Re: [RFC PATCH 03/26] mm: make pageblock_order 2M per default

[Mel Gorman]

On Tue, Apr 18, 2023 at 03:12:50PM -0400, Johannes Weiner wrote:
> pageblock_order can be of various sizes, depending on configuration,
> but the default is MAX_ORDER-1. Given 4k pages, that comes out to
> 4M. This is a large chunk for the allocator/reclaim/compaction to try
> to keep grouped per migratetype. It's also unnecessary as the majority
> of higher order allocations - THP and slab - are smaller than that.
> 
> Before subsequent patches increase the effort that goes into
> maintaining migratetype isolation, it's important to first set the
> defrag block size to what's likely to have common consumers.
> 
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>

This patch may be a distraction in the context of this series. I don't feel
particularly strongly about it but it has strong bikeshed potential. For
configurations that support huge pages of any sort, it should be PMD_ORDER,
for anything else the choice is arbitrary. 2M is as good a guess as
anyway because even if it was tied to the PAGE_ALLOC_COSTLY_ORDER then
the pageblock bitmap overhead might be annoying.

-- 
Mel Gorman
SUSE Labs