should be on a different cache line. They are
>> + * read on every WAL insertion, but updated rarely, and we don't want
>> + * those reads to steal the cache line containing Curr/PrevBytePos.
>> + */
>> + char pad[128];
>
> Do we adjust for cache line lengths anywhere else? PGPROC? Should it
> be a global define?

+1 -- that is, I think it should be.

merlin