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Memory Metrics

Memory Usage

Is free memory really free? What's the difference between cached memory and buffers? Let's get our hands dirty and find out...

You will need 3 open terminals for this task. DO NOT RUN ANY SCRIPTS ON YOUR LAPTOP!

Task M1: Memory usage, Caches and Buffers

  1. Fire up top on Terminal 1, and write down how much free memory you have (keep it running for the rest of this module): 
    (term 1) root:~# top
  2. Start the memory hog hog.sh on Terminal 2, let it run until it gets killed (if it hangs- use Ctrl+c): 
    (term 2) root:~# /bin/sh linux-metrics/scripts/memory/hog.sh
  3. Go to Terminal 1 and compare that to the number you wrote. Are they (almost) the same? If not, why?
  4. Read about the Buffers and Cached values in man 5 proc (under meminfo):
    1. Run the memory hog on Terminal 2 scripts/memory/hog.sh:
    (term 2) root:~# /bin/sh linux-metrics/scripts/memory/hog.sh
    1. Write down the buffer size from Terminal 1.
    2. Now run the buffer balloon buffer.sh on Terminal 2:
    (term 2) root:~# /bin/sh linux-metrics/scripts/memory/buffer.sh
    1. Check the buffer size again.
    2. Read the script, and see if you can make sense of the results.
    3. Repeat all 5 steps above with the cached Mem value.
    4. Repeat all steps for cache.sh:
    (term 2) root:~# /bin/sh linux-metrics/scripts/memory/cache.sh
  5. Let's see how cached Mem affects application performance:
    1. Drop the cache:
    (term 2) root:~# echo 3 > /proc/sys/vm/drop_caches
    1. Time a dummy Python application (you can repeat these 2 steps multiple times):
    (term 2) root:~# time python -c 'print "Hello World"'
    1. Now re-run our dummy Python application, but this time without flushing the cached memory. Can you see the difference?
  6. Run the dentry.py script and observe the memory usage using free. What is using the memory? How does it effect performance? 
    (term 2) root:~# python linux-metrics/scripts/memory/dentry.py
(term 2) root:~# echo 3 > /proc/sys/vm/drop_caches
(term 2) root:~# time ls trash/ >/dev/null
(term 2) root:~# time ls trash/ >/dev/null
  7. Run the dentry2.py script and try dropping the caches. Does it make a difference? 
    (term 2) root:~# python linux-metrics/scripts/memory/dentry2.py

Discussion

  • What's the difference between dentry.py and dentry2.py?
  • Assuming a server has some amount of free memory, can we assume it has enough memory to support it's current workload? If not, why?
  • Why wasn't our memory hog able to grab all the cached memory?
  • Run the following stress test, what do you see? 
    (term 2) root:~# stress -m 18 --vm-bytes 100M  -t 600s

Tools

  • Most tools use /proc/meminfo to fetch memory usage information.
    • A simple example is the free utility.
    • What does the 2nd line of free tell us?
  • To get usage information over some period, use sar -r <delay> <count>.
    • Here you can also see how many dirty pages you have (try running sync while sar is running).

Next: IO Usage