Re: [Xen-devel] [PATCH] xen/arm: io: add function swap_mmio_handler()

[Julien Grall <julien.grall@arm.com>]

Hi,

On 12/06/2019 11:08, chenbaodong wrote:
> 
> On 6/12/19 17:08, Julien Grall wrote:
>> Hi,
>>
>> On 6/12/19 6:42 AM, Baodong Chen wrote:
>>> Swap function can be used when calling sort().
>>> or else, the default swap function generic_swap() is used,
>>> which is a little inefficient.
>>
>> I am not entirely convince this will be more efficient. mmio_handler does not 
>> fit in 64 bit, so the compiler may decide to do either multiple load or 
>> replace with a memcpy.
> 
> Hello Julien,
> 
> I have checked the disassemble result,
> 
> and IIUC generic_swap has a loop so it should be a little inefficient. I'm not 
> expert about hardware, please correct me if i'm wrong.

I am not an hardware expert too... But as I pointed out below this is a 
micro-optimization. In other words, you are tailoring a specific function that 
may run faster now, but this is improvement is going to be lost as this is just 
a very tiny part of the domain creation.

[...]

>>
>> So at best this feels some micro-optimization. But then, this is only call a 
>> limited number of time at each domain build. Is it really worth it?
> 
> It's not hot path here.
> 
> Not sure about worth.
> 
> Personally  i will try my best to do things well according to my understanding.

Micro-optimization are always good, but you also have to factor the cost of 
maintaining and whether this will improve significantly Xen.

> 
>>
>> On a side note, I have noticed you are sending a lot of optimization/clean-up 
>> patch. What is your end goal here?
> 
> My goal is to understand how xen works well.
> 
>>
>> If it is to improve the performance, then there are much bigger fish to fry 
>> within Xen code base. I am happy to point some of them based on where you are 
>> looking to improve.
> 
> Surly i want to improve performance.
> 
> Features like Fast Startup ( I learned from xen summit 2018, samsung automotive 
> presentation).
> 
> But currently i don't understand xen well, only a few weeks experience.

We do have small task for newcomers that would improve Xen code base and also 
allow your to understand more some part of the code.

If you have a specific area of interest, I can see if I have some small tasks there.

Cheers,

-- 
Julien Grall

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