netdev - Re: Page allocator order-0 optimizations merged

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Date:   Wed, 22 Mar 2017 19:39:17 +0200
From:   Tariq Toukan <tariqt@...lanox.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>
CC:     <akpm@...ux-foundation.org>, linux-mm <linux-mm@...ck.org>,
        Saeed Mahameed <saeedm@...lanox.com>
Subject: Re: Page allocator order-0 optimizations merged



On 01/03/2017 7:36 PM, Tariq Toukan wrote:
>
> On 01/03/2017 3:48 PM, Jesper Dangaard Brouer wrote:
>> Hi NetDev community,
>>
>> I just wanted to make net driver people aware that this MM commit[1] got
>> merged and is available in net-next.
>>
>>   commit 374ad05ab64d ("mm, page_alloc: only use per-cpu allocator 
>> for irq-safe requests")
>>   [1] https://git.kernel.org/davem/net-next/c/374ad05ab64d696
>>
>> It provides approx 14% speedup of order-0 page allocations.  I do know
>> most driver do their own page-recycling.  Thus, this gain will only be
>> seen when this page recycling is insufficient, which Tariq was affected
>> by AFAIK.
> Thanks Jesper, this is great news!
> I will start perf testing this tomorrow.
>>
>> We are also playing with a bulk page allocator facility[2], that I've
>> benchmarked[3][4].  While I'm seeing between 34%-46% improvements by
>> bulking, I believe we actually need to do better, before it reach our
>> performance target for high-speed networking.
> Very promising!
> This fits perfectly in our Striding RQ feature (Multi-Packet WQE)
> where we allocate fragmented buffers (of order-0 pages) of 256KB total.
> Big like :)
>
> Thanks,
> Tariq
>> --Jesper
>>
>> [2] 
>> http://lkml.kernel.org/r/20170109163518.6001-5-mgorman%40techsingularity.net
>> [3] http://lkml.kernel.org/r/20170116152518.5519dc1e%40redhat.com
>> [4] 
>> https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/mm/bench/page_bench04_bulk.c
>>
>>
>> On Mon, 27 Feb 2017 12:25:03 -0800 akpm@...ux-foundation.org wrote:
>>
>>> The patch titled
>>>       Subject: mm, page_alloc: only use per-cpu allocator for 
>>> irq-safe requests
>>> has been removed from the -mm tree.  Its filename was
>>> mm-page_alloc-only-use-per-cpu-allocator-for-irq-safe-requests.patch
>>>
>>> This patch was dropped because it was merged into mainline or a 
>>> subsystem tree
>>>
>>> ------------------------------------------------------
>>> From: Mel Gorman <mgorman@...hsingularity.net>
>>> Subject: mm, page_alloc: only use per-cpu allocator for irq-safe 
>>> requests
>>>
>>> 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. It 
>>> could be
>>> slightly modified to avoid soft IRQs using it but it's not clear it's
>>> worthwhile.
>>>
>>> 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 
Hi Mel, Jesper, and all.

This assumption contradicts regular multi-stream traffic that is 
naturally handled
over close numa cores.  I compared iperf TCP multistream (8 streams)
over CX4 (mlx5 driver) with kernels v4.10 (before this series) vs
kernel v4.11-rc1 (with this series).
I disabled the page-cache (recycle) mechanism to stress the page allocator,
and see a drastic degradation in BW, from 47.5 G in v4.10 to 31.4 G in 
v4.11-rc1 (34% drop).
I noticed queued_spin_lock_slowpath occupies 62.87% of CPU time.

Best,
Tariq