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

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Message-ID: <x49r43z837o.fsf@segfault.boston.devel.redhat.com>
Date:	Mon, 12 May 2014 10:46:19 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jens Axboe <axboe@...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

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
curiosity, what workload were you running?

Acked-by: Jeff Moyer <jmoyer@...hat.com>
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