linux-kernel - Fwd: Strange I/O slowdown with 4.x kernels

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Message-ID: <CAKgfjTm=8cwOqJMcABkv3DAQ-RHRUAzOtSirs1auFbLkqqdQmg@mail.gmail.com>
Date:	Fri, 29 Apr 2016 16:36:16 +0300
From:	Rus <rus@...nxsoft.com>
To:	linux-kernel@...r.kernel.org
Subject: Fwd: Strange I/O slowdown with 4.x kernels

Hi,

After switching from 3.x to 4.x I'm observing strange filesystem (ext4
used) slowdown just after fresh boot - it is 100% reproducible. The
simple test just after boot at root fs first shows the good
performance :

dd if=/dev/zero of=/output bs=8k count=10k; rm -f /output
10240+0 records in
10240+0 records out
83886080 bytes (84 MB) copied, 0.5676 s, 150.3 MB/s

But after some small i/o stress (completed updatedb command for ex)
the i/o speed drops dramatically :

dd if=/dev/zero of=/output bs=8k count=10k; rm -f /output
10240+0 records in
10240+0 records out
83886080 bytes (84 MB) copied, 19.5703 s, 4.3 MB/s

What is interesting is that hdparm test still shows normal performance :

hdparm -t /dev/sda1

/dev/sda1:
 Timing buffered disk reads: 448 MB in  3.01 seconds = 148.79 MB/sec

How to debug such thing (bissect can take too much time because of big
version range) ? The config attached, this is ordinary 32-bit system
with 32GB ECC RAM.
 The slowdown clearly triggered with last stable (4.5.2) and latest rc
(4.6-rc5) kernels. The 3.x kernels do not have such bug.
Can supply any additional info.

TIA, Rus

-- 
SfinxSoft
http://sfinxsoft.com

-- 
SfinxSoft
http://sfinxsoft.com

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