Re: Speed-up shared buffers prewarming

On Wed, Mar 15, 2023 at 4:38 PM Konstantin Knizhnik <knizhnik(at)garret(dot)ru> wrote:
> It is well known fact that queries using sequential scan can not be used to prewarm cache, because them are using ring buffer
> even if shared buffers are almost empty.
> I have searched hackers archive but failed to find any discussion about it.
> What are the drawbacks of using free buffers even with BAM_BULKREAD strategy?
> I mean the following trivial patch:

It has been brought up at least in 2014 [1] and 2020 [2]
The part relevant to your patch is in the thread from 2020 here [3].
This quote in particular:

> a) Don't evict buffers when falling off the ringbuffer as long as
> there unused buffers on the freelist. Possibly just set their
> usagecount to zero as long that is the case.

> diff --git a/src/backend/storage/buffer/freelist.c b/src/backend/storage/buffer/freelist.c
> index 6be80476db..243335d0e4 100644
> --- a/src/backend/storage/buffer/freelist.c
> +++ b/src/backend/storage/buffer/freelist.c
> @@ -208,8 +208,15 @@ StrategyGetBuffer(BufferAccessStrategy strategy, uint32 *buf_state)
> /*
> * If given a strategy object, see whether it can select a buffer. We
> * assume strategy objects don't need buffer_strategy_lock.
> */
> - if (strategy != NULL)
> + if (strategy != NULL && StrategyControl->firstFreeBuffer < 0)
> {
> buf = GetBufferFromRing(strategy, buf_state);
> if (buf != NULL)
>
> So if there are free buffers, then use normal buffer allocation instead of GetBufferFromRing.

Similar to what you did.

- Melanie

[1] https://www.postgresql.org/message-id/flat/CAJRYxuL98fE_QN7McnCM5HUo8p9ceNJw%3D20GoN5NVdZdueJFqg%40mail.gmail.com
[2] https://www.postgresql.org/message-id/flat/20200206040026.trjzsmdsbl4gu2b6%40alap3.anarazel.de
[5] https://www.postgresql.org/message-id/20200206040026.trjzsmdsbl4gu2b6%40alap3.anarazel.de