Re: [PATCH] Avoid always dirtying mapping->flags on O_DIRECT

[Jens Axboe <axboe@fb.com>]

On 05/12/2014 08:46 AM, Jeff Moyer wrote:
> Jens Axboe <axboe@fb.com> writes:
> 
>> Hi,
>>
>> In some testing I ran today, we end up spending 40% of the time in
>> filemap_check_errors(). That smells fishy. Looking further, this is
>> basically what happens:
>>
>> blkdev_aio_read()
>>     generic_file_aio_read()
>>         filemap_write_and_wait_range()
>>             if (!mapping->nr_pages)
>>                 filemap_check_errors()
>>
>> and filemap_check_errors() always attempts two test_and_clear_bit() on
>> the mapping flags, thus dirtying it for every single invocation. The
>> patch below tests each of these bits before clearing them, avoiding this
>> issue. In my test case (4-socket box), performance went from 1.7M IOPS
>> to 4.0M IOPS.
> 
> It might help to use the word cacheline somewhere in here.  ;-) Out of

I thought that was self-evident, but yes, I could add that :-)

> curiosity, what workload were you running?

Nothing fancy, just some fio jobs that spread over two nodes.


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