Re: [Patches] [PATCH] ARM: NEON detected memcpy.

["Shih-Yuan Lee (FourDollars)" <sylee@xxxxxxxxxxxxx>]

Hi Ondrej,

I do have some benchmark data.

--- Running benchmarks (average case/perfect alignment case) ---

very small data test:
memcpy_arm     :  (3 bytes copy) =   86.2 MB/s /   88.3 MB/s
memcpy_neon    :  (3 bytes copy) =   53.4 MB/s /   54.5 MB/s
memcpy_arm     :  (4 bytes copy) =   79.8 MB/s /   62.9 MB/s
memcpy_neon    :  (4 bytes copy) =   72.5 MB/s /   73.9 MB/s
memcpy_arm     :  (5 bytes copy) =   91.0 MB/s /   78.7 MB/s
memcpy_neon    :  (5 bytes copy) =   90.2 MB/s /   91.0 MB/s
memcpy_arm     :  (7 bytes copy) =  109.5 MB/s /  104.7 MB/s
memcpy_neon    :  (7 bytes copy) =  122.1 MB/s /  126.6 MB/s
memcpy_arm     :  (8 bytes copy) =  122.4 MB/s /  122.4 MB/s
memcpy_neon    :  (8 bytes copy) =  142.0 MB/s /  148.2 MB/s
memcpy_arm     :  (11 bytes copy) =  157.8 MB/s /  161.3 MB/s
memcpy_neon    :  (11 bytes copy) =  193.8 MB/s /  196.2 MB/s
memcpy_arm     :  (12 bytes copy) =  170.1 MB/s /  172.7 MB/s
memcpy_neon    :  (12 bytes copy) =  206.8 MB/s /  212.5 MB/s
memcpy_arm     :  (15 bytes copy) =  204.0 MB/s /  209.6 MB/s
memcpy_neon    :  (15 bytes copy) =  247.5 MB/s /  270.3 MB/s
memcpy_arm     :  (16 bytes copy) =  212.2 MB/s /  225.6 MB/s
memcpy_neon    :  (16 bytes copy) =  175.3 MB/s /  252.2 MB/s
memcpy_arm     :  (24 bytes copy) =  274.6 MB/s /  326.5 MB/s
memcpy_neon    :  (24 bytes copy) =  244.7 MB/s /  367.8 MB/s
memcpy_arm     :  (31 bytes copy) =  333.3 MB/s /  399.2 MB/s
memcpy_neon    :  (31 bytes copy) =  304.3 MB/s /  463.5 MB/s

L1 cached data:
memcpy_arm     :  (4096 bytes copy) = 1295.5 MB/s / 2691.8 MB/s
memcpy_neon    :  (4096 bytes copy) = 1826.3 MB/s / 2021.8 MB/s
memcpy_arm     :  (6144 bytes copy) = 1306.5 MB/s / 2724.1 MB/s
memcpy_neon    :  (6144 bytes copy) = 1857.8 MB/s / 2053.2 MB/s

L2 cached data:
memcpy_arm     :  (65536 bytes copy) = 1291.5 MB/s / 2304.8 MB/s
memcpy_neon    :  (65536 bytes copy) = 1866.5 MB/s / 2441.7 MB/s
memcpy_arm     :  (98304 bytes copy) = 1285.6 MB/s / 2283.8 MB/s
memcpy_neon    :  (98304 bytes copy) = 1860.7 MB/s / 2454.7 MB/s

SDRAM:
memcpy_arm     :  (2097152 bytes copy) =  466.7 MB/s /  736.5 MB/s
memcpy_neon    :  (2097152 bytes copy) =  727.5 MB/s /  868.8 MB/s
memcpy_arm     :  (3145728 bytes copy) =  507.9 MB/s /  854.7 MB/s
memcpy_neon    :  (3145728 bytes copy) =  852.9 MB/s / 1038.0 MB/s

(*) 1 MB = 1000000 bytes
(*) 'memcpy_arm' - an implementation for older ARM cores from glibc-ports

The similar benchmark is at
http://sourceware.org/ml/libc-ports/2009-07/msg00000.html .

Regards,
$4

On Thu, Apr 4, 2013 at 12:19 AM, OndÅej BÃlka <neleai@xxxxxxxxx> wrote:
> On Wed, Apr 03, 2013 at 11:47:36PM +0800, Shih-Yuan Lee (FourDollars) wrote:
>> Hi Joseph,
>>
> ...
>> > I was previously told by people at ARM that NEON memcpy wasn't a good idea
>> > in practice because of raised power consumption, context switch costs etc.
>> > from using NEON in processes that otherwise didn't use it, even if it
>> > appeared superficially beneficial in benchmarks.
>> >
>> About raised power consumption and context switch costs, I may be able
>> to add some option in configure for the users to decide if they want
>> to use this feature or not.
>> How do you think?
>>
> Configure option is bit overkill.
>
> You need to compare neon/other implementation speed. Then determine
> size where neon is faster if we include energy cost and context switch.
> My first estimate is use neon when larger than 4096 bytes.
>
> However to determine context switch cost of neon you must account network effect.
>
> If you use neon in one function that is called sufficiently often (to
> always save registers) then adding neon implementation for additional functions
> does not increase cost.
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