linux-kernel - Re: [PATCH 4/5] xfs: convert perag lookup to xarray

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Message-ID: <20240822034548.GD32681@lst.de>
Date: Thu, 22 Aug 2024 05:45:48 +0200
From: Christoph Hellwig <hch@....de>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: Christoph Hellwig <hch@....de>,
	Chandan Babu R <chandan.babu@...cle.com>,
	Matthew Wilcox <willy@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 4/5] xfs: convert perag lookup to xarray

On Wed, Aug 21, 2024 at 09:28:10AM -0700, Darrick J. Wong wrote:
> On Wed, Aug 21, 2024 at 08:38:31AM +0200, Christoph Hellwig wrote:
> > Convert the perag lookup from the legacy radix tree to the xarray,
> > which allows for much nicer iteration and bulk lookup semantics.
> 
> Looks like a pretty straightforward covnersion.  Is there a good
> justification for converting the ici radix tree too?  Or is it too
> sparse to be worth doing?

radix trees and xarrays have pretty similar behavior related to
sparseness or waste of interior nodes due to it.  So unless we find a
better data structure for it, it would be worthwhile.

But the ici radix tree does pretty funny things in terms of also
protecting other fields with the lock synchronizing it, so the conversion
is fairly complicated and I don't feel like doing it right now, at least
no without evaluating if for example a rthashtable might actually be
the better data structure here.  The downside of the rthashtable is
that it doens't support tags/masks and isn't great for iteration, so it
might very much not be very suitable.