Re: [PATCH 4/4] mm, page_alloc: Add a bulk page allocator

From: Jesper Dangaard Brouer
Date: Mon Jan 16 2017 - 09:25:27 EST

On Mon, 9 Jan 2017 16:35:18 +0000
Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:

> This patch adds a new page allocator interface via alloc_pages_bulk,
> __alloc_pages_bulk and __alloc_pages_bulk_nodemask. A caller requests a
> number of pages to be allocated and added to a list. They can be freed in
> bulk using free_pages_bulk(). Note that it would theoretically be possible
> to use free_hot_cold_page_list for faster frees if the symbol was exported,
> the refcounts were 0 and the caller guaranteed it was not in an interrupt.
> This would be significantly faster in the free path but also more unsafer
> and a harder API to use.
>
> The API is not guaranteed to return the requested number of pages and
> may fail if the preferred allocation zone has limited free memory, the
> cpuset changes during the allocation or page debugging decides to fail
> an allocation. It's up to the caller to request more pages in batch if
> necessary.
>
> The following compares the allocation cost per page for different batch
> sizes. The baseline is allocating them one at a time and it compares with
> the performance when using the new allocation interface.

I've also played with testing the bulking API here:
[1] https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/mm/bench/page_bench04_bulk.c

My baseline single (order-0 page) show: 158 cycles(tsc) 39.593 ns

Using bulking API:
Bulk: 1 cycles: 128 nanosec: 32.134
Bulk: 2 cycles: 107 nanosec: 26.783
Bulk: 3 cycles: 100 nanosec: 25.047
Bulk: 4 cycles: 95 nanosec: 23.988
Bulk: 8 cycles: 91 nanosec: 22.823
Bulk: 16 cycles: 88 nanosec: 22.093
Bulk: 32 cycles: 85 nanosec: 21.338
Bulk: 64 cycles: 85 nanosec: 21.315
Bulk: 128 cycles: 84 nanosec: 21.214
Bulk: 256 cycles: 115 nanosec: 28.979

This bulk API (and other improvements part of patchset) definitely
moves the speed of the page allocator closer to my (crazy) time budget
target of between 201 to 269 cycles per packet[1]. Remember I was
reporting[2] order-0 cost between 231 to 277 cycles, at MM-summit
2016, so this is a huge improvement since then.

The bulk numbers are great, but it still cannot compete with the
recycles tricks used by drivers. Looking at the code (and as Mel also
mentions) there is room for improvements especially on the bulk free-side.


[1] http://people.netfilter.org/hawk/presentations/devconf2016/net_stack_challenges_100G_Feb2016.pdf
[2] http://people.netfilter.org/hawk/presentations/MM-summit2016/generic_page_pool_mm_summit2016.pdf

> pagealloc
> 4.10.0-rc2 4.10.0-rc2
> one-at-a-time bulk-v2
> Amean alloc-odr0-1 259.54 ( 0.00%) 106.62 ( 58.92%)
> Amean alloc-odr0-2 193.38 ( 0.00%) 76.38 ( 60.50%)
> Amean alloc-odr0-4 162.38 ( 0.00%) 57.23 ( 64.76%)
> Amean alloc-odr0-8 144.31 ( 0.00%) 48.77 ( 66.20%)
> Amean alloc-odr0-16 134.08 ( 0.00%) 45.38 ( 66.15%)
> Amean alloc-odr0-32 128.62 ( 0.00%) 42.77 ( 66.75%)
> Amean alloc-odr0-64 126.00 ( 0.00%) 41.00 ( 67.46%)
> Amean alloc-odr0-128 125.00 ( 0.00%) 40.08 ( 67.94%)
> Amean alloc-odr0-256 136.62 ( 0.00%) 56.00 ( 59.01%)
> Amean alloc-odr0-512 152.00 ( 0.00%) 69.00 ( 54.61%)
> Amean alloc-odr0-1024 158.00 ( 0.00%) 76.23 ( 51.75%)
> Amean alloc-odr0-2048 163.00 ( 0.00%) 81.15 ( 50.21%)
> Amean alloc-odr0-4096 169.77 ( 0.00%) 85.92 ( 49.39%)
> Amean alloc-odr0-8192 170.00 ( 0.00%) 88.00 ( 48.24%)
> Amean alloc-odr0-16384 170.00 ( 0.00%) 89.00 ( 47.65%)
> Amean free-odr0-1 88.69 ( 0.00%) 55.69 ( 37.21%)
> Amean free-odr0-2 66.00 ( 0.00%) 49.38 ( 25.17%)
> Amean free-odr0-4 54.23 ( 0.00%) 45.38 ( 16.31%)
> Amean free-odr0-8 48.23 ( 0.00%) 44.23 ( 8.29%)
> Amean free-odr0-16 47.00 ( 0.00%) 45.00 ( 4.26%)
> Amean free-odr0-32 44.77 ( 0.00%) 43.92 ( 1.89%)
> Amean free-odr0-64 44.00 ( 0.00%) 43.00 ( 2.27%)
> Amean free-odr0-128 43.00 ( 0.00%) 43.00 ( 0.00%)
> Amean free-odr0-256 60.69 ( 0.00%) 60.46 ( 0.38%)
> Amean free-odr0-512 79.23 ( 0.00%) 76.00 ( 4.08%)
> Amean free-odr0-1024 86.00 ( 0.00%) 85.38 ( 0.72%)
> Amean free-odr0-2048 91.00 ( 0.00%) 91.23 ( -0.25%)
> Amean free-odr0-4096 94.85 ( 0.00%) 95.62 ( -0.81%)
> Amean free-odr0-8192 97.00 ( 0.00%) 97.00 ( 0.00%)
> Amean free-odr0-16384 98.00 ( 0.00%) 97.46 ( 0.55%)
> Amean total-odr0-1 348.23 ( 0.00%) 162.31 ( 53.39%)
> Amean total-odr0-2 259.38 ( 0.00%) 125.77 ( 51.51%)
> Amean total-odr0-4 216.62 ( 0.00%) 102.62 ( 52.63%)
> Amean total-odr0-8 192.54 ( 0.00%) 93.00 ( 51.70%)
> Amean total-odr0-16 181.08 ( 0.00%) 90.38 ( 50.08%)
> Amean total-odr0-32 173.38 ( 0.00%) 86.69 ( 50.00%)
> Amean total-odr0-64 170.00 ( 0.00%) 84.00 ( 50.59%)
> Amean total-odr0-128 168.00 ( 0.00%) 83.08 ( 50.55%)
> Amean total-odr0-256 197.31 ( 0.00%) 116.46 ( 40.97%)
> Amean total-odr0-512 231.23 ( 0.00%) 145.00 ( 37.29%)
> Amean total-odr0-1024 244.00 ( 0.00%) 161.62 ( 33.76%)
> Amean total-odr0-2048 254.00 ( 0.00%) 172.38 ( 32.13%)
> Amean total-odr0-4096 264.62 ( 0.00%) 181.54 ( 31.40%)
> Amean total-odr0-8192 267.00 ( 0.00%) 185.00 ( 30.71%)
> Amean total-odr0-16384 268.00 ( 0.00%) 186.46 ( 30.42%)
>
> It shows a roughly 50-60% reduction in the cost of allocating pages.
> The free paths are not improved as much but relatively little can be batched
> there. It's not quite as fast as it could be but taking further shortcuts
> would require making a lot of assumptions about the state of the page and
> the context of the caller.

--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer