Re: optimize several list functions with SIMD intrinsics - Mailing list pgsql-hackers

On 21/04/2023 23:33, Nathan Bossart wrote:
> On Fri, Apr 21, 2023 at 01:50:34PM +0700, John Naylor wrote:
>> On Wed, Mar 8, 2023 at 7:25 AM Nathan Bossart <nathandbossart@gmail.com>
>> wrote:
>>> was mostly a fun weekend project, and I don't presently have any concrete
>>> examples of workloads where this might help.
>>
>> It seems like that should be demonstrated before seriously considering
>> this, like a profile where the relevant list functions show up.
> 
> Agreed.

Grepping for "tlist_member" and "list_delete_ptr", I don't see any 
callers in hot codepaths where this could make a noticeable difference. 
So I've marked this as Returned with Feedback in the commitfest.

> I noticed that several of the List functions do simple linear searches that
> can be optimized with SIMD intrinsics (as was done for XidInMVCCSnapshot in
> 37a6e5d).  The following table shows the time spent iterating over a list
> of n elements (via list_member_int) one billion times on my x86 laptop.
> 
> 
>       n   | head (ms) | patched (ms) 
>     ------+-----------+--------------
>         2 |      3884 |         3001
>         4 |      5506 |         4092
>         8 |      6209 |         3026
>        16 |      8797 |         4458
>        32 |     25051 |         7032
>        64 |     37611 |        12763
>       128 |     61886 |        22770
>       256 |    111170 |        59885
>       512 |    209612 |       103378
>      1024 |    407462 |       189484

I'm surprised to see an improvement with n=2 and n=2. AFAICS, the 
vectorization only kicks in when n >= 8.

-- 
Heikki Linnakangas
Neon (https://neon.tech)