linux-ext4 - Re: [PATCH, RFC] Don't do page stablization if !CONFIG_BLKDEV

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Date:	Thu, 8 Mar 2012 16:24:12 -0500
From:	Ted Ts'o <tytso@....edu>
To:	Boaz Harrosh <bharrosh@...asas.com>
Cc:	Chris Mason <chris.mason@...cle.com>, Zach Brown <zab@...bo.net>,
	Eric Sandeen <sandeen@...hat.com>,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [PATCH, RFC] Don't do page stablization if
 !CONFIG_BLKDEV_INTEGRITY

On Thu, Mar 08, 2012 at 12:20:26PM -0800, Boaz Harrosh wrote:
> 
> I have a theory of how we can fix that 2-sec wait, by avoiding writeback of
> the last n pages of an inode who's mtime is less then 2-sec. This would
> solve any sequential writer wait penalty, which is Ted's case

That won't work in general, *unless* 2 seconds is enough time that the
appending writer is done writing to that particular 4k page and moved
on to the next 4k block, so nothing touches that page and potentially
blocks for however long it takes for the queues to drain.

Let's take another example, suppose you have a file-backed mmap
region, and you modify the page, and now let's suppose the process is
under enough memory pressure that the page cleaner decides to initiate
writeback of the page.  Now suppose you get unlucky (this is the 1% or
0.1% case; remember, 99th or 99.9 percentile latencies matter), and
you try to modify the page in question again.  ***THUNK*** your
process takes a page fault, and is frozen solid in amber for
potentially seconds until the I/O queues drain.

Hmm.... let's turn this around.  If the issue is checksum calculation,
how about trying to solve this problem in some cases by deferring the
checksum calculation until right before the block I/O layer is going
to schedule the write (i.e., have the I/O submitter provide a callback
function which calculates the checksum, which gets called by the BIO
layer at the very last moment)?

This won't work in all cases (I can see this getting really messy in
the software RAID-5/6 case if you don't want to memory copies) but it
might solve the problem in at least some of the cases where people
care about this.

					- Ted
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