Re: [HACKERS] Block level parallel vacuum - Mailing list pgsql-hackers

On Mon, Oct 28, 2019 at 6:08 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> On Fri, Oct 25, 2019 at 9:19 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > On Fri, Oct 25, 2019 at 2:06 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> > >
> > > On Fri, Oct 25, 2019 at 10:22 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > > >
> > > > For more detail of my idea it is that the first worker who entered to
> > > > vacuum_delay_point adds its local value to shared value and reset the
> > > > local value to 0. And then the worker sleeps if it exceeds
> > > > VacuumCostLimit but before sleeping it can subtract VacuumCostLimit
> > > > from the shared value. Since vacuum_delay_point are typically called
> > > > per page processed I expect there will not such problem. Thoughts?
> > >
> > > Oh right, I assumed that when the local balance is exceeding the
> > > VacuumCostLimit that time you are adding it to the shared value but
> > > you are adding it to to shared value every time in vacuum_delay_point.
> > > So I think your idea is correct.
> >
> > I've attached the updated patch set.
> >
> > First three patches add new variables and a callback to index AM.
> >
> > Next two patches are the main part to support parallel vacuum. I've
> > incorporated all review comments I got so far. The memory layout of
> > variable-length index statistics might be complex a bit. It's similar
> > to the format of heap tuple header, having a null bitmap. And both the
> > size of index statistics and actual data for each indexes follows.
> >
> > Last patch is a PoC patch that implements the shared vacuum cost
> > balance. For now it's separated but after testing both approaches it
> > will be merged to 0004 patch. I'll test both next week.
> >
> > This patch set can be applied on top of the patch[1] that improves
> > gist index bulk-deletion. So canparallelvacuum of gist index is true.
> >
>
> + /* Get the space for IndexBulkDeleteResult */
> + bulkdelete_res = GetIndexBulkDeleteResult(shared_indstats);
> +
> + /*
> + * Update the pointer to the corresponding bulk-deletion result
> + * if someone has already updated it.
> + */
> + if (shared_indstats->updated && stats[idx] == NULL)
> + stats[idx] = bulkdelete_res;
> +
>
> I have a doubt in this hunk,  I do not understand when this condition
> will be hit?  Because whenever we are setting shared_indstats->updated
> to true at the same time we are setting stats[idx] to shared stat.  So
> I am not sure in what case the shared_indstats->updated will be true
> but stats[idx] is still pointing to NULL?
>

I think it can be true in the case where one parallel vacuum worker
vacuums the index that was vacuumed by other workers in previous index
vacuum cycle. Suppose that worker-A and worker-B vacuumed index-A and
index-B respectively. After that worker-A vacuum index-B in the next
index vacuum cycle. In this case, shared_indstats->updated is true
because worker-B already vacuumed in the previous vacuum cycle. On the
other hand stats[idx] on worker-A is NULL because it's first time for
worker-A to vacuum index-B. Therefore worker-A updates its stats[idx]
to the bulk-deletion result on DSM in order to pass it to the index
AM.

Regards,

--
Masahiko Sawada