From: Jesper Dangaard Brouer <brouer@redhat.com>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: linux-mm@kvack.org, chuck.lever@oracle.com,
netdev@vger.kernel.org, linux-nfs@vger.kernel.org,
linux-kernel@vger.kernel.org, brouer@redhat.com
Subject: Re: [PATCH RFC net-next 3/3] mm: make zone->free_area[order] access faster
Date: Thu, 25 Feb 2021 16:16:33 +0100 [thread overview]
Message-ID: <20210225161633.53e5f910@carbon> (raw)
In-Reply-To: <20210225112849.GM3697@techsingularity.net>
On Thu, 25 Feb 2021 11:28:49 +0000
Mel Gorman <mgorman@techsingularity.net> wrote:
> As a side-node, I didn't pick up the other patches as there is review
> feedback and I didn't have strong opinions either way. Patch 3 is curious
> though, it probably should be split out and sent separetly but still;
>
> On Wed, Feb 24, 2021 at 07:56:51PM +0100, Jesper Dangaard Brouer wrote:
> > Avoid multiplication (imul) operations when accessing:
> > zone->free_area[order].nr_free
> >
> > This was really tricky to find. I was puzzled why perf reported that
> > rmqueue_bulk was using 44% of the time in an imul operation:
> >
> > ??? del_page_from_free_list():
> > 44,54 ??? e2: imul $0x58,%rax,%rax
> >
> > This operation was generated (by compiler) because the struct free_area have
> > size 88 bytes or 0x58 hex. The compiler cannot find a shift operation to use
> > and instead choose to use a more expensive imul, to find the offset into the
> > array free_area[].
> >
> > The patch align struct free_area to a cache-line, which cause the
> > compiler avoid the imul operation. The imul operation is very fast on
> > modern Intel CPUs. To help fast-path that decrement 'nr_free' move the
> > member 'nr_free' to be first element, which saves one 'add' operation.
> >
> > Looking up instruction latency this exchange a 3-cycle imul with a
> > 1-cycle shl, saving 2-cycles. It does trade some space to do this.
> >
> > Used: gcc (GCC) 9.3.1 20200408 (Red Hat 9.3.1-2)
> >
>
> I'm having some trouble parsing this and matching it to the patch itself.
>
> First off, on my system (x86-64), the size of struct free area is 72,
> not 88 bytes. For either size, cache-aligning the structure is a big
> increase in the struct size.
Yes, the increase in size is big. For the struct free_area 40 bytes for
my case and 56 bytes for your case. The real problem is that this is
multiplied by 11 (MAX_ORDER) and multiplied by number of zone structs
(is it 5?). Thus, 56*11*5 = 3080 bytes.
Thus, I'm not sure it is worth it! As I'm only saving 2-cycles, for
something that depends on the compiler generating specific code. And
the compiler can easily change, and "fix" this on-its-own in a later
release, and then we are just wasting memory.
I did notice this imul happens 45 times in mm/page_alloc.o, with this
offset 0x58, but still this is likely not on hot-path.
> struct free_area {
> struct list_head free_list[4]; /* 0 64 */
> /* --- cacheline 1 boundary (64 bytes) --- */
> long unsigned int nr_free; /* 64 8 */
>
> /* size: 72, cachelines: 2, members: 2 */
> /* last cacheline: 8 bytes */
> };
>
> Are there other patches in the tree? What does pahole say?
The size of size of struct free_area varies based on some CONFIG
setting, as free_list[] array size is determined by MIGRATE_TYPES,
which on my system is 5, and not 4 as on your system.
struct list_head free_list[MIGRATE_TYPES];
CONFIG_CMA and CONFIG_MEMORY_ISOLATION both increase MIGRATE_TYPES with one.
Thus, the array size can vary from 4 to 6.
> With gcc-9, I'm also not seeing the imul instruction outputted like you
> described in rmqueue_pcplist which inlines rmqueue_bulk. At the point
> where it calls get_page_from_free_area, it's using shl for the page list
> operation. This might be a compiler glitch but given that free_area is a
> different size, I'm less certain and wonder if something else is going on.
I think it is the size variation.
> Finally, moving nr_free to the end and cache aligning it will make the
> started of each free_list cache-aligned because of its location in the
> struct zone so what purpose does __pad_to_align_free_list serve?
The purpose of purpose of __pad_to_align_free_list is because struct
list_head is 16 bytes, thus I wanted to align free_list to 16, given we
already have wasted the space.
Notice I added some more detailed notes in[1]:
[1] https://github.com/xdp-project/xdp-project/blob/master/areas/mem/page_pool06_alloc_pages_bulk.org#micro-optimisations
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
next prev parent reply other threads:[~2021-02-25 15:16 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-24 10:26 [RFC PATCH 0/3] Introduce a bulk order-0 page allocator for sunrpc Mel Gorman
2021-02-24 10:26 ` [PATCH 1/3] SUNRPC: Set rq_page_end differently Mel Gorman
2021-02-24 10:26 ` [PATCH 2/3] mm, page_alloc: Add a bulk page allocator Mel Gorman
2021-02-24 10:26 ` [PATCH 3/3] SUNRPC: Refresh rq_pages using " Mel Gorman
2021-02-24 11:27 ` [RFC PATCH 0/3] Introduce a bulk order-0 page allocator for sunrpc Jesper Dangaard Brouer
2021-02-24 11:55 ` Mel Gorman
2021-02-24 13:20 ` Chuck Lever
2021-02-24 18:56 ` [PATCH RFC net-next 0/3] Use bulk order-0 page allocator API for page_pool Jesper Dangaard Brouer
2021-02-24 18:56 ` [PATCH RFC net-next 1/3] net: page_pool: refactor dma_map into own function page_pool_dma_map Jesper Dangaard Brouer
2021-02-24 20:11 ` Ilias Apalodimas
2021-02-24 18:56 ` [PATCH RFC net-next 2/3] net: page_pool: use alloc_pages_bulk in refill code path Jesper Dangaard Brouer
2021-02-24 20:15 ` Ilias Apalodimas
2021-02-26 14:31 ` Jesper Dangaard Brouer
2021-02-24 18:56 ` [PATCH RFC net-next 3/3] mm: make zone->free_area[order] access faster Jesper Dangaard Brouer
2021-02-25 11:28 ` Mel Gorman
2021-02-25 15:16 ` Jesper Dangaard Brouer [this message]
2021-02-25 15:38 ` Mel Gorman
2021-02-26 14:34 ` Jesper Dangaard Brouer
2021-03-01 13:29 ` [PATCH RFC V2 net-next 0/2] Use bulk order-0 page allocator API for page_pool Jesper Dangaard Brouer
2021-03-01 13:29 ` [PATCH RFC V2 net-next 1/2] net: page_pool: refactor dma_map into own function page_pool_dma_map Jesper Dangaard Brouer
2021-03-01 13:29 ` [PATCH RFC V2 net-next 2/2] net: page_pool: use alloc_pages_bulk in refill code path Jesper Dangaard Brouer
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