Re: Read-Write optimistic lock (Re: sinvaladt.c: remove msgnumLock, use atomic operations on maxMsgNum)

04.06.2025 00:04, Andres Freund пишет:
> Hi,
> 
> On 2025-06-02 21:20:33 +0300, Yura Sokolov wrote:
>> But still problem of spin lock contention is here.
> 
> I still would like to see a reproducer for this.

For problem in sinvaladt.c we have no synthetic reproducer. But version
with changed maxMsgNum to atomic solved customer's issue.

For problem on XLogRecoveryCtlData->info_lck we have reproducer which shows
follower with a lot of logical walsenders with patched version lags less
and uses less CPU... but only 4-socket Xeon(R) Gold 6348H. There is no
benefit from patch on 2-socket Intel(R) Xeon(R) Gold 5220R.

>> So I propose to introduce another spin lock type capable for Exclusive and
>> Shared lock modes (i.e. Write/Read modes) and use it in this two places.
> 
> I am vehemently opposed to that. We should work towards getting rid of
> spinlocks, not introduce more versions of spinlocks. Userspace spinlocks
> largely are a bad idea. We should instead make properly queued locks cheaper
> (e.g. by having an exclusive-only lock, which can be cheaper to release on
> common platforms).

1. when faster lock will arrive?
2. "exclusive-only" lock will never scale at scenario when there are few
writers and a lot of readers. It is just direct consequence of Amdahl's Law.


To be honestly, our version of XLogRecoveryCtlData->info_lck fix is based
on optimistic read-write lock. I've tried to make more regular read-write
spin lock in previous patch version to make it more familiar.

But after playing a bit with variants using simple C program [1], I found
optimistic lock is only really scalable solution for the problem (aside of
direct use of atomic operations).

So attached version contains optimistic read-write lock used in these two
places.

[1] https://pastebin.com/CwSPkeGi

-- 
regards
Yura Sokolov aka funny-falcon