Re: [PoC] Improve dead tuple storage for lazy vacuum

On Wed, Sep 21, 2022 at 01:17:21PM +0700, John Naylor wrote:
> In trying to wrap the SIMD code behind layers of abstraction, the latest
> patch (and Nathan's cleanup) threw it away in almost all cases. To explain,
> we need to talk about how vectorized code deals with the "tail" that is too
> small for the register:
>
> 1. Use a one-by-one algorithm, like we do for the pg_lfind* variants.
> 2. Read some junk into the register and mask off false positives from the
> result.
>
> There are advantages to both depending on the situation.
>
> Patch v5 and earlier used #2. Patch v6 used #1, so if a node16 has 15
> elements or less, it will iterate over them one-by-one exactly like a
> node4. Only when full with 16 will the vector path be taken. When another
> entry is added, the elements are copied to the next bigger node, so there's
> a *small* window where it's fast.
>
> In short, this code needs to be lower level so that we still have full
> control while being portable. I will work on this, and also the related
> code for node dispatch.

Is it possible to use approach #2 here, too? AFAICT space is allocated for
all of the chunks, so there wouldn't be any danger in searching all them
and discarding any results >= node->count. Granted, we're depending on the
number of chunks always being a multiple of elements-per-vector in order to
avoid the tail path, but that seems like a reasonably safe assumption that
can be covered with comments.

--
Nathan Bossart
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