linux-ext4 - Re: ext4: performance regression introduced by the cgroup writeback support

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Message-ID: <20150923185337.GK26647@mtj.duckdns.org>
Date:	Wed, 23 Sep 2015 14:53:37 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Chris Mason <clm@...com>, Dexuan Cui <decui@...rosoft.com>,
	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: ext4: performance regression introduced by the cgroup writeback
 support

On Wed, Sep 23, 2015 at 12:13:59PM -0400, Chris Mason wrote:
> > The issue is: under high CPU and disk I/O pressure, *some* processes can suffer from a very slow write speed (e.g., <1MB/s or even only 20KB/s), while the normal write speed should be at least dozens of MB/s.

So, I think I know what caused this regression.  Separate wb domains
shouldn't have been enabled on traditional hierarchies.  It doesn't
work there and leads to multiple wb domains competing on the same
blkcg and the bw estimation would go completely haywire.  Will update
soon.

Thanks.

-- 
tejun
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