From: Andy Lutomirski <luto@amacapital.net>
To: Andreas Dilger <adilger@dilger.ca>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"linux-ext4@vger.kernel.org" <linux-ext4@vger.kernel.org>
Subject: Re: Latency writing to an mlocked ext4 mapping
Date: Wed, 19 Oct 2011 22:59:55 -0700 [thread overview]
Message-ID: <CALCETrUcOKQAJTTmCSD3Q3wYS-zLqv6tBa4AdkK50bNobRhDUQ@mail.gmail.com> (raw)
In-Reply-To: <CALCETrX=-CnNQ9+4tRbqMG4mfuy2FBPXXoJeBVDVPnEiRJYRFQ@mail.gmail.com>
On Wed, Oct 19, 2011 at 7:17 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Wed, Oct 19, 2011 at 6:15 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>> On Wed, Oct 19, 2011 at 6:02 PM, Andreas Dilger <adilger@dilger.ca> wrote:
>>> What kernel are you using? A change to keep pages consistent during writeout was landed not too long ago (maybe Linux 3.0) in order to allow checksumming of the data.
>>
>> 3.0.6, with no relevant patches. (I have a one-liner added to the tcp
>> code that I'll submit sometime soon.) Would this explain the latency
>> in file_update_time or is that a separate issue? file_update_time
>> seems like a good thing to make fully asynchronous (especially if the
>> file in question is a fifo, but I've already moved my fifos to tmpfs).
>
> On 2.6.39.4, I got one instance of:
>
> call_rwsem_down_read_failed ext4_map_blocks ext4_da_get_block_prep
> __block_write_begin ext4_da_write_begin ext4_page_mkwrite do_wp_page
> handle_pte_fault handle_mm_fault do_page_fault page_fault
>
> but I'm not seeing the large numbers of the ext4_page_mkwrite trace
> that I get on 3.0.6. file_update_time is now by far the dominant
> cause of latency.
The culprit seems to be do_wp_page -> file_update_time ->
mark_inode_dirty_sync. This surprises me for two reasons:
- Why the _sync? Are we worried that data will be written out before
the metadata? If so, surely there's a better way than adding latency
here.
- Why are we calling file_update_time at all? Presumably we also
update the time when the page is written back (if not, that sounds
like a bug, since the contents may be changed after something saw the
mtime update), and, if so, why bother updating it on the first write?
Anything that relies on this behavior is, I think, unreliable, because
the page could be made writable arbitrarily early by another program
that changes nothing.
--Andy
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next prev parent reply other threads:[~2011-10-20 5:59 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-10-20 0:39 Latency writing to an mlocked ext4 mapping Andy Lutomirski
2011-10-20 1:02 ` Andreas Dilger
2011-10-20 1:15 ` Andy Lutomirski
2011-10-20 2:17 ` Andy Lutomirski
2011-10-20 5:59 ` Andy Lutomirski [this message]
2011-10-25 12:26 ` Jan Kara
2011-10-28 23:37 ` Andy Lutomirski
2011-10-28 23:39 ` [PATCH] mm: Improve cmtime update on shared writable mmaps Andy Lutomirski
2011-11-01 22:53 ` Jan Kara
2011-11-01 23:02 ` Andy Lutomirski
2011-11-02 7:38 ` Christoph Hellwig
2011-11-02 15:02 ` Jan Kara
2011-11-02 15:19 ` Ted Ts'o
2011-10-31 23:10 ` Latency writing to an mlocked ext4 mapping Jan Kara
2011-10-31 23:14 ` Andy Lutomirski
2011-11-01 23:03 ` Jan Kara
2011-11-01 23:10 ` Andy Lutomirski
2011-11-02 1:51 ` Andy Lutomirski
2011-11-02 20:17 ` Jan Kara
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