From: Ming Lei <ming.lei@redhat.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Pavel Begunkov <asml.silence@gmail.com>,
linux-fsdevel@vger.kernel.org,
Alexander Viro <viro@zeniv.linux.org.uk>,
Jens Axboe <axboe@kernel.dk>,
linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/2] iov_iter: optimise iov_iter_npages for bvec
Date: Fri, 20 Nov 2020 10:24:26 +0800 [thread overview]
Message-ID: <20201120022426.GC333150@T590> (raw)
In-Reply-To: <20201120020610.GL29991@casper.infradead.org>
On Fri, Nov 20, 2020 at 02:06:10AM +0000, Matthew Wilcox wrote:
> On Fri, Nov 20, 2020 at 01:56:22AM +0000, Pavel Begunkov wrote:
> > On 20/11/2020 01:49, Matthew Wilcox wrote:
> > > On Fri, Nov 20, 2020 at 01:39:05AM +0000, Pavel Begunkov wrote:
> > >> On 20/11/2020 01:20, Matthew Wilcox wrote:
> > >>> On Thu, Nov 19, 2020 at 11:24:38PM +0000, Pavel Begunkov wrote:
> > >>>> The block layer spends quite a while in iov_iter_npages(), but for the
> > >>>> bvec case the number of pages is already known and stored in
> > >>>> iter->nr_segs, so it can be returned immediately as an optimisation
> > >>>
> > >>> Er ... no, it doesn't. nr_segs is the number of bvecs. Each bvec can
> > >>> store up to 4GB of contiguous physical memory.
> > >>
> > >> Ah, really, missed min() with PAGE_SIZE in bvec_iter_len(), then it's a
> > >> stupid statement. Thanks!
> > >>
> > >> Are there many users of that? All these iterators are a huge burden,
> > >> just to count one 4KB page in bvec it takes 2% of CPU time for me.
> > >
> > > __bio_try_merge_page() will create multipage BIOs, and that's
> > > called from a number of places including
> > > bio_try_merge_hw_seg(), bio_add_page(), and __bio_iov_iter_get_pages()
> >
> > I get it that there are a lot of places, more interesting how often
> > it's actually triggered and if that's performance critical for anybody.
> > Not like I'm going to change it, just out of curiosity, but bvec.h
> > can be nicely optimised without it.
>
> Typically when you're allocating pages for the page cache, they'll get
> allocated in order and then you'll read or write them in order, so yes,
> it ends up triggering quite a lot. There was once a bug in the page
> allocator which caused them to get allocated in reverse order and it
> was a noticable performance hit (this was 15-20 years ago).
hugepage use cases can benefit much from this way too.
Thanks,
Ming
next prev parent reply other threads:[~2020-11-20 2:25 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-19 23:24 [PATCH v2 0/2] optimise iov_iter Pavel Begunkov
2020-11-19 23:24 ` [PATCH v2 1/2] iov_iter: optimise iov_iter_npages for bvec Pavel Begunkov
2020-11-20 1:20 ` Matthew Wilcox
2020-11-20 1:39 ` Pavel Begunkov
2020-11-20 1:49 ` Matthew Wilcox
2020-11-20 1:56 ` Pavel Begunkov
2020-11-20 2:06 ` Matthew Wilcox
2020-11-20 2:08 ` Pavel Begunkov
2020-11-20 2:24 ` Ming Lei [this message]
2020-11-20 17:22 ` Pavel Begunkov
2020-11-20 17:23 ` Pavel Begunkov
2020-11-20 2:22 ` Ming Lei
2020-11-20 2:25 ` Pavel Begunkov
2020-11-20 2:54 ` Matthew Wilcox
2020-11-20 8:14 ` Christoph Hellwig
2020-11-20 12:39 ` Matthew Wilcox
2020-11-20 13:00 ` Pavel Begunkov
2020-11-20 13:13 ` Matthew Wilcox
2020-11-20 9:57 ` Pavel Begunkov
2020-11-20 13:29 ` David Laight
2020-11-19 23:24 ` [PATCH v2 2/2] iov_iter: optimise iter type checking Pavel Begunkov
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