linux-kernel - Re: [PATCH] nvme_core: scan namespaces asynchronously

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Date: Thu, 4 Jan 2024 09:47:36 -0700
From: Keith Busch <kbusch@...nel.org>
To: Stuart Hayes <stuart.w.hayes@...il.com>
Cc: linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
	Christoph Hellwig <hch@....de>, Sagi Grimberg <sagi@...mberg.me>,
	linux-nvme@...ts.infradead.org
Subject: Re: [PATCH] nvme_core: scan namespaces asynchronously

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.