On Sat, Oct 1, 2016 at 2:44 AM, Andres Freund <and...@anarazel.de> wrote:

> Hi,
> On 2016-07-26 17:43:33 -0700, Andres Freund wrote:
> > In the attached patch I've attached simplehash.h, which can be
> > customized by a bunch of macros, before being inlined.  There's also a
> > patch using this for tidbitmap.c and nodeAgg/nodeSubplan/... via
> > execGrouping.c.
> Attached is a significantly improved version of this series.  The main
> changes are:
> - The hash table now uses robin-hood style hash collision handling. See the
>   commit message of 0002 (or simplehash.h) for an introduction. That
>   significantly reduces the impact of "clustering" due to linear
>   addressing.
> - Significant comment and correctness fixes, both in simplehash.h
> - itself, and 0003/0004.
> - a lot of other random performance improvements for the hashing code.
> Unfortunately I'm running out battery right now, so I don't want to
> re-run the benchmarks posted upthread (loading takes a while). But the
> last time I ran them all the results after the patches were better than
> before.
> This patch series currently consists out of four patches:
> - 0001 - add unlikely/likely() macros. The use here is not entirely
>   mandatory, but they do provide a quite consistent improvement. There
>   are other threads where they proved to be beneficial, so I see little
>   reason not to introduce them.
> - 0002 - the customizable hashtable itself. It now contains documentation.
> - 0003 - convert tidbitmap.c to use the new hashmap. This includes a fix
>   for the issue mentioned in [1], to improve peformance in heavily lossy
>   scenarios (otherwise we could regress in some extreme cases)
> - 0004 - convert execGrouping.c, e.g. used by hash aggregates
> While not quite perfect yet, I do think this is at a state where input
> is needed.  I don't want to go a lot deeper into this rabbit hole,
> before we have some agreement that this is a sensible course of action.
> I think there's several possible additional users of simplehash.h,
> e.g. catcache.c - which has an open coded, not particularly fast hash
> table and frequently shows up in profiles - but I think the above two
> conversions are plenty to start with.
> Comments?
> Greetings,
> Andres Freund
> [1] http://archives.postgresql.org/message-id/20160923205843.zcs
> 533sqdzlh4cpo%40alap3.anarazel.de
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This is a great addition.

A couple of comments.

* 80% occupancy is a bit conservative for RH hashing, it works well up to
90% if you use the early stops due to distance. So that TODO is worth

* An optimized memory layout for RH hashmap is along the lines of
HHHKVKVKV, using one bit of the hash as the present/absent flag. That plays
specially well with large occupancies (~90%). Due to RH the probings on the
H array are very short and within a single cache line. Even with a 31bit
hash it's reallyyy rare to have to probe more than 1 entry in the KV array.
Essentially guaranteeing that the 99% percentile of lookups will only hit a
couple of cache lines (if you ignore crossing cache lines and other

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