linux-ext4 - Re: [PATCH 3/3] ext4: Avoid unnecessary transaction stalls during writeback

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Message-ID: <20170430004743.rjwkzuwoyzewxj27@thunk.org>
Date:   Sat, 29 Apr 2017 20:47:43 -0400
From:   Theodore Ts'o <tytso@....edu>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: [PATCH 3/3] ext4: Avoid unnecessary transaction stalls during
 writeback

On Tue, Apr 11, 2017 at 03:54:18PM +0200, Jan Kara wrote:
> Currently ext4_writepages() submits all pages with transaction started.
> When no page needs block allocation or extent conversion we can submit
> all dirty pages in the inode while holding a single transaction handle
> and when device is congested this can take significant amount of time.
> Thus ext4_writepages() can block transaction commits for extended
> periods of time.
> 
> Take for example a simple benchmark simulating PostgreSQL database
> (pgioperf in mmtest). The benchmark runs 16 processes doing random reads
> from a huge file, one process doing random writes to the huge file, and
> one process doing sequential writes to a small writes and frequently
> running fsync. With unpatched kernel transaction commits take on average
> ~18s with standard deviation of ~41s, top 5 commit times are:
> 
> 274.466639s, 126.467347s, 86.992429s, 34.351563s, 31.517653s.
> 
> After this patch transaction commits take on average 0.1s with standard
> deviation of 0.15s, top 5 commit times are:
> 
> 0.563792s, 0.519980s, 0.509841s, 0.471700s, 0.469899s
> 
> Signed-off-by: Jan Kara <jack@...e.cz>

Great work!   Many thanks.

Apologies for the late application; April has been crazy busy.  I'm
kicking off a full regression test, but it looks good.

	      	   	      	    	- Ted