Обсуждение: Update comments in multixact.c

Hi,

I noticed that commit 5212d447fa updated some comments in multixact.c because
SLRU truncation for multixacts is performed during VACUUM, instead of
checkpoint. Should the following comments which mentioned checkpointer be
changed, too?

1.
* we compute it (using nextMXact if none are valid).  Each backend is
* required not to attempt to access any SLRU data for MultiXactIds older
* than its own OldestVisibleMXactId[] setting; this is necessary because
* the checkpointer could truncate away such data at any instant.

2.
 * We set the OldestVisibleMXactId for a given transaction the first time
 * it's going to inspect any MultiXactId.  Once we have set this, we are
 * guaranteed that the checkpointer won't truncate off SLRU data for
 * MultiXactIds at or after our OldestVisibleMXactId.

Regards,
Shi yu

On Tue, Jan 17, 2023 at 1:33 AM shiy.fnst@fujitsu.com
<shiy.fnst@fujitsu.com> wrote:
> I noticed that commit 5212d447fa updated some comments in multixact.c because
> SLRU truncation for multixacts is performed during VACUUM, instead of
> checkpoint. Should the following comments which mentioned checkpointer be
> changed, too?

Yes, I think so.

-- 
Peter Geoghegan

On Wed, Jan 18, 2023 6:04 AM Peter Geoghegan <pg@bowt.ie> wrote:
> 
> On Tue, Jan 17, 2023 at 1:33 AM shiy.fnst@fujitsu.com
> <shiy.fnst@fujitsu.com> wrote:
> > I noticed that commit 5212d447fa updated some comments in multixact.c
> because
> > SLRU truncation for multixacts is performed during VACUUM, instead of
> > checkpoint. Should the following comments which mentioned checkpointer be
> > changed, too?
> 
> Yes, I think so.

Thanks for your reply.

Attach a patch which fixed them.

Regards,
Shi yu

On Wed, Jan 18, 2023 at 2:02 AM shiy.fnst@fujitsu.com
<shiy.fnst@fujitsu.com> wrote:
> Thanks for your reply.
>
> Attach a patch which fixed them.

Pushed something close to that just now. I decided that it was better
to not specify when truncation happened in these two places at all,
though. The important detail is that it can happen if certain rules
are not followed.

Thanks
-- 
Peter Geoghegan