Ram upgrade vs. adding a swapfile

ordinary

I have a VPS with around 35 Wordpress installs on Linode. It's currently on the 4GB RAM / 80GB HDD plan. Disk space is under 50% but even when the server is sitting on <10% CPU the memory usage reported in Enhance is 75-85%:

free -m
total used free shared buff/cache available
Mem: 3911 1213 160 1524 2537 886
Swap: 511 511 0

I know there are a number of great threads here regarding memory usage (and reminders not to panic about Linux managing memory) but I'm pretty sure this is a problem.

I found Linode a community post explaining how to create an additional swapfile: https://www.linode.com/community/questions/19273/how-can-i-get-more-swap-space

I could just upgrade the server but otherwise performance seems fine and I don't need more disk space. It seems that adding a swapfile could be a good stop-gap solution, but I don't want to cause myself new headaches.

I would appreciate any advice!

Zoinkies

I’ve tried to avoid swap at all opportunity. Swap isn’t good for performance and additional ram would be better.

twest

Zoinkies yeah I actually ended up setting my swappiness down to 0 or 1 (I think 0). So it's basically OFF, but still theoretically available if the kernel has an emergency, help prevent kernel panic. I'm running 250GB of ram per server though, so if I ever ran low enough on real memory to need swap I'd be utterly fugged anyways. Disabling swap was one of the things that helped me get rid of random load spikes that were plaguing me for months.

twest

RAM usage doesn't really have a lot to do with CPU usage.

You're also confused about how much RAM your instance is consuming. According to your data paste you're only using 1213MB of 3911MB, 31% of RAM used. The rest is cache, Linux is very good about caching unused RAM to help speed things up, if your system needed to use RAM then it would reduce the cache to allow the usage.

You also already have a 511MB swap file, which is sufficient for your small server. Resizing your swap file to be larger to create some "artificial ram" would be a terrible idea, end up causing your entire system to run at a crawl.

You didn't seem to post anything about a problem, so it seems you're just worried based on lack of understanding of basic Linux operation. From what you've pasted, everything looks good and normal. You might want to reduce your swappiness level if your system is using a lot of the swap, because swap memory is so slow.

If anything, I'd be upgrading the base server to get more CPU and RAM. 35 sites with so little available system resources is just asking for trouble.

ordinary

twest You're absolutely right - I need to learn more about how Linux manages memory. I was focussed on Enhance's server page showing me "85%" RAM utilisation without realising much of that is buffer cache.

The sites I've chosen to move to this server don't see a lot of activity and there's no problem with performance (yet). Based on your advice I'll definitely make a server upgrade my next action.

Thanks again for your help!

xyzulu

I always go with a zero swap setup now. I’d recommend it for anyone serious about proper server management.

twest

xyzulu that's not really "proper server management" though. Linux expects there to be swap, swap is a normal function of the kernel, disabling swap is abnormal. It's more dangerous on smaller servers where the chance of OOM is greater, like little vps servers. I have 200GB of free ram and most other sydadmins I spoke with about it still insisted on keeping swap, avoid catastrophic kernel panic situations.

In my opinion the safest config is keeping swap available, and lowering swapiness to prevent all but the most critical of swapping. A small server I'd recommend swapiness 1-5.

I do like to prevent any swapping, but also worry about kernel panic. Everyone should definitely weigh the pros and cons and monitor their own system to see what works best.

AdamM

twest You're right. I always use swap because it helps avoid OOM

xyzulu
Just run tests that load the server heavily with and without swap and see the results...

xyzulu

Swapping always reduces performance. I’d prefer a service gets killed (and restarted) instead.

AdamM

xyzulu This will not restart the services but will freeze the entire server... Sometimes even a restart is required. I have done many tests, filling up all the RAM can cause many problems.

But of course it's your choice.

xyzulu

I get it..I have tried things both ways.. and tested again over the years. If you need swap space, you don't have enough ram.

In my experience on enhance I am yet to need to power cycle a server. I have no swap set on all my servers.. some, for example are 6-8core CPU with 16Gb ram and 75 website (some reasonable traffic). Loads are never over 1.. and while I can get some OOM kills, these are due to the enhance screenshot service (getting better) and mysql/mariadb.

I'm always open to learn more.. but I am fairly set on this "no swap is a good idea" opinion at this time.

SharedGrid

xyzulu while I can get some OOM kills, these are due to the enhance screenshot service (getting better) and mysql/mariadb.

You let MySQL/MariaDB get killed due to OOM?

xyzulu

SharedGrid Not on purpose 😉
While using mysqltuner to set the correct my.cnf (mariadb) variables I have seen a few OOM mariadb kills. Again, it's not a good thing.. and a sign your ram/mysql/server setup needs to be adjusted.

JohnB

In my experience set swapiness to almost 0 and have monitoring that alerts you when swap is being used. Any respectable hosting enviroment should have enough real memory but if something goes wrong like a memory leak you have swap to fall back on and alert you so that hopefully the issue can be fixed before swap is also exhausted.

Goes without saying that swap will never ever be close to the performance (or stability) of real memory.

mike

just my 2 cents, do not avoid swap and do not set it to 0 because it will only change the way swap works.

we set all our swapiness to 5 (depends how much ram u have in)

if you saw it in mariab db to set a database server to 1 it assumes ur server does only one thing .. a db.

watch your used ram and upgrade.