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From: Matthew Wilcox <willy@infradead.org>
To: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: Mel Gorman <mgorman@techsingularity.net>,
	Andrew Morton <akpm@linux-foundation.org>,
	Chuck Lever <chuck.lever@oracle.com>,
	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 <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH 2/5] mm/page_alloc: Add a bulk page allocator
Date: Fri, 12 Mar 2021 14:58:14 +0000	[thread overview]
Message-ID: <20210312145814.GA2577561@casper.infradead.org> (raw)
In-Reply-To: <20210312124609.33d4d4ba@carbon>

On Fri, Mar 12, 2021 at 12:46:09PM +0100, Jesper Dangaard Brouer wrote:
> In my page_pool patch I'm bulk allocating 64 pages. I wanted to ask if
> this is too much? (PP_ALLOC_CACHE_REFILL=64).
> 
> The mlx5 driver have a while loop for allocation 64 pages, which it
> used in this case, that is why 64 is chosen.  If we choose a lower
> bulk number, then the bulk-alloc will just be called more times.

The thing about batching is that smaller batches are often better.
Let's suppose you need to allocate 100 pages for something, and the page
allocator takes up 90% of your latency budget.  Batching just ten pages
at a time is going to reduce the overhead to 9%.  Going to 64 pages
reduces the overhead from 9% to 2% -- maybe that's important, but
possibly not.

> The result of the API is to deliver pages as a double-linked list via
> LRU (page->lru member).  If you are planning to use llist, then how to
> handle this API change later?
> 
> Have you notice that the two users store the struct-page pointers in an
> array?  We could have the caller provide the array to store struct-page
> pointers, like we do with kmem_cache_alloc_bulk API.

My preference would be for a pagevec.  That does limit you to 15 pages
per call [1], but I do think that might be enough.  And the overhead of
manipulating a linked list isn't free.

[1] patches exist to increase this, because it turns out that 15 may
not be enough for all systems!  but it would limit to 255 as an absolute
hard cap.

  parent reply	other threads:[~2021-03-12 14:59 UTC|newest]

Thread overview: 37+ 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 [this message]
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
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
2021-03-11 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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