0017078: High await times for IO operations on single queue devices.

ID	Project	Category	View Status	Date Submitted	Last Update

0017078	CentOS-7	kernel	public	2020-02-21 18:12	2020-03-04 10:57

Reporter	miw
Priority	normal	Severity	minor	Reproducibility	always
Status	new	Resolution	open
Platform	GCE	OS	Centos7	OS Version	Linux 3.10.0-106
Product Version	7.7-1908
Target Version		Fixed in Version

Summary	0017078: High await times for IO operations on single queue devices.
Description	We are observing high await/w_await/r_await values (in our case, up to ~150ms, about 8x higher result when ran on same setup with CentOS 8) reported by iostat for disk IO operations. The issue can be easily reproduced by creating a sample file, decompressing it using gzip and running "iostat -x 1" in parallel. When the benchmark was changed to write from /dev/zero to a file the results are consistent. It suggests gzip is not important here. Our setup was running inside virtual machine with 16 cores with 16gb memory (to make sure that there are no other constrains) and IO tests were running on single queue (virtual) SCSI disk. This issue is visible with Centos7 but not with CentOS 8.
Steps To Reproduce	yum install -yq sysstat head -c 2G </dev/urandom > 2G.bin iostat -x 1 > iostat.txt & sleep 10 time gzip -c -d -f 2G.bin > 2G.bin.gz sleep 10 # high await/w_await/r_await values should be visible in IO stat output. cat iostat.txt
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