linux-kernel - Re: [linus:master] [readahead] ab4443fe3c: vm-scalability.throughput -21.4% regression

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Date: Wed, 21 Feb 2024 12:14:25 +0100
From: Jan Kara <jack@...e.cz>
To: kernel test robot <oliver.sang@...el.com>
Cc: Jan Kara <jack@...e.cz>, oe-lkp@...ts.linux.dev, lkp@...el.com,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Matthew Wilcox <willy@...radead.org>,
	Guo Xuenan <guoxuenan@...wei.com>, linux-fsdevel@...r.kernel.org,
	ying.huang@...el.com, feng.tang@...el.com, fengwei.yin@...el.com
Subject: Re: [linus:master] [readahead]  ab4443fe3c:
 vm-scalability.throughput -21.4% regression

On Tue 20-02-24 16:25:37, kernel test robot wrote:
> Hello,
> 
> kernel test robot noticed a -21.4% regression of vm-scalability.throughput on:
> 
> 
> commit: ab4443fe3ca6298663a55c4a70efc6c3ce913ca6 ("readahead: avoid multiple marked readahead pages")
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
> 
> testcase: vm-scalability
> test machine: 224 threads 2 sockets Intel(R) Xeon(R) Platinum 8480CTDX (Sapphire Rapids) with 512G memory
> parameters:
> 
> 	runtime: 300s
> 	test: lru-file-readtwice
> 	cpufreq_governor: performance

JFYI I had a look into this. What the test seems to do is that it creates
image files on tmpfs, loopmounts XFS there, and does reads over file on
XFS. But I was not able to find what lru-file-readtwice exactly does,
neither I was able to reproduce it because I got stuck on some missing Ruby
dependencies on my test system yesterday.

Given the workload is over tmpfs, I'm not very concerned about what
readahead does and how it performs but still I'd like to investigate where
the regression is coming from because it is unexpected.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR