Well, this post made me reassess using zswap vs zram. I've expanded my zswap and disabled zram. Not sure if that makes any difference to others, but it's probably good for me.
Stop making swap partitions—use swap files instead
151–160 of 257 posts
Re: Stop making swap partitions—use swap files instead
#152For a while since RAM sizes have gotten so big I would set up linux servers without swap at all. I figured what is a 2GB swap file going to do if the system has already exhausted 128GB of real RAM? I never saw any issues doing this, but I learned that linux ideally wants some swap space, which it will use for some kind of housekeeping stuff.
It does not need it at all. I’ve run a million+ hosts with no swap. You need to be really disciplined about memory allocations (us cgroups) and ok with what happens if you run out (something in that cgroup dies). But there is no “housekeeping “ that requires it.
This comes up every time swap is mentioned.
Even if you have plenty of ram, having swap can help with performance.
Why? Because the OS can page rarely used or seemingly downright unused dirty pages to swap, freeing up precious RAM for caches.
I'd hazard a guess that especially if you run a ton of containers this can help, as there's probably a lot of memory getting dirtied on initialization of containers and their processes, which ends up never read afterwards.
I always set up swap, because sacrificing some disk space in potential exchange of free RAM, which is orders of magnitudes more expensive, is a nice prospect.
Re: Stop making swap partitions—use swap files instead
#153For a while since RAM sizes have gotten so big I would set up linux servers without swap at all. I figured what is a 2GB swap file going to do if the system has already exhausted 128GB of real RAM? I never saw any issues doing this, but I learned that linux ideally wants some swap space, which it will use for some kind of housekeeping stuff.
Any tips on measuring the impact of not having swap? I never use it on my machines for similar reasons.
It highly depends on your specific workload, but freeing up RAM by paging out rarely or downright unused pages to swap can definitely give a performance advantage.
In consequence, I always give swap. Even on the off-chance it ends up unused, disk space is orders of magnitude cheaper than RAM, so I don't mind sacrificing it for potential gains.
Re: Stop making swap partitions—use swap files instead
#154For a while since RAM sizes have gotten so big I would set up linux servers without swap at all. I figured what is a 2GB swap file going to do if the system has already exhausted 128GB of real RAM? I never saw any issues doing this, but I learned that linux ideally wants some swap space, which it will use for some kind of housekeeping stuff.
You never saw any issues, but your systems might have been faster with swap. Even if you're nowhere any threshing threshold, RAM used for caches is better than RAM used for unused pages, which could have been paged out to swapspace.
Note that RAM is much, much more expensive than disk space. If I can sacrifice some of the latter to potentially get some of the former, it's usually a nice trade for me.
Re: Stop making swap partitions—use swap files instead
#155Earlier quoted context omitted.
I have tried to forego swap because I had hoped that it would avoid swap thrashing, where performance goes into the toilet; I'd rather a process was killed but I could still access the system, than be unable to login to it. But I've found that even with no/little swap, the system can get into high memory pressure situations (even easier because it can't swap out unused pages of libraries and the like), and still go i…
Read only pages (most of libraries and binaries) get “paged out” to the file they came from, not swap. (They obviously don’t page out, they are just discarded and read again when needed)
Yes, only dirty (i.e. modified) pages profit from getting paged out to swap.
But while I don't have any hard data, I imagine that depending on the workload, there can be a punch of pages that are read during initialization, or a one-off workload, and never being read or freed up afterwards.
Just try activating a swap file, and see if it gets any usage, despite memory pressure being low.
Re: Stop making swap partitions—use swap files instead
#156Couple of things this didn't consider.
* If you have a small root partition, the default config puts the swap file on there, wasting precious space.
* With a partition, it's easier to put swap on a separate physical device, which is good for performance.
* If you dual boot 2+ Linux distributions or installations on a single machine, as I do routinely, you can share a single swap partition between distros no problem. This means it's more space efficient and keeps down partitioning complexity.
* And of course there's hibernation, as others have mentioned.
Conclusion: no, bad advice, except for very simplistic scenarios.
Re: Stop making swap partitions—use swap files instead
#157Earlier quoted context omitted.
I have tried to forego swap because I had hoped that it would avoid swap thrashing, where performance goes into the toilet; I'd rather a process was killed but I could still access the system, than be unable to login to it. But I've found that even with no/little swap, the system can get into high memory pressure situations (even easier because it can't swap out unused pages of libraries and the like), and still go i…
The old rule of thumb used to be 2x actual RAM for your swap file/partition size. That's probably not right with today's RAM sizes but I'm not sure what the current rule is. I just go with the Ubuntu installer default but I don't know if that's just a fixed number or computed in some way from amount of RAM available.
> The old rule of thumb used to be 2x actual RAM for your swap file/partition size.
Which was always a dumbass rule that made no logical sense.
Like, I remember a time when I had 4 GB of RAM and had my swap set to 8 GB. Then I upgraded to 16 GB of RAM. Theoretically, I shouldn't even need a swap file anymore, since my old setup would max at 12 GB and yet I never had OOM problems, but that "rule of thumb" would dictate that I should increase my swap to 32 GB.
Re: Stop making swap partitions—use swap files instead
#158Earlier quoted context omitted.
It does not need it at all. I’ve run a million+ hosts with no swap. You need to be really disciplined about memory allocations (us cgroups) and ok with what happens if you run out (something in that cgroup dies). But there is no “housekeeping “ that requires it.
This comes up every time swap is mentioned. Even if you have plenty of ram, having swap can help with performance. Why? Because the OS can page rarely used or seemingly downright unused dirty pages to swap, freeing up precious RAM for caches. I'd hazard a guess that especially if you run a ton of containers this can help, as there's probably a lot of memory getting dirtied on initialization of containers and their pr…
We looked at this a lot (it was a lot of money at our scale). A cgroup itself is just a few small data structures. And a reasonable process inside will only ever have a small amount of dirty memory it won’t ever use. Even then you can’t be sure of it packing neatly into whole pages.
So, the typical saving is tiny, and the cost in unpredictable performance is quite bad.
If you really do have an issue with “lost” dirty memory, your time is best spent addressing that directly.
Re: Stop making swap partitions—use swap files instead
#159Earlier quoted context omitted.
Read only pages (most of libraries and binaries) get “paged out” to the file they came from, not swap. (They obviously don’t page out, they are just discarded and read again when needed)
Yes, only dirty (i.e. modified) pages profit from getting paged out to swap. But while I don't have any hard data, I imagine that depending on the workload, there can be a punch of pages that are read during initialization, or a one-off workload, and never being read or freed up afterwards. Just try activating a swap file, and see if it gets any usage, despite memory pressure being low.
When we looked it was always tiny
Re: Stop making swap partitions—use swap files instead
#160Are there people still using swap partitions? I've been using swap files on Linux for years now.