linux-kernel - Re: [PATCH] Avoid always dirtying mapping->flags on O

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Date:	Mon, 12 May 2014 09:08:16 -0600
From:	Jens Axboe <axboe@...com>
To:	Jeff Moyer <jmoyer@...hat.com>
CC:	<linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>, <linux-mm@...ck.org>
Subject: Re: [PATCH] Avoid always dirtying mapping->flags on O_DIRECT

On 05/12/2014 08:46 AM, Jeff Moyer wrote:
> Jens Axboe <axboe@...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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