linux-kernel - Re: Latency writing to an mlocked ext4 mapping

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Date:	Wed, 19 Oct 2011 19:17:18 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: Latency writing to an mlocked ext4 mapping

On Wed, Oct 19, 2011 at 6:15 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Wed, Oct 19, 2011 at 6:02 PM, Andreas Dilger <adilger@...ger.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.

I'll leave it running overnight and see what happens.


--Andy
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