linux-kernel - Re: [LKP] [lkp] [xfs] 68a9f5e700: aim7.jobs-per-min -13.6% regression

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Date:	Fri, 12 Aug 2016 04:52:18 +0200
From:	Christoph Hellwig <hch@....de>
To:	Dave Chinner <david@...morbit.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christoph Hellwig <hch@....de>,
	"Huang, Ying" <ying.huang@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Bob Peterson <rpeterso@...hat.com>,
	Wu Fengguang <fengguang.wu@...el.com>, LKP <lkp@...org>
Subject: Re: [LKP] [lkp] [xfs] 68a9f5e700: aim7.jobs-per-min -13.6%
	regression

On Fri, Aug 12, 2016 at 12:23:29PM +1000, Dave Chinner wrote:
> Christoph, maybe there's something we can do to only trigger
> speculative prealloc growth checks if the new file size crosses the end of
> the currently allocated block at the EOF. That would chop out a fair
> chunk of the xfs_bmapi_read calls being done in this workload. I'm
> not sure how much effort we should spend optimising this slow path,
> though....

I can look at that, but indeed optimizing this patch seems a bit
stupid.  The other thing we could do is to optimize xfs_bmapi_read - even
if it shouldn't be called this often it seems like it should waste a whole
lot less CPU cycles.