Re: [PATCH] Batched clock sweep to reduce cross-socket atomic contention

Hi,

FWIW, I think it's quite unhelpful to combine a thread to make a relatively
local optimization (batched clock sweep) with huge changes to how buffer
replacement works. I don't see how those things are related.

On 2026-07-22 17:52:02 +0300, Ants Aasma wrote:
> I think the initial results are a promising indication that G-CLOCKs
> counters are not providing much value.

I am doubtful that the benchmarks in this thread actually shows anything
particularly interesting wrt that. There's just not enough interesting IO in
pgbench -S:

- The indexes are too small compared to the table data to be interesting, even
index leaf pages are unlikely to be evicted.

- The fact that realistically there's at max two misses for a single query
(one index leaf page and the heap page) rather substantially limits the max
impact the wrong buffer replacement triggers.

- I don't think the hit ratio is an all that interesting way to evaluate the
cache replacement, particularly for something like pgbench -S. It really
matters *which* buffers are evicted.

- pgbench -S has about the same penalty for an index miss as for a heap miss
(since both trigger one synchronous IO wait and since there's 1 leaf page
and one heap access per query). That's not true for many other workloads.

Greetings,

Andres Freund