Horton

Re: Change GUC hashtable to use simplehash?

John Naylor <johncnaylorls@gmail.com>

View thread
From: John Naylor <johncnaylorls@gmail.com>
To: Jeff Davis <pgsql@j-davis.com>
Cc: Andres Freund <andres@anarazel.de>, Tom Lane <tgl@sss.pgh.pa.us>, Gurjeet Singh <gurjeet@singh.im>, pgsql-hackers@postgresql.org
Date: 2023-11-22T13:06:34Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Silence warning in older versions of Valgrind

  2. Revert "Speed up tail processing when hashing aligned C strings, take two"

  3. Speed up tail processing when hashing aligned C strings, take two

  4. Teach fasthash_accum to use platform endianness for bytewise loads

  5. Add macro to disable address safety instrumentation

  6. Convert uses of hash_string_pointer to fasthash equivalent

  7. Speed up tail processing when hashing aligned C strings

  8. Add helper functions for dshash tables with string keys.

  9. Fix warnings in cpluspluscheck

  10. Further cosmetic review of hashfn_unstable.h

  11. Simplify initialization of incremental hash state

  12. Add optimized C string hashing

  13. Add inline incremental hash functions for in-memory use

  14. Make all Perl warnings fatal

Attachments

I wrote:

> Thinking some more, I'm not quite comfortable with the number of
> places in these patches that have to know about the pre-downcased
> strings, or whether we need that in the first place. If lower case is
> common enough to optimize for, it seems the equality function can just
> check strict equality on the char and only on mismatch try downcasing
> before returning false. Doing our own function would allow the
> compiler to inline it, or at least keep it on the same page. Further,
> the old hash function shouldn't need to branch to do the same
> downcasing, since hashing is lossy anyway. In the keyword hashes, we
> just do "*ch |= 0x20", which downcases letters and turns undercores to
> DEL. I can take a stab at that later.

v4 is a quick POC for that. I haven't verified that it's correct for
the case of the probe and the entry don't match, but in case it
doesn't it should be easy to fix. I also didn't bother with
SH_STORE_HASH in my testing.

0001 adds the murmur32 finalizer -- we should do that regardless of
anything else in this thread.
0002 is just Jeff's 0001
0003 adds an equality function that downcases lazily, and teaches the
hash function about the 0x20 trick.

master:
latency average = 581.765 ms

v3 0001-0005:
latency average = 544.576 ms

v4 0001-0003:
latency average = 547.489 ms

This gives similar results with a tiny amount of code (excluding the
simplehash conversion). I didn't check if the compiler inlined these
functions, but we can hint it if necessary. We could use the new
equality function in all the call sites that currently test for
"guc_name_compare() == 0", in which case it might not end up inlined,
but that's probably okay.

We could also try to improve the hash function's collision behavior by
collecting the bytes on a uint64 and calling our new murmur64 before
returning the lower half, but that's speculative.