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Subjectioband: Writer starves reader even without competitors (Re: Regarding dm-ioband tests)
On Tue, Sep 08, 2009 at 01:54:00PM -0400, Vivek Goyal wrote:

[..]
> I ran a test to show how readers can be starved in certain cases. I launched
> one reader and three writers. I ran this test twice. First without dm-ioband
> and then with dm-ioband.
>
> Following are few lines from the script to launch readers and writers.
>
> **************************************************************
> sync
> echo 3 > /proc/sys/vm/drop_caches
>
> # Launch writers on sdd2
> dd if=/dev/zero of=/mnt/sdd2/writezerofile1 bs=4K count=262144 &
>
> # Launch writers on sdd1
> dd if=/dev/zero of=/mnt/sdd1/writezerofile1 bs=4K count=262144 &
> dd if=/dev/zero of=/mnt/sdd1/writezerofile2 bs=4K count=262144 &
>
> echo "sleeping for 5 seconds"
> sleep 5
>
> # launch reader on sdd1
> time dd if=/mnt/sdd1/testzerofile1 of=/dev/zero &
> echo "launched reader $!"
> *********************************************************************
>
> Without dm-ioband, reader finished in roughly 5 seconds.
>
> 289533952 bytes (290 MB) copied, 5.16765 s, 56.0 MB/s
> real 0m5.300s
> user 0m0.098s
> sys 0m0.492s
>
> With dm-ioband, reader took, more than 2 minutes to finish.
>
> 289533952 bytes (290 MB) copied, 122.386 s, 2.4 MB/s
>
> real 2m2.569s
> user 0m0.107s
> sys 0m0.548s
>
> I had created ioband1 on /dev/sdd1 and ioband2 on /dev/sdd2 with weights
> 200 and 100 respectively.

Hi Ryo,

I notice that with-in a single ioband device, a single writer starves the
reader even without any competitor groups being present.

I ran following two tests with and without dm-ioband devices

Test1
====
Try to use fio for a sequential reader job. First fio will lay out the
file and do write operation. While those writes are going on, try to do
ls on that partition and observe latency of ls operation.
with dm-ioband (ls test)
------------------------
# cd /mnt/sdd2
# time ls
real 0m9.483s
user 0m0.000s
sys 0m0.002s

without dm-ioband (ls test)
---------------------------
# cd /mnt/sdd2
# time ls
256M-file1 256M-file5 2G-file1 2G-file5 writefile1 writezerofile
256M-file2 256M-file6 2G-file2 files writefile2
256M-file3 256M-file7 2G-file3 fio writefile3
256M-file4 256M-file8 2G-file4 lost+found writefile4

real 0m0.067s
user 0m0.000s
sys 0m0.002s

Notice the time simle "ls" operation took in two cases.


Test2
=====
Same case where fio is laying out a file and then try to read some small
files on that partition at the interval of 1 second.
small file read with dm-ioband
------------------------------
[root@chilli fairness-tests]# ./small-file-read.sh
file # 0, plain reading it took: 0.24 seconds
file # 1, plain reading it took: 13.40 seconds
file # 2, plain reading it took: 6.27 seconds
file # 3, plain reading it took: 13.84 seconds
file # 4, plain reading it took: 5.63 seconds

small file read with-out dm-ioband
==================================
[root@chilli fairness-tests]# ./small-file-read.sh
file # 0, plain reading it took: 0.04 seconds
file # 1, plain reading it took: 0.03 seconds
file # 2, plain reading it took: 0.04 seconds
file # 3, plain reading it took: 0.03 seconds
file # 4, plain reading it took: 0.03 seconds
Notice how small file read latencies have shot up.

Looks like a single writer is completely starving a reader even without
any IO going in any of the other groups.

setup
=====
I created an two ioband device of weight 100 each on partition /dev/sdd1
and /dev/sdd2 respectively. I am doing IO only on partition /dev/sdd2
(ioband2).
Following is fio job script.

[seqread]
runtime=60
rw=read
size=2G
directory=/mnt/sdd2/fio/
numjobs=1
group_reporting


Following is small file read script.

echo 3 > /proc/sys/vm/drop_caches
for ((i=0;i<5;i++)); do
printf "file #%4d, plain reading it took: " $i
/usr/bin/time -f "%e seconds" cat /mnt/sdd2/files/$i >/dev/null
sleep 1
done


Thanks
Vivek



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