From: Matthew Wilcox <willy@infradead.org>
To: Chuck Lever III <chuck.lever@oracle.com>
Cc: Mel Gorman <mgorman@techsingularity.net>,
Jesper Dangaard Brouer <brouer@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
Christoph Hellwig <hch@infradead.org>,
LKML <linux-kernel@vger.kernel.org>,
Linux-Net <netdev@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>,
Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH 2/5] mm/page_alloc: Add a bulk page allocator
Date: Sat, 13 Mar 2021 19:33:43 +0000 [thread overview]
Message-ID: <20210313193343.GJ2577561@casper.infradead.org> (raw)
In-Reply-To: <7D8C62E1-77FD-4B41-90D7-253D13715A6F@oracle.com>
On Sat, Mar 13, 2021 at 04:56:31PM +0000, Chuck Lever III wrote:
> IME lists are indeed less CPU-efficient, but I wonder if that
> expense is insignificant compared to serialization primitives like
> disabling and re-enabling IRQs, which we are avoiding by using
> bulk page allocation.
Cache misses are a worse problem than serialisation. Paul McKenney had
a neat demonstration where he took a sheet of toilet paper to represent
an instruction, and then unrolled two rolls of toilet paper around the
lecture theatre to represent an L3 cache miss. Obviously a serialising
instruction is worse than an add instruction, but i'm thinking maybe
50-100 sheets of paper, not an entire roll?
Anyway, I'm not arguing against a bulk allocator, nor even saying this
is a bad interface. It just maybe could be better.
> My initial experience with the current interface left me feeling
> uneasy about re-using the lru list field. That seems to expose an
> internal API feature to consumers of the page allocator. If we
> continue with a list-centric bulk allocator API I hope there can
> be some conveniently-placed documentation that explains when it is
> safe to use that field. Or perhaps the field should be renamed.
Heh. Spoken like a filesystem developer who's never been exposed to the
->readpages API (it's almost dead). It's fairly common in the memory
management world to string pages together through the lru list_head.
Slab does it, as does put_pages_list() in mm/swap.c. It's natural for
Mel to keep using this pattern ... and I dislike it intensely.
> I have a mild preference for an array-style interface because that's
> more natural for the NFSD consumer, but I'm happy to have a bulk
> allocator either way. Purely from a code-reuse point of view, I
> wonder how many consumers of alloc_pages_bulk() will be like
> svc_alloc_arg(), where they need to fill in pages in an array. Each
> such consumer would need to repeat the logic to convert the returned
> list into an array. We have, for instance, release_pages(), which is
> an array-centric page allocator API. Maybe a helper function or two
> might prevent duplication of the list conversion logic.
>
> And I agree with Mel that passing a single large array seems more
> useful then having to build code at each consumer call-site to
> iterate over smaller page_vecs until that array is filled.
So how about this?
You provide the interface you'd _actually_ like to use (array-based) and
implement it on top of Mel's lru-list implementation. If it's general
enough to be used by Jesper's use-case, we lift it to page_alloc.c.
If we go a year and there are no users of the lru-list interface, we
can just change the implementation.
next prev parent reply other threads:[~2021-03-13 19:35 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-10 10:46 [PATCH 0/5] Introduce a bulk order-0 page allocator with two in-tree users Mel Gorman
2021-03-10 10:46 ` [PATCH 1/5] SUNRPC: Set rq_page_end differently Mel Gorman
2021-03-10 10:46 ` [PATCH 2/5] mm/page_alloc: Add a bulk page allocator Mel Gorman
2021-03-10 23:46 ` Andrew Morton
2021-03-11 8:42 ` Mel Gorman
2021-03-12 11:46 ` Jesper Dangaard Brouer
2021-03-12 13:44 ` Mel Gorman
2021-03-12 14:58 ` Matthew Wilcox
2021-03-12 16:03 ` Mel Gorman
2021-03-12 21:08 ` Matthew Wilcox
2021-03-13 13:16 ` Mel Gorman
2021-03-13 16:39 ` Matthew Wilcox
2021-03-13 16:56 ` Chuck Lever III
2021-03-13 19:33 ` Matthew Wilcox [this message]
2021-03-14 12:52 ` Mel Gorman
2021-03-14 15:22 ` Chuck Lever III
2021-03-15 10:42 ` Mel Gorman
2021-03-15 16:42 ` Jesper Dangaard Brouer
2021-03-19 17:10 ` Jesper Dangaard Brouer
2021-03-12 12:43 ` Matthew Wilcox
2021-03-12 14:15 ` Mel Gorman
2021-03-10 10:46 ` [PATCH 3/5] SUNRPC: Refresh rq_pages using " Mel Gorman
2021-03-10 10:46 ` [PATCH 4/5] net: page_pool: refactor dma_map into own function page_pool_dma_map Mel Gorman
2021-03-10 10:46 ` [PATCH 5/5] net: page_pool: use alloc_pages_bulk in refill code path Mel Gorman
2021-03-10 23:47 ` [PATCH 0/5] Introduce a bulk order-0 page allocator with two in-tree users Andrew Morton
2021-03-11 8:48 ` Mel Gorman
2021-03-12 15:10 ` Matthew Wilcox
-- strict thread matches above, loose matches on Subject: below --
2021-03-11 11:49 [PATCH 0/5 v3] " Mel Gorman
2021-03-11 11:49 ` [PATCH 2/5] mm/page_alloc: Add a bulk page allocator Mel Gorman
2021-03-11 16:42 ` Alexander Duyck
2021-03-12 7:32 ` Mel Gorman
2021-03-01 16:11 [PATCH 0/5] Introduce a bulk order-0 page allocator with two in-tree users Mel Gorman
2021-03-01 16:11 ` [PATCH 2/5] mm/page_alloc: Add a bulk page allocator Mel Gorman
2021-03-09 17:12 ` Christoph Hellwig
2021-03-09 18:10 ` Mel Gorman
2021-03-10 11:04 ` Shay Agroskin
2021-03-10 11:38 ` Mel Gorman
2021-03-12 12:01 ` Jesper Dangaard Brouer
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