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Date:   Fri, 21 Apr 2023 13:37:19 +0100
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     linux-mm@kvack.org, Kaiyang Zhao <kaiyang2@cs.cmu.edu>,
        Vlastimil Babka <vbabka@suse.cz>,
        David Rientjes <rientjes@google.com>,
        linux-kernel@vger.kernel.org, kernel-team@fb.com
Subject: Re: [RFC PATCH 03/26] mm: make pageblock_order 2M per default
Message-ID: <20230421123719.w7tufd6asqdpxcf3@techsingularity.net>
References: <20230418191313.268131-1-hannes@cmpxchg.org>
 <20230418191313.268131-4-hannes@cmpxchg.org>
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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@cmpxchg.org>

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