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ep:labs:02:contents:tasks:ex2 [2020/10/24 22:22] radu.mantu [02. [20p] Swap space] |
ep:labs:02:contents:tasks:ex2 [2025/02/11 23:22] (current) cezar.craciunoiu [02. [10p] Feedback] |
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| - | ==== 02. [20p] Swap space ==== | + | ==== [10p] Feedback ==== |
| - | + | ||
| - | <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> | + | |
| - | + | ||
| - | Now that we created a //swap file//, what are the advantages / disadvantages when compared to a //swap partition//? | + | |
| - | * advantages: | + | |
| - | * easier to manage | + | |
| - | * similar performance | + | |
| - | * disadvantage: | + | |
| - | * can be affected by disk fragmentation (not the case for a partition) | + | |
| - | + | ||
| - | === [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 |}} | + | |
| - | </solution> | + | |
| - | + | ||
| - | === NON-DEMO TASK === | + | |
| - | + | ||
| - | Create two swap files. Set their priorities to 10 and 20, respectively. \\ | + | |
| - | Include the commands (copy+paste) or a screenshot of the terminal. | + | |
| + | Please take a minute to fill in the **[[https://forms.gle/NpSRnoEh9NLYowFr5 | feedback form]]** for this lab. | ||
