Re: Inaccurate comments in ReorderBufferCheckMemoryLimit()

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.

Regards,

--
Masahiko Sawada
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