Re: Speeding up GIST index creation for tsvectors

On Thu, 11 Mar 2021 at 04:25, John Naylor <john(dot)naylor(at)enterprisedb(dot)com> wrote:
> Okay, so it's hard-coded in various functions in contrib modules. In that
> case, let's just keep the existing coding for those. In fact, the comments
> that got removed by your patch specifically say: /* Using the popcount
> functions here isn't likely to win */ ...which your testing confirmed. The
> new function should be used only for Gist and possibly Intarray, whose
> default is 124. That means we can't get rid of hemdistsign(), but that's
> fine.

The comment is there for all types. Since I get the performance better
on all the types, I have kept the pg_xorcount() call for all these
contrib modules. I understand that since for some types default
siglen is small, we won't get benefit. But I think we should call
pg_xorcount() for the benefit of non-default siglen case.

Have replaced hemdistsign() by pg_xorcount() everywhere; but with
that, the call looks a bit clumsy because of having to type-cast the
parameters to const unsigned char *. I noticed that the cast to
"unsigned char *" is required only when we use the value in the
pg_number_of_ones array. Elsewhere we can safely use 'a' and 'b' as
"char *". So I changed the pg_xorcount() parameters from unsigned char
* to char *.

> I think the way to go is a simplified version of your 0001 (not 0002), with
> only a single function, for gist and intarray only, and a style that better
> matches the surrounding code. If you look at my xor functions in the attached
> text file, you'll get an idea of what it should look like. Note that it got
> the above performance without ever trying to massage the pointer alignment.
> I'm a bit uncomfortable with the fact that we can't rely on alignment, but
> maybe there's a simple fix somewhere in the gist code.

In the attached 0001-v3 patch, I have updated the code to match it
with surrounding code; specifically the usage of *a++ rather than
a[i].

Regarding the alignment changes... I have removed the code that
handled the leading identically unaligned bytes, for lack of evidence
that percentage of such cases is significant. Like I noted earlier,
for the tsearch data I used, identically unaligned cases were only 6%.
If I find scenarios where these cases can be significant after all and
if we cannot do anything in the gist index code, then we might have to
bring back the unaligned byte handling. I didn't get a chance to dig
deeper into the gist index implementation to see why they are not
always 8-byte aligned.

I have kept the 0002 patch as-is. Due to significant *additional*
speedup, over and above the 0001 improvement, I think the code
re-arrangement done is worth it for non-x86 platforms.

-Amit Khandekar