Re: ProcArrayGroupClearXid() compare-exchange style

On Tue, Oct 15, 2019 at 9:23 AM Noah Misch <noah(at)leadboat(dot)com> wrote:
>
> ProcArrayGroupClearXid() has this:
>
> while (true)
> {
> nextidx = pg_atomic_read_u32(&procglobal->procArrayGroupFirst);
>
> ...
>
> if (pg_atomic_compare_exchange_u32(&procglobal->procArrayGroupFirst,
> &nextidx,
> (uint32) proc->pgprocno))
> break;
> }
>
> This, from UnpinBuffer(), is our more-typical style:
>
> old_buf_state = pg_atomic_read_u32(&buf->state);
> for (;;)
> {
> ...
>
> if (pg_atomic_compare_exchange_u32(&buf->state, &old_buf_state,
> buf_state))
> break;
> }
>
> That is, we typically put the pg_atomic_read_u32() outside the loop. After
> the first iteration, it is redundant with the side effect of
> pg_atomic_compare_exchange_u32(). I haven't checked whether this materially
> improves performance, but, for style, I would like to change it in HEAD.
>

+1. I am not sure if it would improve performance as this whole
optimization was to reduce the number of attempts to acquire LWLock,
but definitely, it makes the code consistent.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com