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Date: Thu, 4 Jan 2024 09:47:36 -0700
From: Keith Busch <kbusch@kernel.org>
To: Stuart Hayes <stuart.w.hayes@gmail.com>
Cc: linux-kernel@vger.kernel.org, Jens Axboe <axboe@kernel.dk>,
	Christoph Hellwig <hch@lst.de>, Sagi Grimberg <sagi@grimberg.me>,
	linux-nvme@lists.infradead.org
Subject: Re: [PATCH] nvme_core: scan namespaces asynchronously
Message-ID: <ZZbhKM0L8pFYX_zd@kbusch-mbp>
References: <20240104163826.10561-1-stuart.w.hayes@gmail.com>
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In-Reply-To: <20240104163826.10561-1-stuart.w.hayes@gmail.com>

On Thu, Jan 04, 2024 at 10:38:26AM -0600, Stuart Hayes wrote:
> Currently NVME namespaces are scanned serially, so it can take a long time
> for all of a controller's namespaces to become available, especially with a
> slower (fabrics) interface with large number (~1000) of namespaces.
> 
> Use async function calls to make namespace scanning happen in parallel,
> and add a (boolean) module parameter "async_ns_scan" to enable this.

Hm, we're not doing a whole lot of blocking IO to bring up a namespace,
so I'm a little surprised it makes a noticable difference. How much time
improvement are you observing by parallelizing the scan? Is there a
tipping point in Number of Namespaces where inline scanning is better
than asynchronous? And if it is a meaningful gain, let's not introduce
another module parameter to disable it.