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Date:   Tue, 27 Jun 2023 09:30:09 +1000
From:   Dave Chinner <david@fromorbit.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Marcelo Tosatti <mtosatti@redhat.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>,
        Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        Frederic Weisbecker <frederic@kernel.org>,
        Valentin Schneider <vschneid@redhat.com>,
        Leonardo Bras <leobras@redhat.com>,
        Yair Podemsky <ypodemsk@redhat.com>, P J P <ppandit@redhat.com>
Subject: Re: [PATCH] fs/buffer.c: remove per-CPU buffer_head lookup cache
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References: <ZJnTRfHND0Wi4YcU@tpad>
 <ZJndTjktg17nulcs@casper.infradead.org>
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On Mon, Jun 26, 2023 at 07:47:42PM +0100, Matthew Wilcox wrote:
> On Mon, Jun 26, 2023 at 03:04:53PM -0300, Marcelo Tosatti wrote:
> > Upon closer investigation, it was found that in current codebase, lookup_bh_lru
> > is slower than __find_get_block_slow:
> > 
> >  114 ns per __find_get_block
> >  68 ns per __find_get_block_slow
> > 
> > So remove the per-CPU buffer_head caching.
> 
> LOL.  That's amazing.  I can't even see why it's so expensive.  The
> local_irq_disable(), perhaps?  Your test case is the best possible
> one for lookup_bh_lru() where you're not even doing the copy.

I think it's even simpler than that.

i.e. the lookaside cache is being missed, so it's a pure cost and
the code is always having to call __find_get_block_slow() anyway.
Peeking at 16 buffers to not find a match is just as expensive as
walking 3-4 tree levels in an Xarray to find the buffer in the first
place....

IMO, this is an example of how lookaside caches are only a benefit
if the working set of items largely fits in the lookaside cache and
the cache lookup itself is much, much slower than a lookaside cache
miss.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com