Re: Improve eviction algorithm in ReorderBuffer

On Fri, Mar 29, 2024 at 12:13 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Fri, Mar 29, 2024 at 2:09 PM vignesh C <vignesh21(at)gmail(dot)com> wrote:
> >
> > On Tue, 26 Mar 2024 at 10:05, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > >
> > > On Thu, Mar 14, 2024 at 12:02 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > > >
> > > >
> > > > I've attached new version patches.
> > >
> > > Since the previous patch conflicts with the current HEAD, I've
> > > attached the rebased patches.
> >
> > Thanks for the updated patch.
> > One comment:
> > I felt we can mention the improvement where we update memory
> > accounting info at transaction level instead of per change level which
> > is done in ReorderBufferCleanupTXN, ReorderBufferTruncateTXN, and
> > ReorderBufferSerializeTXN also in the commit message:
>
> Agreed.
>
> I think the patch is in good shape. I'll push the patch with the
> suggestion next week, barring any objections.
>

Few minor comments:
1.
@@ -3636,6 +3801,8 @@ ReorderBufferCheckMemoryLimit(ReorderBuffer *rb)
Assert(txn->nentries_mem == 0);
}

+ ReorderBufferMaybeResetMaxHeap(rb);
+

Can we write a comment about why this reset is required here?
Otherwise, the reason is not apparent.

2.
Although using max-heap to select the largest
+ * transaction is effective when there are many transactions being decoded,
+ * there is generally no need to use it as long as all transactions being
+ * decoded are top-level transactions. Therefore, we use MaxConnections as the
+ * threshold so we can prevent switching to the state unless we use
+ * subtransactions.
+ */
+#define MAX_HEAP_TXN_COUNT_THRESHOLD MaxConnections

Isn't using max-heap equally effective in finding the largest
transaction whether there are top-level or top-level plus
subtransactions? This comment indicates it is only effective when
there are subtransactions.

--
With Regards,
Amit Kapila.