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

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Date:	Thu, 24 Sep 2015 00:15:37 +0000
From:	Dexuan Cui <decui@...rosoft.com>
To:	Tejun Heo <tj@...nel.org>, Chris Mason <clm@...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

> -----Original Message-----
> From: Tejun Heo [mailto:htejun@...il.com] On Behalf Of Tejun Heo
> Sent: Thursday, September 24, 2015 2:54
> 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-fsdevel@...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

Thanks a lot for the quick fix, Tejun!

I'll test the fix.
I'll report back in case it can't fix the issue --I think this is unlikely. :-)

-- Dexuan
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