Re: [PATCH 1/3] md/raid10: optimize fix_read_error

[Li Nan]

在 2023/6/23 18:03, Paul Menzel 写道:
> Dear Li,
>
>
> Thank you for your patch.
>
> Am 23.06.23 um 19:32 schrieb linan666@xxxxxxxxxxxxxxx:
> > From: Li Nan <linan122@xxxxxxxxxx>
> >
> > We dereference r10_bio->read_slot too many times in fix_read_error().
> > Optimize it by using a variable to store read_slot.
>
> I am always cautious reading about optimizations without any benchmarks 
> or object code analysis. Although your explanation makes sense, did you 
> check, that performance didn’t decrease in some way? (Maybe the compiler 
> even generates the same code.)
>
>
> Kind regards,
>
> Paul

Compared assembly code before and after optimization:
 - With gcc 8.3.0, both are consistent.
 - With gcc 12.2.1, 'r10_bio->read_slot' mostly uses r10bio dirctly:
     2853    while (sl != r10_bio->read_slot) {
       0xffffffff8213f2a0 <+1328>:  cmp    %r14d,0x38(%rbp)

   'slot' is mostly saved to a register individually:
     2819    while (sl != slot) {
       0xffffffff8213f08a <+794>:   cmp    %r14d,%ebx

I have not tested the performance, and it won't bring significant 
performance optimization, which can also be seen from the analysis of 
the assembly code. In fact, I just want to make code more readable.

--
Thanks,
Nan