Re: [PATCH v2 1/2] mm/page_alloc: Clarify batch tuning in zone_batchsize

[Vlastimil Babka]

On 10/9/25 21:29, Joshua Hahn wrote:
> Recently while working on another patch about batching
> free_pcppages_bulk [1], I was curious why pcp->batch was always 63 on my
> machine. This led me to zone_batchsize(), where I found this set of
> lines to determine what the batch size should be for the host:
> 
> 	batch = min(zone_managed_pages(zone) >> 10, SZ_1M / PAGE_SIZE);
> 	batch /= 4;		/* We effectively *= 4 below */
> 	if (batch < 1)
> 		batch = 1;
> 
> All of this is good, except the comment above which says "We effectively
> *= 4 below". Nowhere else in the function zone_batchsize(), is there a
> corresponding multipliation by 4. Looking into the history of this, it
> seems like Dave Hansen had also noticed this back in 2013 [1]. Turns out
> there *used* to be a corresponding *= 4, which was turned into a *= 6
> later on to be used in pageset_setup_from_batch_size(), which no longer
> exists.
> 
> Despite this mismatch not being corrected in the comments, it seems that
> getting rid of the /= 4 leads to a performance regression on machines
> with less than 250G memory and 176 processors. As such, let us preserve
> the functionality but clean up the comments.
> 
> Fold the /= 4 into the calculation above: bitshift by 10+2=12, and
> instead of dividing 1MB, divide 256KB and adjust the comments
> accordingly. No functional change intended.
> 
> Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxx>
> Signed-off-by: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>