linux-kernel - Re: FYI: path walking optimizations pending for 6.11

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Date: Wed, 19 Jun 2024 21:45:33 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Christian Brauner <brauner@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux ARM <linux-arm-kernel@...ts.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: FYI: path walking optimizations pending for 6.11

On Wed, Jun 19, 2024 at 01:25:02PM -0700, Linus Torvalds wrote:
> For example, making d_hash() avoid indirection just means that now
> pretty much _all_ the cost of __d_lookup_rcu() is in the cache misses
> on the hash table itself. Which was always the bulk of it. And on my
> arm64 machine, it turns out that the best optimization for the load I
> tested would be to make that hash table smaller to actually be a bit
> denser in the cache, But that's such a load-dependent optimization
> that I'm not doing this.
> 
> Tuning the hash table size or data structure cacheline layouts might
> be worthwhile - and likely a bigger deal - but is _not_ what these
> patches are about.

Funnily, I'm working on rosebush v2 today.  It's in no shape to send out
(it's failing ~all of its selftests) but *should* greatly improve the
cache friendliness of the hash table.  And it's being written with the
dcache as its first customer.