Re: -falign-functions=16 for i386/amd64

[Ryota Ozaki <ozaki-r%netbsd.org@localhost>]

On Thu, Sep 1, 2016 at 4:04 PM, matthew green <mrg%eterna.com.au@localhost> wrote:
> have you tested other values than 1 and 16?  what about 4 or 8?

4 and 8 are not so good; their performance fluctuations are
similar to the unaligned case in my experiments.

>
> can you post the size difference of kernels?  particularly the
> kernel without DIAGNOSTIC or DEBUG (since those are the ones
> where performance matters most.)

I measured the sizes of GENERIC kernels, i.e., DIAGNOSTIC on
and DEBUG off.

The sizes of kernel binaries don't change in most cases because
the alignment of __rodata_start that begins just after kernel text
hides the changes due to -falign-functions.

The sizes of the actual kernel text (from kernel_text to _etext)
slightly changes. The difference between that of GENERIC kernels
w/ and w/o -falign-functions=16 is 200kB. That is 1% of the total
kernel text size.

BTW, as I noted, I'm not exploring an alignment size that provides
best performance, I just want to reduce performance fluctuations.

Thanks,
  ozaki-r