Large delay starting “dd” write in background in bash even when using nohup

Question

I wrote a small script to print the memory usage during a large sequential write of a file.

#!/bin/bash
rm result
echo 3 > /proc/sys/vm/drop_caches
sync;
echo start
nohup time dd if=/dev/zero of=mem bs=1M count=2000 & 
for i in {1..200}
do
  sleep 0.2
  cat /proc/meminfo | grep Dirty >> result
  cat /proc/meminfo | grep Dirty
done
cat nohup.out
cat result

I should see the increase of the "Dirty" size from the beginning of the run. But when I ran the script, I often see a big delay (up to several seconds), during which the "Dirty" size does not increase, which possibly means the start of "dd" program is delayed. A sample problematic output is:

Dirty:                20 kB
Dirty:                20 kB
Dirty:                20 kB
Dirty:                20 kB
Dirty:                20 kB
Dirty:                24 kB
Dirty:                24 kB
Dirty:                24 kB
Dirty:                24 kB
Dirty:                28 kB
Dirty:                28 kB
Dirty:                28 kB
Dirty:                28 kB
Dirty:                28 kB
Dirty:             16528 kB
Dirty:            140608 kB
Dirty:            277228 kB
Dirty:            311768 kB
Dirty:            434308 kB
Dirty:            563352 kB
Dirty:            690952 kB
...

The length of the delay is uncertain, sometimes there's no delay at all. And in contrast, when I ran

time dd if=/dev/zero of=mem bs=1M count=2000

with some real time meminfo viewer, such as:

#!/bin/bash
clear
while true
do
  sleep 0.2
  tput home
  cat /proc/meminfo
done

I always see the "Dirty" size increases immediately. Is there something wrong with my script? I also doubt about how the "write" operation is executed by the OS, because I also tested the file read and detected the "Cached" field in /proc/meminfo, and it seems to have no delay at all.

Thanks,

Answer 1

nohup time dd if=/dev/zero of=mem bs=1M count=2000 & 

do you have a "time" binary on your system? otherwise nohup wouldn't know how to run a bash internal by itself and it will fail

Answer 2

What if you simply remove 'nohup' from your script - does it change anything?