linux-kernel - Re: [PATCH 3/3] apple-nvme: defer cache flushes by a specified amount

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Message-ID: <20250213062013.GC19608@lst.de>
Date: Thu, 13 Feb 2025 07:20:13 +0100
From: Christoph Hellwig <hch@....de>
To: Alyssa Rosenzweig <alyssa@...enzweig.io>
Cc: Hector Martin <marcan@...can.st>, Sven Peter <sven@...npeter.dev>,
	Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...nel.dk>,
	Christoph Hellwig <hch@....de>, Sagi Grimberg <sagi@...mberg.me>,
	Philipp Zabel <p.zabel@...gutronix.de>, asahi@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org,
	linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] apple-nvme: defer cache flushes by a specified
 amount

On Tue, Feb 11, 2025 at 01:25:59PM -0500, Alyssa Rosenzweig wrote:
> From: Jens Axboe <axboe@...nel.dk>
> 
> Cache flushes on the M1 nvme are really slow, taking 17-18 msec to
> complete. This can slow down workloads considerably, pure random writes
> end up being bound by the flush latency and hence run at 55-60 IOPS.
> 
> Add a deferred flush work around to provide better performance, at a
> minimal risk. By default, flushes are delayed at most 1 second, but this
> is configurable.
> 
> With this work-around, a pure random write workload runs at ~12K IOPS
> rather than 56 IOPS.

Just as last time this really is not a driver feature.  Cache flushes
are slow on consumer hardware, it's just apple is worse than usual.
Breaking file system transactional guarantee by ignoring data integrity
command in the driver is a no-go.

If we want to allow an opt-in policy for those whole feel adventurous,
it belongs into the core flush state machine.  Fortunately the patch
author seems qualified to touch that :)