Re: Inaccurate comments in ReorderBufferCheckMemoryLimit()

On Tue, Aug 1, 2023 at 2:06 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Tue, Aug 1, 2023 at 11:33 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Mon, Jul 31, 2023 at 8:46 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > >
> > > While reading the code, I realized that the following code comments
> > > might not be accurate:
> > >
> > > /*
> > > * Pick the largest transaction (or subtransaction) and evict it from
> > > * memory by streaming, if possible. Otherwise, spill to disk.
> > > */
> > > if (ReorderBufferCanStartStreaming(rb) &&
> > > (txn = ReorderBufferLargestStreamableTopTXN(rb)) != NULL)
> > > {
> > > /* we know there has to be one, because the size is not zero */
> > > Assert(txn && rbtxn_is_toptxn(txn));
> > > Assert(txn->total_size > 0);
> > > Assert(rb->size >= txn->total_size);
> > >
> > > ReorderBufferStreamTXN(rb, txn);
> > > }
> > >
> > > AFAICS since ReorderBufferLargestStreamableTopTXN() returns only
> > > top-level transactions, the comment above the if statement is not
> > > right. It would not pick a subtransaction.
> > >
> >
> > I think the subtransaction case is for the spill-to-disk case as both
> > cases are explained in the same comment.
> >
> > > Also, I'm not sure that the second comment "we know there has to be
> > > one, because the size is not zero" is right since there might not be
> > > top-transactions that are streamable.
> > >
> >
> > I think this comment is probably referring to asserts related to the
> > size similar to spill to disk case.
> >
> > How about if we just remove (or subtransaction) from the following
> > comment: "Pick the largest transaction (or subtransaction) and evict
> > it from memory by streaming, if possible. Otherwise, spill to disk."?
> > Then by referring to streaming/spill-to-disk cases, one can understand
> > in which cases only top-level xacts are involved and in which cases
> > both are involved.
>
> Sounds good. I've updated the patch accordingly.
>

LGTM.

--
With Regards,
Amit Kapila.