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SubjectRe: ARM router NAT performance affected by random/unrelated commits
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On 22.05.2019 14:17, Russell King - ARM Linux admin wrote:
> On Wed, May 22, 2019 at 01:51:01PM +0200, Rafał Miłecki wrote:
>> On 21.05.2019 12:45, Russell King - ARM Linux admin wrote:> On Tue, May 21, 2019 at 12:28:48PM +0200, Rafał Miłecki wrote:
>>>> I work on home routers based on Broadcom's Northstar SoCs. Those devices
>>>> have ARM Cortex-A9 and most of them are dual-core.
>>>>
>>>> As for home routers, my main concern is network performance. That CPU
>>>> isn't powerful enough to handle gigabit traffic so all kind of
>>>> optimizations do matter. I noticed some unexpected changes in NAT
>>>> performance when switching between kernels.
>>>>
>>>> My hardware is BCM47094 SoC (dual core ARM) with integrated network
>>>> controller and external BCM53012 switch.
>>>
>>> Guessing, I'd say it's to do with the placement of code wrt cachelines.
>>> You could try aligning some of the cache flushing code to a cache line
>>> and see what effect that has.
>>
>> Is System.map a good place to check for functions code alignment?
>>
>> With Linux 4.19 + OpenWrt mtd patches I have:
>> (...)
>> c010ea94 t v7_dma_inv_range
>> c010eae0 t v7_dma_clean_range
>> (...)
>> c02ca3d0 T blk_mq_update_nr_hw_queues
>> c02ca69c T blk_mq_alloc_tag_set
>> c02ca94c T blk_mq_release
>> c02ca9b4 T blk_mq_free_queue
>> c02caa88 T blk_mq_update_nr_requests
>> c02cab50 T blk_mq_unique_tag
>> (...)
>>
>> After cherry-picking 9316a9ed6895 ("blk-mq: provide helper for setting
>> up an SQ queue and tag set"):
>> (...)
>> c010ea94 t v7_dma_inv_range
>> c010eae0 t v7_dma_clean_range
>> (...)
>> c02ca3d0 T blk_mq_update_nr_hw_queues
>> c02ca69c T blk_mq_alloc_tag_set
>> c02ca94c T blk_mq_init_sq_queue <-- NEW
>> c02ca9c0 T blk_mq_release <-- Different address of this & all below
>> c02caa28 T blk_mq_free_queue
>> c02caafc T blk_mq_update_nr_requests
>> c02cabc4 T blk_mq_unique_tag
>> (...)
>>
>> As you can see blk_mq_init_sq_queue has appeared in the System.map and
>> it affected addresses of ~30000 symbols. I can believe some frequently
>> used symbols got luckily aligned and that improved overall performance.
>>
>> Interestingly v7_dma_inv_range() and v7_dma_clean_range() were not
>> relocated.
>>
>> *****
>>
>> I followed Russell's suggestion and added .align 5 to cache-v7.S (see
>> two attached diffs).
>>
>> 1) v4.19 + OpenWrt mtd patches
>>> egrep -B 1 -A 1 "v7_dma_(inv|clean)_range" System.map
>> c010ea58 T v7_flush_kern_dcache_area
>> c010ea94 t v7_dma_inv_range
>> c010eae0 t v7_dma_clean_range
>> c010eb18 T b15_dma_flush_range
>>
>> 2) v4.19 + OpenWrt mtd patches + two .align 5 in cache-v7.S
>> c010ea6c T v7_flush_kern_dcache_area
>> c010eac0 t v7_dma_inv_range
>> c010eb20 t v7_dma_clean_range
>> c010eb58 T b15_dma_flush_range
>> (actually 15 symbols above v7_dma_inv_range were replaced)
>>
>> This method seems to be somehow working (at least affects addresses in
>> System.map).
>>
>> *****
>>
>> I run 2 tests for each combination of changes. Each test consisted of
>> 10 sequences of: 30 seconds iperf session + reboot.
>>
>>
>>> git reset --hard v4.19
>>> git am OpenWrt-mtd-chages.patch
>> Test #1: 738 Mb/s
>> Test #2: 737 Mb/s
>>
>>> git reset --hard v4.19
>>> git am OpenWrt-mtd-chages.patch
>> patch -p1 < v7_dma_clean_range-align.diff
>> Test #1: 746 Mb/s
>> Test #2: 747 Mb/s
>>
>>> git reset --hard v4.19
>>> git am OpenWrt-mtd-chages.patch
>>> patch -p1 < v7_dma_inv_range-align.diff
>> Test #1: 745 Mb/s
>> Test #2: 746 Mb/s
>>
>>> git reset --hard v4.19
>>> git am OpenWrt-mtd-chages.patch
>>> patch -p1 < v7_dma_clean_range-align.diff
>>> patch -p1 < v7_dma_inv_range-align.diff
>> Test #1: 762 Mb/s
>> Test #2: 761 Mb/s
>>
>> As you can see I got a quite nice performance improvement after aligning
>> both: v7_dma_clean_range() and v7_dma_inv_range().
>
> This is an improvement of about 3.3%.
>
>> It still wasn't as good as with 9316a9ed6895 cherry-picked but pretty
>> close.
>>
>>
>>> git reset --hard v4.19
>>> git am OpenWrt-mtd-chages.patch
>>> git cherry-pick -x 9316a9ed6895
>> Test #1: 770 Mb/s
>> Test #2: 766 Mb/s
>>
>>> git reset --hard v4.19
>>> git am OpenWrt-mtd-chages.patch
>>> git cherry-pick -x 9316a9ed6895
>>> patch -p1 < v7_dma_clean_range-align.diff
>> Test #1: 756 Mb/s
>> Test #2: 759 Mb/s
>>
>>> git reset --hard v4.19
>>> git am OpenWrt-mtd-chages.patch
>>> git cherry-pick -x 9316a9ed6895
>>> patch -p1 < v7_dma_inv_range-align.diff
>> Test #1: 758 Mb/s
>> Test #2: 759 Mb/s
>>
>>> git reset --hard v4.19
>>> git am OpenWrt-mtd-chages.patch
>>> git cherry-pick -x 9316a9ed6895
>>> patch -p1 < v7_dma_clean_range-align.diff
>>> patch -p1 < v7_dma_inv_range-align.diff
>> Test #1: 767 Mb/s
>> Test #2: 763 Mb/s
>>
>> Now you can see how unpredictable it is. If I cherry-pick 9316a9ed6895
>> and do an extra alignment of v7_dma_clean_range() and v7_dma_inv_range()
>> that extra alignment can actually *hurt* NAT performance.
>
> You have a maximum variance of 4Mb/s in your tests which is around
> 0.5%, and this shows a reduction of 3Mb/s, or 0.4%.
>
> If we look at it a different way:
> - Without the alignment patches, there is a difference of 4% in
> performance depending on whether 9316a9ed6895 is applied.
> - With the alignment patches, there is a difference of 0.4% in
> performance depending on whether 9316a9ed6895 is applied.
>
> How can this not be beneficial?

Aligning v7_dma_clean_range() and v7_dma_inv_range() is definitely
beneficial! I'm sorry I wasn't clear enough.

I redid testing of 2 most important setups with few more iterations.

> git reset --hard v4.19
> git am OpenWrt-mtd-chages.patch
> git cherry-pick -x 9316a9ed6895
[ 3] 0.0-30.0 sec 2.71 GBytes 776 Mbits/sec
[ 3] 0.0-30.0 sec 2.71 GBytes 775 Mbits/sec
[ 3] 0.0-30.0 sec 2.70 GBytes 774 Mbits/sec
[ 3] 0.0-30.0 sec 2.70 GBytes 774 Mbits/sec
[ 3] 0.0-30.0 sec 2.70 GBytes 773 Mbits/sec
[ 3] 0.0-30.0 sec 2.70 GBytes 773 Mbits/sec
[ 3] 0.0-30.0 sec 2.70 GBytes 773 Mbits/sec
[ 3] 0.0-30.0 sec 2.69 GBytes 771 Mbits/sec
[ 3] 0.0-30.0 sec 2.69 GBytes 771 Mbits/sec
[ 3] 0.0-30.0 sec 2.69 GBytes 771 Mbits/sec
[ 3] 0.0-30.0 sec 2.69 GBytes 771 Mbits/sec
[ 3] 0.0-30.0 sec 2.69 GBytes 770 Mbits/sec
[ 3] 0.0-30.0 sec 2.69 GBytes 770 Mbits/sec
[ 3] 0.0-30.0 sec 2.69 GBytes 770 Mbits/sec
[ 3] 0.0-30.0 sec 2.69 GBytes 770 Mbits/sec
[ 3] 0.0-30.0 sec 2.69 GBytes 769 Mbits/sec
[ 3] 0.0-30.0 sec 2.69 GBytes 769 Mbits/sec
[ 3] 0.0-30.0 sec 2.68 GBytes 768 Mbits/sec
[ 3] 0.0-30.0 sec 2.68 GBytes 768 Mbits/sec
[ 3] 0.0-30.0 sec 2.68 GBytes 767 Mbits/sec
[ 3] 0.0-30.0 sec 2.68 GBytes 767 Mbits/sec
[ 3] 0.0-30.0 sec 2.68 GBytes 767 Mbits/sec
[ 3] 0.0-30.0 sec 2.67 GBytes 765 Mbits/sec
[ 3] 0.0-30.0 sec 2.67 GBytes 765 Mbits/sec
[ 3] 0.0-30.0 sec 2.67 GBytes 764 Mbits/sec
[ 3] 0.0-30.0 sec 2.67 GBytes 763 Mbits/sec
[ 3] 0.0-30.0 sec 2.67 GBytes 763 Mbits/sec
[ 3] 0.0-30.0 sec 2.66 GBytes 762 Mbits/sec
[ 3] 0.0-30.0 sec 2.66 GBytes 760 Mbits/sec
[ 3] 0.0-30.0 sec 2.66 GBytes 760 Mbits/sec
Average: 769 Mb/s (+4,10%)
Previous results: 773 Mb/s, 770 Mb/s, 766 Mb/s

> git reset --hard v4.19
> git am OpenWrt-mtd-chages.patch
> patch -p1 < v7_dma_clean_range-align.diff
> patch -p1 < v7_dma_inv_range-align.diff
[ 3] 0.0-30.0 sec 2.69 GBytes 769 Mbits/sec
[ 3] 0.0-30.0 sec 2.69 GBytes 769 Mbits/sec
[ 3] 0.0-30.0 sec 2.68 GBytes 767 Mbits/sec
[ 3] 0.0-30.0 sec 2.68 GBytes 766 Mbits/sec
[ 3] 0.0-30.0 sec 2.67 GBytes 766 Mbits/sec
[ 3] 0.0-30.0 sec 2.67 GBytes 765 Mbits/sec
[ 3] 0.0-30.0 sec 2.67 GBytes 765 Mbits/sec
[ 3] 0.0-30.0 sec 2.67 GBytes 765 Mbits/sec
[ 3] 0.0-30.0 sec 2.67 GBytes 764 Mbits/sec
[ 3] 0.0-30.0 sec 2.67 GBytes 763 Mbits/sec
[ 3] 0.0-30.0 sec 2.67 GBytes 763 Mbits/sec
[ 3] 0.0-30.0 sec 2.66 GBytes 762 Mbits/sec
[ 3] 0.0-30.0 sec 2.66 GBytes 762 Mbits/sec
[ 3] 0.0-30.0 sec 2.66 GBytes 762 Mbits/sec
[ 3] 0.0-30.0 sec 2.66 GBytes 762 Mbits/sec
[ 3] 0.0-30.0 sec 2.66 GBytes 761 Mbits/sec
[ 3] 0.0-30.0 sec 2.66 GBytes 761 Mbits/sec
[ 3] 0.0-30.0 sec 2.66 GBytes 760 Mbits/sec
[ 3] 0.0-30.0 sec 2.66 GBytes 760 Mbits/sec
[ 3] 0.0-30.0 sec 2.66 GBytes 760 Mbits/sec
[ 3] 0.0-30.0 sec 2.65 GBytes 760 Mbits/sec
[ 3] 0.0-30.0 sec 2.65 GBytes 759 Mbits/sec
[ 3] 0.0-30.0 sec 2.65 GBytes 759 Mbits/sec
[ 3] 0.0-30.0 sec 2.65 GBytes 758 Mbits/sec
[ 3] 0.0-30.0 sec 2.65 GBytes 758 Mbits/sec
[ 3] 0.0-30.0 sec 2.65 GBytes 757 Mbits/sec
[ 3] 0.0-30.0 sec 2.65 GBytes 757 Mbits/sec
[ 3] 0.0-30.0 sec 2.65 GBytes 757 Mbits/sec
[ 3] 0.0-30.0 sec 2.64 GBytes 757 Mbits/sec
[ 3] 0.0-30.0 sec 2.64 GBytes 756 Mbits/sec
Average: 762 Mb/s (+3,16%)
Previous results: 767 Mb/s, 763 Mb/s

So let me explain why I keep researching on this. There are two reasons:

1) Realignment done by cherry-picking 9316a9ed6895 was providing a
*marginally* better performance than aligning v7_dma_clean_range() and
v7_dma_inv_range(). It's a *very* minimal difference but I can't stop
thinking I can still do better.

2) Cherry-picking 9316a9ed6895 doesn't change v7_dma_clean_range or
v7_dma_inv_range addresses at all. Yet it still improves NAT
performance. That makes me believe there are more functions that (if
properly aligned) can bump NAT performance.
I hope that aligning all:
* v7_dma_clean_range
* v7_dma_inv_range
* [some unrevealed functions]
could result in even better NAT performance.

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