Re: heavily contended lwlocks with long wait queues scale badly

On 11/1/22 1:41 PM, Andres Freund wrote:

>> Andres: when you suggested backpatching, were you thinking of the Nov 2022
>> release or the Feb 2023 release?
> 
> I wasn't thinking that concretely. Even if we decide to backpatch, I'd be very
> hesitant to do it in a few days.

Yeah this was my thinking (and also why I took a few days to reply given 
the lack of urgency for this release). It would at least give some more 
time for others to test it to feel confident that we're not introducing 
noticeable regressions.

> 
> 
> 
> I tested with browser etc running, so this is plenty noisy. I used the best of
> the two pgbench -T21 -P5 tps, after ignoring the first two periods (they're
> too noisy). I used an ok-ish NVMe SSD, rather than the the expensive one that
> has "free" fsync.
> 
> synchronous_commit=on:
> 
> clients   master	     fix
> 16          6196            6202
> 64         25716           25545
> 256        90131           90240
> 1024      128556          151487
> 2048       59417          157050
> 4096       32252          178823
> 
> 
> synchronous_commit=off:
> 
> clients   master	     fix
> 16        409828	  409016
> 64        454257          455804
> 256       304175          452160
> 1024      135081          334979
> 2048       66124          291582
> 4096       27019          245701
> 
> 
> Hm. That's a bigger effect than I anticipated. I guess sc=off isn't actually
> required, due to the level of concurrency making group commit very
> effective.
> 
> This is without an index, serial column or anything. But a quick comparison
> for just 4096 clients shows that to still be a big difference if I create an
> serial primary key:
> master: 26172
> fix: 155813

🤯 (seeing if my exploding head makes it into the archives).

Given the lack of ABI changes (hesitant to say low-risk until after more 
testing, but seemingly low-risk), I can get behind backpatching esp if 
we're targeting Feb 2023 so we can tests some more.

With my advocacy hat on, it bums me that we may not get as much buzz 
about this change given it's not in a major release, but 1/ it'll fix an 
issue that will help users with high-concurrency and 2/ users would be 
able to perform a simpler update to get the change.

Thanks,

Jonathan