Re: [Patch] Optimize dropping of relation buffers using dlist

On Fri, Aug 7, 2020 at 3:03 AM Tomas Vondra
<tomas.vondra@2ndquadrant.com> wrote:
>
> >But I understand the sentiment on the added overhead on BufferAlloc.
> >Regarding the case where the patch would potentially affect workloads
> >that fit into RAM but not into shared buffers, could one of Andres'
> >suggested idea/s above address that, in addition to this patch's
> >possible shared invalidation fix? Could that settle the added overhead
> >in BufferAlloc() as temporary fix?
>
> Not sure.
>
> >Thomas Munro is also working on caching relation sizes [1], maybe that
> >way we could get the latest known relation size. Currently, it's
> >possible only during recovery in smgrnblocks.
>
> It's not clear to me how would knowing the relation size help reducing
> the overhead of this patch?
>

AFAICU the idea is to directly call BufTableLookup (similar to how we
do in BufferAlloc) to find the buf_id in function
DropRelFileNodeBuffers and then invalidate the required buffers.  And,
we need to do this when the size of the relation is less than some
threshold. So, I think the crux would be to reliably get the number of
blocks information. So, probably relation size cache stuff might help.

-- 
With Regards,
Amit Kapila.