From: Kent Overstreet <kent.overstreet@linux.dev>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthew Wilcox <willy@infradead.org>,
Luis Chamberlain <mcgrof@kernel.org>,
lsf-pc@lists.linux-foundation.org, linux-fsdevel@vger.kernel.org,
linux-mm <linux-mm@kvack.org>,
Daniel Gomez <da.gomez@samsung.com>,
Pankaj Raghav <p.raghav@samsung.com>,
Jens Axboe <axboe@kernel.dk>, Dave Chinner <david@fromorbit.com>,
Christoph Hellwig <hch@lst.de>, Chris Mason <clm@fb.com>,
Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [LSF/MM/BPF TOPIC] Measuring limits and enhancing buffered IO
Date: Sun, 25 Feb 2024 01:04:16 -0500 [thread overview]
Message-ID: <vtbdx77a7kzibpi543abxmu3jjhpztuhna6epzleu4e5wmkrrf@roejpebs23cm> (raw)
In-Reply-To: <o4a6577t2z5xytjwmixqkl33h23vfnjypwbx7jaaldtldpvjf5@dzbzkhrzyobb>
On Sun, Feb 25, 2024 at 12:18:23AM -0500, Kent Overstreet wrote:
> On Sat, Feb 24, 2024 at 09:31:44AM -0800, Linus Torvalds wrote:
> Before large folios, we had people very much bottlenecked by 4k page
> overhead on sequential IO; my customer/sponsor was one of them.
>
> Factor of 2 or 3, IIRC; it was _bad_. And when you looked at the
> profiles and looked at the filemap.c code it wasn't hard to see why;
> we'd walk a radix tree, do an atomic op (get the page), then do a 4k
> usercopy... hence the work I did to break up
> generic_file_buffered_read() and vectorize it, which was a huge
> improvement.
>
> It's definitely less of a factor when post large folios and when we're
> talking about workloads that don't fit in cache, but I always wanted to
> do a generic version of the vectorized write path that brfs and bcachefs
> have.
to expound further, our buffered io performance really is crap vs.
direct in lots of real world scenarios, and what was going on in
generic_file_buffered_read() was just one instance of a larger theme -
walking data structures, taking locks/atomics/barriers, then doing work
on the page/folio with cacheline bounces, in a loop - lots of places
where batching/vectorizing would help a lot but it tends to be
insufficient.
i had patches that went further than the generic_file_buffered_read()
rework to vectorize add_to_page_cache_lru(), and that was another
significant improvement.
the pagecache lru operations were another hot spot... willy and I at one
point were spitballing getting rid of the linked list for a dequeue,
more for getting rid of the list_head in struct page/folio and replacing
it with a single size_t index, but it'd open up more vectorizing
possibilities
i give willy crap about the .readahead interface... the way we make the
filesystem code walk the xarray to get the folios instead of just
passing it a vector is stupid
folio_batch is stupid, it shouldn't be fixed size. there's no reason for
that to be a small fixed size array on the stack, the slub fastpath has
no atomic ops and doesn't disable preemption or interrupts - it's
_fast_. just use a darray and vectorize the whole operation
but that wouldn't be the big gains, bigger would be hunting down all the
places that aren't vectorized and should be.
i haven't reviewed the recent .writepages work christoph et all are
doing, if that's properly vectorized now that'll help
next prev parent reply other threads:[~2024-02-25 6:04 UTC|newest]
Thread overview: 87+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-02-23 23:59 [LSF/MM/BPF TOPIC] Measuring limits and enhancing buffered IO Luis Chamberlain
2024-02-24 4:12 ` Matthew Wilcox
2024-02-24 17:31 ` Linus Torvalds
2024-02-24 18:13 ` Matthew Wilcox
2024-02-24 18:24 ` Linus Torvalds
2024-02-24 18:20 ` Linus Torvalds
2024-02-24 19:11 ` Linus Torvalds
2024-02-24 21:42 ` Theodore Ts'o
2024-02-24 22:57 ` Chris Mason
2024-02-24 23:40 ` Linus Torvalds
2024-05-10 23:57 ` Luis Chamberlain
2024-02-25 5:18 ` Kent Overstreet
2024-02-25 6:04 ` Kent Overstreet [this message]
2024-02-25 13:10 ` Matthew Wilcox
2024-02-25 17:03 ` Linus Torvalds
2024-02-25 21:14 ` Matthew Wilcox
2024-02-25 23:45 ` Linus Torvalds
2024-02-26 1:02 ` Kent Overstreet
2024-02-26 1:32 ` Linus Torvalds
2024-02-26 1:58 ` Kent Overstreet
2024-02-26 2:06 ` Kent Overstreet
2024-02-26 2:34 ` Linus Torvalds
2024-02-26 2:50 ` Al Viro
2024-02-26 17:17 ` Linus Torvalds
2024-02-26 21:07 ` Matthew Wilcox
2024-02-26 21:17 ` Kent Overstreet
2024-02-26 21:19 ` Kent Overstreet
2024-02-26 21:55 ` Paul E. McKenney
2024-02-26 23:29 ` Kent Overstreet
2024-02-27 0:05 ` Paul E. McKenney
2024-02-27 0:29 ` Kent Overstreet
2024-02-27 0:55 ` Paul E. McKenney
2024-02-27 1:08 ` Kent Overstreet
2024-02-27 5:17 ` Paul E. McKenney
2024-02-27 6:21 ` Kent Overstreet
2024-02-27 15:32 ` Paul E. McKenney
2024-02-27 15:52 ` Kent Overstreet
2024-02-27 16:06 ` Paul E. McKenney
2024-02-27 15:54 ` Matthew Wilcox
2024-02-27 16:21 ` Paul E. McKenney
2024-02-27 16:34 ` Kent Overstreet
2024-02-27 17:58 ` Paul E. McKenney
2024-02-28 23:55 ` Kent Overstreet
2024-02-29 19:42 ` Paul E. McKenney
2024-02-29 20:51 ` Kent Overstreet
2024-03-05 2:19 ` Paul E. McKenney
2024-02-27 0:43 ` Dave Chinner
2024-02-26 22:46 ` Linus Torvalds
2024-02-26 23:48 ` Linus Torvalds
2024-02-27 7:21 ` Kent Overstreet
2024-02-27 15:39 ` Matthew Wilcox
2024-02-27 15:54 ` Kent Overstreet
2024-02-27 16:34 ` Linus Torvalds
2024-02-27 16:47 ` Kent Overstreet
2024-02-27 17:07 ` Linus Torvalds
2024-02-27 17:20 ` Kent Overstreet
2024-02-27 18:02 ` Linus Torvalds
2024-05-14 11:52 ` Luis Chamberlain
2024-05-14 16:04 ` Linus Torvalds
2024-02-25 21:29 ` Kent Overstreet
2024-02-25 17:32 ` Kent Overstreet
2024-02-24 17:55 ` Luis Chamberlain
2024-02-25 5:24 ` Kent Overstreet
2024-02-26 12:22 ` Dave Chinner
2024-02-27 10:07 ` Kent Overstreet
2024-02-27 14:08 ` Luis Chamberlain
2024-02-27 14:57 ` Kent Overstreet
2024-02-27 22:13 ` Dave Chinner
2024-02-27 22:21 ` Kent Overstreet
2024-02-27 22:42 ` Dave Chinner
2024-02-28 7:48 ` [Lsf-pc] " Amir Goldstein
2024-02-28 14:01 ` Chris Mason
2024-02-29 0:25 ` Dave Chinner
2024-02-29 0:57 ` Kent Overstreet
2024-03-04 0:46 ` Dave Chinner
2024-02-27 22:46 ` Linus Torvalds
2024-02-27 23:00 ` Linus Torvalds
2024-02-28 2:22 ` Kent Overstreet
2024-02-28 3:00 ` Matthew Wilcox
2024-02-28 4:22 ` Matthew Wilcox
2024-02-28 17:34 ` Kent Overstreet
2024-02-28 18:04 ` Matthew Wilcox
2024-02-28 18:18 ` Kent Overstreet
2024-02-28 19:09 ` Linus Torvalds
2024-02-28 19:29 ` Kent Overstreet
2024-02-28 20:17 ` Linus Torvalds
2024-02-28 23:21 ` Kent Overstreet
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