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Date: Mon, 10 Mar 2025 14:18:55 +0100
From: Christoph Hellwig <hch@lst.de>
To: Dave Chinner <david@fromorbit.com>
Cc: Christoph Hellwig <hch@lst.de>, Carlos Maiolino <cem@kernel.org>,
	"Darrick J. Wong" <djwong@kernel.org>,
	Dave Chinner <dchinner@redhat.com>, linux-xfs@vger.kernel.org
Subject: Re: [PATCH 07/12] xfs: convert buffer cache to use high order
 folios
Message-ID: <20250310131854.GB8546@lst.de>
References: <20250305140532.158563-1-hch@lst.de> <20250305140532.158563-8-hch@lst.de> <Z8i5HSS1ppbtQiJc@dread.disaster.area>
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On Thu, Mar 06, 2025 at 07:50:37AM +1100, Dave Chinner wrote:
> The only thing extra that I would do here is take a leaf from the
> kmalloc() call in xlog_kvmalloc() and turn off direct reclaim for
> this allocation because >= 32kB allocations are considered "costly"
> and so will enter the compaction code if direct reclaim is enabled.
> 
> Given that we fall back to vmalloc, clearing __GFP_DIRECT_RECLAIM
> and setting __GFP_NORETRY here means that we don't burn lots of CPU
> on memory compaction if there is no high order folios available for
> immediate allocation. And on a busy machine, compaction is likely to
> fail frequently and so this is all wasted CPU time.
> 
> This may be one of the reasons why you don't see any change in real
> performance with 64kB directory blocks - we spend more time in
> folio allocation because of compaction overhead than we gain back
> from avoiding the use of vmapped buffers....

FYI, this did not make any difference in my testing.