Re: [PATCH 1/6] bio: optimize initialization of a bio

[Jens Axboe <axboe@kernel.dk>]

On 8/12/21 12:51 AM, Christoph Hellwig wrote:
> On Wed, Aug 11, 2021 at 01:35:28PM -0600, Jens Axboe wrote:
>> The memset() used is measurably slower in targeted benchmarks. Get rid
>> of it and fill in the bio manually, in a separate helper.
> 
> If you have some numbers if would be great to throw them in here.

It's about 1% of the overhead of the alloc after the cache, which
comes later in the series.

Percent│    return __underlying_memset(p, c, size);
       │      lea    0x8(%r8),%rdi
       │    bio_alloc_kiocb():
  2.18 │      cmove  %rax,%r9
       │    memset():
       │      mov    %r8,%rcx
       │      and    $0xfffffffffffffff8,%rdi
       │      movq   $0x0,(%r8)
       │      sub    %rdi,%rcx
       │      add    $0x60,%ecx
       │      shr    $0x3,%ecx
 55.02 │      rep    stos %rax,%es:(%rdi)

This is on AMD, might look different on Intel, the manual clear seems
like a nice win on both. As a minor detail, avoids things like
re-setting bio->bi_pool for cached entries, as it never changes.


>> +static inline void __bio_init(struct bio *bio)
> 
> Why is this split from bio_init and what are the criteria where an
> initialization goes?

Got rid of the helper.

>> +	bio->bi_flags = bio->bi_ioprio = bio->bi_write_hint = 0;
> 
> Please keep each initialization on a separate line.

Done

-- 
Jens Axboe