Re: [PATCH 3/4] mm, page_allocator: Only use per-cpu allocator for irq-safe requests

From: Jesper Dangaard Brouer
Date: Wed Jan 04 2017 - 09:23:26 EST

On Wed, 4 Jan 2017 11:10:48 +0000 Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:

> Many workloads that allocate pages are not handling an interrupt at a
> time. As allocation requests may be from IRQ context, it's necessary to
> disable/enable IRQs for every page allocation. This cost is the bulk
> of the free path but also a significant percentage of the allocation
> path.
>
> This patch alters the locking and checks such that only irq-safe allocation
> requests use the per-cpu allocator. All others acquire the irq-safe
> zone->lock and allocate from the buddy allocator. It relies on disabling
> preemption to safely access the per-cpu structures.

I love this idea and patch :-)

> It could be slightly
> modified to avoid soft IRQs using it but it's not clear it's worthwhile.

NICs usually refill their RX-ring from SoftIRQ context (NAPI).
Thus, we do want this optimization to work in softirq.


> This modification may slow allocations from IRQ context slightly but the main
> gain from the per-cpu allocator is that it scales better for allocations
> from multiple contexts. There is an implicit assumption that intensive
> allocations from IRQ contexts on multiple CPUs from a single NUMA node are
> rare and that the fast majority of scaling issues are encountered in !IRQ
> contexts such as page faulting.

IHMO, I agree with this implicit assumption.


> It's worth noting that this patch is not
> required for a bulk page allocator but it significantly reduces the overhead.
>
> The following is results from a page allocator micro-benchmark. Only
> order-0 is interesting as higher orders do not use the per-cpu allocator

I'm seeing approx 34% reduction in a order-0 micro-benchmark! amazing! :-)
[1] https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/mm/bench/

> 4.10.0-rc2 4.10.0-rc2
> vanilla irqsafe-v1r5
> Amean alloc-odr0-1 287.15 ( 0.00%) 219.00 ( 23.73%)
> Amean alloc-odr0-2 221.23 ( 0.00%) 183.23 ( 17.18%)
> Amean alloc-odr0-4 187.00 ( 0.00%) 151.38 ( 19.05%)
> Amean alloc-odr0-8 167.54 ( 0.00%) 132.77 ( 20.75%)
> Amean alloc-odr0-16 156.00 ( 0.00%) 123.00 ( 21.15%)
> Amean alloc-odr0-32 149.00 ( 0.00%) 118.31 ( 20.60%)
> Amean alloc-odr0-64 138.77 ( 0.00%) 116.00 ( 16.41%)
> Amean alloc-odr0-128 145.00 ( 0.00%) 118.00 ( 18.62%)
> Amean alloc-odr0-256 136.15 ( 0.00%) 125.00 ( 8.19%)
> Amean alloc-odr0-512 147.92 ( 0.00%) 121.77 ( 17.68%)
> Amean alloc-odr0-1024 147.23 ( 0.00%) 126.15 ( 14.32%)
> Amean alloc-odr0-2048 155.15 ( 0.00%) 129.92 ( 16.26%)
> Amean alloc-odr0-4096 164.00 ( 0.00%) 136.77 ( 16.60%)
> Amean alloc-odr0-8192 166.92 ( 0.00%) 138.08 ( 17.28%)
> Amean alloc-odr0-16384 159.00 ( 0.00%) 138.00 ( 13.21%)
> Amean free-odr0-1 165.00 ( 0.00%) 89.00 ( 46.06%)
> Amean free-odr0-2 113.00 ( 0.00%) 63.00 ( 44.25%)
> Amean free-odr0-4 99.00 ( 0.00%) 54.00 ( 45.45%)
> Amean free-odr0-8 88.00 ( 0.00%) 47.38 ( 46.15%)
> Amean free-odr0-16 83.00 ( 0.00%) 46.00 ( 44.58%)
> Amean free-odr0-32 80.00 ( 0.00%) 44.38 ( 44.52%)
> Amean free-odr0-64 72.62 ( 0.00%) 43.00 ( 40.78%)
> Amean free-odr0-128 78.00 ( 0.00%) 42.00 ( 46.15%)
> Amean free-odr0-256 80.46 ( 0.00%) 57.00 ( 29.16%)
> Amean free-odr0-512 96.38 ( 0.00%) 64.69 ( 32.88%)
> Amean free-odr0-1024 107.31 ( 0.00%) 72.54 ( 32.40%)
> Amean free-odr0-2048 108.92 ( 0.00%) 78.08 ( 28.32%)
> Amean free-odr0-4096 113.38 ( 0.00%) 82.23 ( 27.48%)
> Amean free-odr0-8192 112.08 ( 0.00%) 82.85 ( 26.08%)
> Amean free-odr0-16384 110.38 ( 0.00%) 81.92 ( 25.78%)
> Amean total-odr0-1 452.15 ( 0.00%) 308.00 ( 31.88%)
> Amean total-odr0-2 334.23 ( 0.00%) 246.23 ( 26.33%)
> Amean total-odr0-4 286.00 ( 0.00%) 205.38 ( 28.19%)
> Amean total-odr0-8 255.54 ( 0.00%) 180.15 ( 29.50%)
> Amean total-odr0-16 239.00 ( 0.00%) 169.00 ( 29.29%)
> Amean total-odr0-32 229.00 ( 0.00%) 162.69 ( 28.96%)
> Amean total-odr0-64 211.38 ( 0.00%) 159.00 ( 24.78%)
> Amean total-odr0-128 223.00 ( 0.00%) 160.00 ( 28.25%)
> Amean total-odr0-256 216.62 ( 0.00%) 182.00 ( 15.98%)
> Amean total-odr0-512 244.31 ( 0.00%) 186.46 ( 23.68%)
> Amean total-odr0-1024 254.54 ( 0.00%) 198.69 ( 21.94%)
> Amean total-odr0-2048 264.08 ( 0.00%) 208.00 ( 21.24%)
> Amean total-odr0-4096 277.38 ( 0.00%) 219.00 ( 21.05%)
> Amean total-odr0-8192 279.00 ( 0.00%) 220.92 ( 20.82%)
> Amean total-odr0-16384 269.38 ( 0.00%) 219.92 ( 18.36%)
>
> This is the alloc, free and total overhead of allocating order-0 pages in
> batches of 1 page up to 16384 pages. Avoiding disabling/enabling overhead
> massively reduces overhead. Alloc overhead is roughly reduced by 14-20% in
> most cases. The free path is reduced by 26-46% and the total reduction
> is significant.
>
[...]
>
> Similarly, little benefit was seen on networking benchmarks both localhost
> and between physical server/clients where other costs dominate. It's
> possible that this will only be noticable on very high speed networks.

The networking results highly depend on NIC drivers. As you mention in
the cover-letter, (1) some drivers (e.g mlx4) alloc high-order pages to
work-around order-0 pages and DMA-map being too slow (for their HW
use-case), (2) drivers that do use order-0 pages have driver specific
page-recycling tricks (e.g. mlx5 and ixgbe). The page_pool target
making a more generic recycle mechanism for drivers to use.

I'm very excited to see improvements in this area! :-)))
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer