Re: [PATCH] block: Optimize bio_init()

[Jens Axboe <axboe@kernel.dk>]

On 9/12/21 4:01 PM, Bart Van Assche wrote:
> On 9/12/21 06:03, Jens Axboe wrote:
>> On 9/11/21 9:19 PM, Bart Van Assche wrote:
>>> The performance numbers in the patch description come from a
>>> Intel Xeon Gold 6154 CPU. I reran the test today on an old Intel
>>> Core i7-4790 CPU and obtained the opposite result: higher IOPS
>>> without this patch than with this patch although the assembler
>>> code looks to be the same. It seems like how fast "rep stos"
>>> runs depends on the CPU type?
>>
>> It does appear so. Which is a bit frustrating...
> 
> Further measurements have shown that this behavior is specific to
> gcc and also that clang always generates faster code for the version
> of bio_init() in my patch. I have reported this as a bug to the gcc
> project. See also https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102294.

Interesting! Here are some results from my end. First the 3970X again:

gcc-11.1
Elapsed time: 0.980807 s
Elapsed time: 0.452951 s
Elapsed time: 0.949918 s

clang-11.0
Elapsed time: 0.284734 s
Elapsed time: 0.356595 s
Elapsed time: 0.285459 s

And my laptop, which is using:

11th Gen Intel(R) Core(TM) i7-1185G7 @ 3.00GHz

gcc-11.1
Elapsed time: 0.218427 s
Elapsed time: 0.235000 s
Elapsed time: 0.214217 s

clang-11.0
Elapsed time: 0.217436 s
Elapsed time: 0.170959 s
Elapsed time: 0.149630 s

All compiles done with -O2 -march=native

Now I kind of want to compile the kernel with clang and see how that
goes...

-- 
Jens Axboe