Re: [PATCH] buffer: Avoid setting buffer bits that are already set

[kemi <kemi.wang@intel.com>]

On 2017年10月24日 00:19, Jens Axboe wrote:
> On 10/23/2017 10:27 AM, Kemi Wang wrote:
>> It's expensive to set buffer flags that are already set, because that
>> causes a costly cache line transition.
>>
>> A common case is setting the "verified" flag during ext4 writes.
>> This patch checks for the flag being set first.
>>
>> With the AIM7/creat-clo benchmark testing on a 48G ramdisk based-on ext4
>> file system, we see 3.3%(15431->15936) improvement of aim7.jobs-per-min on
>> a 2-sockets broadwell platform.
>>
>> What the benchmark does is: it forks 3000 processes, and each  process do
>> the following:
>> a) open a new file
>> b) close the file
>> c) delete the file
>> until loop=100*1000 times.
>>
>> The original patch is contributed by Andi Kleen.
> 
> We discussed this recently, in reference to this commit:
> 
> commit 7fcbbaf18392f0b17c95e2f033c8ccf87eecde1d
> Author: Jens Axboe <axboe@fb.com>
> Date:   Thu May 22 11:54:16 2014 -0700
> 
>     mm/filemap.c: avoid always dirtying mapping->flags on O_DIRECT
> 
> which made a massive difference, as the changelog details.
> 
> blk-mq uses this extensively as well, where possible. The problem is
> that it always has to be explained, hence the recent discussion was
> around perhaps adding
> 
> set_bit_if_not_set()
> clear_bit_if_set()
> 
> or similar functions, to document in a single location why this matters.
> Additionally, some archs may be able to implement that in an efficient
> manner.
> 
> You can add my reviewed-by to the below, 

Thanks.

I'll see if I can find some
> time to implement the above in a nice way.

Agree. Maybe something like test_and_set_bit() would be more suitable.

 In the mean time, you may
> want to consider adding a comment to the function explaining why you
> have done it that way.
> 

Sure.