Re: [PATCH RFC net-next 3/3] mm: make zone->free_area[order] access faster
From: Jesper Dangaard Brouer
Date: Thu Feb 25 2021 - 10:18:17 EST
On 2021年2月25日 11:28:49 +0000
Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:
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As a side-node, I didn't pick up the other patches as there is review
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feedback and I didn't have strong opinions either way. Patch 3 is curious
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though, it probably should be split out and sent separetly but still;
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On Wed, Feb 24, 2021 at 07:56:51PM +0100, Jesper Dangaard Brouer wrote:
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> Avoid multiplication (imul) operations when accessing:
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> zone->free_area[order].nr_free
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> This was really tricky to find. I was puzzled why perf reported that
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> rmqueue_bulk was using 44% of the time in an imul operation:
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> ??? del_page_from_free_list():
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> 44,54 ??? e2: imul 0ドルx58,%rax,%rax
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> This operation was generated (by compiler) because the struct free_area have
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> size 88 bytes or 0x58 hex. The compiler cannot find a shift operation to use
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> and instead choose to use a more expensive imul, to find the offset into the
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> array free_area[].
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> The patch align struct free_area to a cache-line, which cause the
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> compiler avoid the imul operation. The imul operation is very fast on
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> modern Intel CPUs. To help fast-path that decrement 'nr_free' move the
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> member 'nr_free' to be first element, which saves one 'add' operation.
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> Looking up instruction latency this exchange a 3-cycle imul with a
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> 1-cycle shl, saving 2-cycles. It does trade some space to do this.
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> Used: gcc (GCC) 9.3.1 20200408 (Red Hat 9.3.1-2)
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I'm having some trouble parsing this and matching it to the patch itself.
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First off, on my system (x86-64), the size of struct free area is 72,
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not 88 bytes. For either size, cache-aligning the structure is a big
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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.
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struct free_area {
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struct list_head free_list[4]; /* 0 64 */
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/* --- cacheline 1 boundary (64 bytes) --- */
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long unsigned int nr_free; /* 64 8 */
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/* size: 72, cachelines: 2, members: 2 */
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/* last cacheline: 8 bytes */
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};
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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.
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With gcc-9, I'm also not seeing the imul instruction outputted like you
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described in rmqueue_pcplist which inlines rmqueue_bulk. At the point
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where it calls get_page_from_free_area, it's using shl for the page list
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operation. This might be a compiler glitch but given that free_area is a
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different size, I'm less certain and wonder if something else is going on.
I think it is the size variation.
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Finally, moving nr_free to the end and cache aligning it will make the
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started of each free_list cache-aligned because of its location in the
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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