Re: Fix memory counter update in reorderbuffer

On Fri, Aug 9, 2024 at 3:30 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Thu, Aug 8, 2024 at 9:43 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > On Wed, Aug 7, 2024 at 3:17 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > > On Wed, Aug 7, 2024 at 7:42 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > >
> > > >
> > > > > BTW, commit 5bec1d6bc5e also introduced
> > > > > ReorderBufferChangeMemoryUpdate() in ReorderBufferTruncateTXN() which
> > > > > is also worth considering while fixing the reported problem. It may
> > > > > not have the same problem as you have reported but we can consider
> > > > > whether setting txn size as zero explicitly is required or not.
> > > >
> > > > The reason why it introduced ReorderBufferChangeMemoryUpdate() is the
> > > > same as I mentioned above. And yes, as you mentioned, it doesn't have
> > > > the same problem that I reported here.
> > > >
> > >
> > > I checked again and found that ReorderBufferResetTXN() first calls
> > > ReorderBufferTruncateTXN() and then ReorderBufferToastReset(). After
> > > that, it also tries to free spec_insert change which should have the
> > > same problem. So, what saves this path from the same problem?
> >
> > Good catch. I've not created a test case for that but it seems to be
> > possible to happen.
> >
> > I think that subtracting txn->size to reduce the memory counter to
> > zero seems to be a wrong idea in the first place. If we want to save
> > updating memory counter and max-heap, we should use the exact memory
> > size that we freed. In other words, just change the memory usage
> > update to a batch operation.
> >
>
> Sounds reasonable but how would you find the size for a batch update
> operation? Are you planning to track it while freeing the individual
> changes?

Yes, one idea is to make ReorderBufferReturnChange() return the memory
size that it just freed. Then the caller who wants to update the
memory counter in a batch sums up the memory size.

Regards,

--
Masahiko Sawada
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