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ep:labs:04:contents:tasks:ex2 [2021/09/27 13:36] radu.mantu [02. [20p] Traffic monitoring - Tcpdump] |
ep:labs:04:contents:tasks:ex2 [2025/02/11 23:36] (current) cezar.craciunoiu |
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| - | ==== 02. [??p] iperf3 | + | ==== 02. [20p] Swap space ==== |
| - | TODO | + | <note warning> |
| + | Before starting this task, call the assistant to show him your progress. If you manage to freeze your PC, it might prove tricky to do so afterwards. | ||
| + | </note> | ||
| + | === [10p] Task A - Swap File === | ||
| + | |||
| + | First, let us check what swap devices we have enabled. Check the //NAME// and //SIZE// columns of the following command: | ||
| + | <code> | ||
| + | $ swapon --show | ||
| + | </code> | ||
| + | No output means that there are no swap devices available. | ||
| + | |||
| + | If you ever installed a Linux distro, you may remember creating a separate //swap partition//. This, however, is only one method of creating swap space. The other is by adding a //swap file//. Run the following commands: | ||
| + | <code> | ||
| + | $ sudo swapoff -a | ||
| + | $ sudo dd if=/dev/zero of=/swapfile bs=1024 count=$((4 * 1024 * 1024)) | ||
| + | $ sudo chmod 600 /swapfile | ||
| + | $ sudo mkswap /swapfile | ||
| + | $ sudo swapon /swapfile | ||
| + | |||
| + | $ swapon --show | ||
| + | </code> | ||
| + | |||
| + | Just to clarify what we did: | ||
| + | * disabled all swap devices | ||
| + | * created a 4Gb zero-initialized file | ||
| + | * set the permission to the file so only //root// can edit it | ||
| + | * created a swap area from the file using **mkswap** (works on devices too) | ||
| + | * activated the swap area | ||
| + | |||
| + | The new swap area is temporary and will not survive a reboot. To make it permanent, we need to register it in [[https://en.wikipedia.org/wiki/Fstab|/etc/fstab]] by adding a line such as this: | ||
| + | |||
| + | <code> | ||
| + | /swapfile swap swap defaults 0 0 | ||
| + | </code> | ||
| + | |||
| + | === [10p] Task B - Does it work? === | ||
| + | |||
| + | In one terminal run **vmstat** and look at the //swpd// and //free// columns. | ||
| + | <code> | ||
| + | $ vmstat -w 1 | ||
| + | </code> | ||
| + | |||
| + | In another terminal, open a python shell and allocate a bit more memory than the available RAM. Identify the moment when the newly created swap space is being used. | ||
| + | |||
| + | One thing you might notice is that the value in **vmstat**'s //free// column is lower than before. This does not mean that you have less available RAM after creating the swap file. Remember using the **dd** command to create a 4GB file? A big chunk of RAM was used to buffer the data that was written to disk. If //free// drops to unacceptable levels, the kernel will make sure to reclaim some of this buffer/cache memory. To get a clear view of how much available memory you actually have, try running the following command: | ||
| + | |||
| + | <code bash> | ||
| + | $ free -h | ||
| + | </code> | ||
| + | |||
| + | Observe that once you close the python shell and the memory is freed, //swpd// still displays a non-zero value. Why? There simply isn't a reason to clear the data from the swap area. If you really want to clean up the used swap space, try the following: | ||
| + | <code> | ||
| + | $ vmstat | ||
| + | $ sudo swapoff -a && sudo swapon -a | ||
| + | $ vmstat | ||
| + | </code> | ||
| + | |||
| + | <solution -hidden> | ||
| + | Output here: | ||
| + | |||
| + | {{ :ep:labs:ep2017_l2_ex05.png?550 |}} | ||
| + | |||
| + | Free memory goes down, swap usage goes up. | ||
| + | </solution> | ||
| + | |||
| + | Create two swap files. Set their priorities to 10 and 20, respectively. \\ | ||
| + | Include the commands (copy+paste) or a screenshot of the terminal. \\ | ||
| + | Also add 2 advantages and disadvantages when using a //swap file// comparing with a //swap partition//. | ||
