I got a cheap netbook style laptop for traveling some weeks ago (HP Stream 11" with 4 GB of RAM and a N4120). Didn’t expect much more from this hardware than opening a few browser tabs and doing some retro gaming via Steam.
Shared RAM with graphics card means that 3.64 GB of RAM are effectively usable for the OS. This was even too little RAM to open a handful of tabs w/o having tabs being unresponsive for seconds sometimes in a very annoying way. Another thing which made trouble was the Wifi - I guess it went into power saving, was swapped and didn’t load fast enough to provide a good experience. (Of course I wasted an hour checking for Wifi drivers/support.)
In short: Even for my low expectations for this laptop it was an underwhelming experience.
First step was to look at my vm.swappiness and set it to 10, which already helped, but still the machine had hiccups and annoying timeouts.
In a last, desperate effort I enabled ZRAM on the laptop… and literally WTF: Saying it is a night and day difference doesn’t do the experience justice. Typing this words now on the Stream, which I use exactly the same way as my much more beefy other machines (my next worst computer has 8G of RAM and an Intel Core i3), browsing with 10 open tabs, e-mail client open on another virtual desktop… it is crazy, it makes the Stream fun to use and I use it at home for everything which isn’t heavily CPU/IO bound.
What surprised me the most: No hiccups, no timeouts and it even fixed the Wifi issues on this little machine. Didn’t expect this would be possible, especially with a N4120 and 3.64 GB of RAM.
In short, my laptop changed from not even reaching my low/realistic expectations to being my favorite technical purchase of the last years, thanks to ZRAM.
Besides making this a ZRAM appreciation post, I really want to spread the word about it. Especially for old hardware and limited RAM situations, IMHO it should be the first thing which comes to mind/is recommended.
Fedora and PopOS use it by default, so it is well tested and should IMHO again, be a default at least for desktop setups.
Give it a try - supposedly it even improved the experience on much more beefy computers for gaming etc.
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Thanks, great input.
I totally agree - ZRAM isn’t magic and of course it will fall flat on its face for loads of encrypted, compressed or pure random data. In my limited day to day usage I just never hit that situation, so far.
Again, I fully agree, I wouldn’t have expected that the N4120 works so well with ZRAM. For work I am forced to use a recent mac with loads of RAM. When just browsing the web/checking emails I don’t feel any noticeable difference between the mac and the Stream for the CPU. (Of course, CPU/IO bound tasks are another story, and the display of the mac is in another league.) Usually I would consider myself to be quite sensitive to speed, I notice a real difference between using Gnome (with impatience etc. extensions) and Xfce, concerning the responsiveness of my desktop.
I’ll check the BIOS settings, I expect the same as you. Not sure, if I will lower the reserved RAM for the iGPU, everything works fine at the moment and I want to try some light gaming on this machine.
I’m not going to remember the right terminology, but you can also configure it with a chunk of disk to stick files that it can’t compress into so they don’t end up clogging up your swapspace.
Not sure if you are referring to ZSWAP, which is backed by physical swap and writes uncompressable pages to the physical swap. ZRAM has AFAIK an option called WRITEBACK, which allows it to also use physical swap, but I didn’t find that ZRAM is concerned if it can compress the page or not. (Grain of salt and if someone is more knowledgeable I happily be corrected.)
Anyone know of an eli5 on zram? I don’t see why just compressing your RAM would make it run better.
Let me give it a try:
Imagine you are having breakfast and sitting on your breakfast table. Everything on your table and reachable w/o getting up is what your CPU holds in its register. When you need something from the fridge in your home, this is your RAM. If you need something that is not in your fridge, you have to get dressed, get out of our home, walk to the groceries store which is half an hour away, find what you are looking for, pay for it, walk home for around half an our, switch back to your relaxed clothing and finally you can continue your breakfast. The groceries store is your hard disk/ssd whatever. With compression, imagine you have a big second fridge in the basement (or the house next to yours, you get the idea). Not as good as having stuff on your table or in your fridge, but usually at least an order of magnitude better than having to visit the groceries store.
Turns out usually a significant amount of RAM is compressible. I was surprised at it too, and actually still am. But of course it also depends on how you use your system. If you run a media player that caches a lot to RAM, its cache wont be compressible, but they say its efficient for example for memory of web browsers.
I’m running ZRAM on my old Netgear ReadyNAS’s, which has 512MB or 256MB RAM. It enables them to do a lot more than they otherwise would be able to, running a modern linux distribution.
I’ve been so satisfied with it that I even started running it on my modern desktop with 32GB RAM, it helps with my tab addiction :)
Rolling out ZRAM to all my boxes right now! 🙃
Do you also tweak other settings for ZRAM? According to ArchWiki PopOS settled for the following settings:
vm.swappiness = 180
vm.watermark_boost_factor = 0
vm.watermark_scale_factor = 125
vm.page-cluster = 0
I am testing this settings right now and cannot say I experience a difference compared to the defaults.
Kernel 5.8 changed this, so the value is from 0 to 200.
https://docs.kernel.org/admin-guide/sysctl/vm.html#swappiness
I read several articles on this topic, and it seems there are urban legends and misunderstandings about setting vm.swappiness. The 180 was an experimental result from PopOS and Fedora people, and it only makes sense with ZRAM AFAIK. … anyway, long story short, I ended up with the default of 60 after some testing. ;-)
Has anyone ever actually benchmarked vm.page-cluster = 0? Makes no sense to me to suggest a cpu is so bottlenecked that disabling read-ahead would actually help. If anything it would mitigate the decompression time if it guessed correctly as the work would already be done if left at the default of 3. Normally cpu is not bound when using zram because its quite low cpu anyway.
For my own usage, I just enabled ZRAM, 50% with zstd and left all other options unchanged, which seems to be the conservative approach and more important: It works on my machines™.
I would love to see some benchmarks or even a structured run down about the available options and configurations.
zstd compression algorithm can be overkill depending of your use case. I’d try lz4 compression algorithm just to see if it sits well with you, since it’s overally faster, less CPU intensive, than zstd at a possibly negligible compression rate.
Have been using zstd for compression with ZRAM at least for my raspberry pi for years now, and I don’t experience any CPU bottleneck in usual usage, but memory is really scarce, so I’ll stick to it.
If I remember correctly, other people on reddit, from Fedora and PopOS did some benchmarks. Fedora defaults to lz4, AFAIK and PopOS to zstd.
If you like ZRAM, make sure you also enable MG-LRU and consider using ZSWAP with z3fold allocator instead because it’s capable of dumping older compressed pages to swap file whereas ZRAM, once full, is simply bypassed until pages in the store are freed.
I do number crunching on memory constrained systems. MG-LRU improves the efficiency of page reclaim, and ZSWAP interacts much more nicely if you have a swap file also.
Thank you for your advice. Right now my setup works so I’ll try to not waste my entire weekend playing with technology, but if I need further tweaks in the future I’ll look into ZSWAP.
One question: Do you know why Fedora and PopOS decided for ZRAM and against ZSWAP? As I wrote already somewhere else: I did choose ZRAM for it being default in Fedora, which gave me some trust that people better informed/experienced choose ZRAM over ZSWAP.
Totally understandable. I have done the wasting of weekends just to go back to where I started.
I think that ZRAM has a simpler implementation and has a history of being more widely used whereas ZSWAP is only recently seeing more usage as a default. I suspect it’s in the interest of stability and because the implementation is better characterized.
With that said, MG-LRU is not enabled by default for the same reason, but it has a big impact such that it’s the default on newer Android devices. Stability is a relative term.
This is a theoretical advantage of ZSWAP over ZRAM, but when I researched it, every real world comparison I found seemed to find that ZRAM performed better even when this advantage should have come into play.
I’d be very interested in references to that research so I can improve the performance of my memory constrained systems.
Sorry, it’s been a while since I read this stuff and I don’t have the links. The state of web searching these days sucks and I can’t easily find them.
One bit I remember was that a lot of the concern about LRU inversion in ZRAM that might make ZSWAP look preferable is out of date since the addition of a writeback option to ZRAM. I also remember people claiming that ZRAM had an advantage in being multithreaded.
FWIW I find this three year old answer saying the kswapd that ZSWAP uses is single threaded but there is a patch to make it multithreaded that significantly improves it’s performance. No idea if this is out of date.
So ZRAM is RAM, but compressed? I didn’t know about it, thanks for sharing!
