On Tue, Dec 22, 2020 at 5:41 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Tue, Dec 22, 2020 at 2:55 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > Apart from tests, do let me know if you are happy with the changes in
> > the patch? Next, I'll look into DropRelFileNodesAllBuffers()
> > optimization patch.
> >
>
> Review of v35-0004-Optimize-DropRelFileNodesAllBuffers-in-recovery [1]
> ========================================================
>
> In this code, I am slightly worried about the additional cost of each
> time checking smgrexists. Consider a case where there are many
> relations and only one or few of them have not cached the information,
> in such a case we will pay the cost of smgrexists for many relations
> without even going to the optimized path. Can we avoid that in some
> way or at least reduce its usage to only when it is required? One idea
> could be that we first check if the nblocks information is cached and
> if so then we don't need to call smgrnblocks, otherwise, check if it
> exists. For this, we need an API like smgrnblocks_cahced, something we
> discussed earlier but preferred the current API. Do you have any
> better ideas?
>
One more idea which is not better than what I mentioned above is that
we completely avoid calling smgrexists and rely on smgrnblocks. It
will throw an error in case the particular fork doesn't exist and we
can use try .. catch to handle it. I just mentioned it as it came
across my mind but I don't think it is better than the previous one.
One more thing about patch:
+ /* Get the number of blocks for a relation's fork */
+ block[i][j] = smgrnblocks(smgr_reln[i], j, &cached);
+
+ if (!cached)
+ goto buffer_full_scan;
Why do we need to use goto here? We can simply break from the loop and
then check if (cached && nBlocksToInvalidate <
BUF_DROP_FULL_SCAN_THRESHOLD). I think we should try to avoid goto if
possible without much complexity.
--
With Regards,
Amit Kapila.
